brokenhii/l, 14-17 n 1994 - … 14-17 n 1994 abstracts by c f pain, ... kprd lnn dtr ttrn indt rr...

AUSTRALIAN REGOLITH

CONFERENCE '94

BROKENHII/L, 14-17 N 1994

ABSTRACTS

By C F PAIN, M A CRAIG, & I D CAMPBELL (EDITORS)

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AUSTRALIAN GEOLOGICAL SURVEY ORGANISATION

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CARS

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Centre for Australian Regolith Studies

AUSTRALIAN REGOLITH CONFERENCE '94

Broken Hill, 14 - 17 November 1994

ABSTRACTS

Record 1994/56

Edited by

C F Pain and M A Craig Division of Regional Geology and Minerals, AGSO

and

lain D Campbell Centre for Australian Regolith Studies

University of Canberra

AGSO


Conference Convenors: C F Pain, Division of Regional Geology and Minerals, AGSO lain D. Campbell, Centre for Australian Regolith Studies, University of Canberra

Organising Committee: Graham Taylor, Centre for Australian Regolith Studies, University of Canberra K G McQueen, Centre for Australian Regolith Studies, University of Canberra Tony Eggleton, Centre for Australian Regolith Studies, Australian National University M A Craig, Division of Regional Geology and Minerals, AGSO

Fieldtrip: S M Hill, Centre for Australian Regolith Studies, Australian National University

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DEPARTMENT OF PRIMARY INDUSTRIES AND ENERGY

Minister for Resources: Hon. David Beddall, MPSecretary: Greg Taylor


Executive Director: Harvey Jacka

C Commonwealth of Australia 1994

ISSN: 1039-0073ISBN: 0 642 22146 4

This work is copyright. Apart from any fair dealings for the purposes of study,research, criticism or review, as permitted under the Copyright Act, no part may bereproduced by any process without written permission. Copyright is the responsibilityof the Executive Director, Australian Geological Survey Organisation. Inquiriesshould be directed to the Principal Information Officer, Australian GeologicalSurvey Organisation, GPO Box 378, Canberra City, ACT, 2601.

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The Australian Regoflith Conference '94 is sponsored and supported by the following organisations:

Aberfoyle Limited

Australian Geological Survey Organisation

Australian National University

BHP

Broken Hill Regional Tourist Association

Centre for Australian Regolith Studies

Council of the City of Broken Hill

CRA Exploration

Geological Society of Australia

Normanby Poseidon

North Exploration

University of Canberra

Western Mining Corporation

The conference also benefited from two predecessors:

Cainozoic Evolution of Southeastern Australia Conference, 1980

The Age of Landforms in Eastern Australia Conference, 1986

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TABLE OF CONTENTS•Alipour, S., A.C. Dunlop and D.R. Cohen. Morphology of Lags in the Cobar Area, NSW^ 1

• Anand, R.R., R.E. Smith and L.D.M. Robertson. Classification and Origin of Laterites and Ferruginous

•Regolith Materials in the Yilgarn Craton, Western Austarlia^Error! Bookmark not defined.

Anand, R.R., C. Phang and M.J. Lintern. Morphology and Genesis of Pedogenic Carbonates in some• Areas of the Yilgarn Craton^ 4

•Make!, Aro. Quaternary Vadose Calcretes Revisited^ 5

Bourman, R.P. The Genesis of Ferricretes in the Mount Lofty Ranges of South Australia.^ 641^Bourman, R.P. Towards Distinguishing Transported and in situ Ferricretes: Data from Southern Australia^7

• Britt, A.F. and Tony Eggleton. Duricrust and its relationship to the landscape at Johnson Creek, NorthwesternQueensland^ 9

• Butt, C.R.M. Regolith Evolution at Mount Percy, Kalgoorlie, Western Australia^ 10

• Butt, C.R.M. and R.R. Anand. Terminology of Deeply Weathered Regoliths^ 11

•Cairns, Chris, K.G. McQueen and Graham Taylor. Regolith Geochemistry and Mineralogy Over

Two Mineralised Shear Zones near Cobar, NSW^ 13• Campbell, lain D. and Graham M. Taylor. Bauxite Genesis and Landscape Evolution at Gove,

Northern Territory^ 14Chan, Roslyn. Geomorphic and Regolith Evolution of the Bathurst Region, NSW^ 15

• Chapman, G.A. Soil Landscape Maps and Regolith Information^ 16

• Chen , X.Y. (Pedogenic) Gyperete Formation in Arid Central Australia^ 17Chivas, Alan R. and Michael I. Bird. The Use of Oxygen- and Hydrogen-Isotopes in Weathering

• Profiles to Indicate Past Climates, Latitudes, Altitudes and Ages of Formation^ 18

• Chivas, Alan R. and Colin Wood. Stable-Isotope Indicators of the Origin and Age of the WeatheringZones on the Broken Hill Orebody^ 19

• Clarke, Jonathan D.A. Pisolites all over: Classification, Genesis, and Evolution of Ferruginous Surface Grains ^20

• Craig, M.A. and H.M. Churchward. Regolith Landform Features of the Northeast Yilgarn, WA^21

•Crick, I.H. Evolution of some Landforms and Regolith in Pastern Arnhem Land^ 22Dammer, D., I. McDougall and A.R. Chivas. Episodic Weathering: Evidence from 40Ar/39Ar Laser

• Microprobe and Two-Stage K-Ar Dating of Weathering-Related Cryptomelane-Hollandite andAlunite/Jarosite in Western and Northern Australian Regolith^ 23

• Edgoose, C.J. and K. T. Winstanly. Landscape Evolution on the Barkly Tableland^ 24

• Golding, Suzanne D. and John A. Webb. Stable Isotope Constraints on Silcrete Formation^26

•Goldrick, Geoff. Stream Morphology and Neogene Landscape Evolution in the Lachlan Valley, NSW^27Grant, A.R. Landscape Processes in the Upper Todd River Catchment, Central Australia, and Implications

• for Assessing Land Use Impacts^ 28

•Greene, R.S.B. and W.D. Nettleton. Soil Genesis in a Longitudinal Dune-Swale Landscape, NSW, Australia^29Greffie, C., M. Amouric, M. Benedetti and C. Parron. High Resolution Microscopy of Synthetic

• Ferrihydrites and their Transformation into Goethites^ 30

•Hazel!, Murray. RTMAP - AGSO's Regolith Mapping Database^ 31Hill, S.M. The Differential Weathering of Granitic Rocks in Victoria, Australia^ 32

• Joyce, E.B. and D. Lulofs. Mobility of Base Metals through Regolith at Broken Hill, NSW, based ondetailed Regolith Mapping and Chemical Analyses^ 33

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Joyce, E.B., J.A. Webb and N.G. Collins. The Geochemistry of Sub-Basaltic Silcretes in Central Victoria 34 •Kamprad, Julienne. Radiometric Pattern Indicates Prior Alluvial Material 35 •Kotsonis, Andrew. The Karoonda Pedoderm: Plio-Pleistocene Lateritic Profile of the Western

Murray Basin, Southeastern Australia. 36 •Lawie, David and Paul M. Ashley. Surficial Geochemical Expression of Mineralised Systems in the

Glary Block, South Australia 37 •

Lawrance, L.M. Signposts Old and New: Prediction of Present and Palaeo-Supergene Geochemical •Environments using Iron Redox Chemistry 38

Le Gleuher, MaIte, lain D. Campbell, Tony Eggleton and Graham Taylor. Evidence for a Buried Paleosol •in the Weipa Region,North Queensland. 39

Ma, Chi and Tony Eggleton. Structural Characteristics of Kaolin Minerals from Eastern Australian Regolith 40

McGowran, Brian. Chronicling the Australian Duricrusts: Relevance of the Marine Succession 41 •

McNally, G.H. and I.R. Wilson. Some Observations on the Miracicina Pale,ochannel 42McQueen, KG. Geochemical Exploration of Retrograde Schist Zones: Could this Approach Detect

Blind Broken Hill Type Orebodies? 43 •

Mora, Surendra and Aro Arakel. Genesis of Laterite Deposits in Western India 45 •Munday, Tim, Stewart Rodrigues and Andy Gabell. Getting the most out of Geophysical Data

Sets for Regolith Mapping Purposes - the Application of Forward Modelling and Residual Analysis 46 •

Nott, Jonathan. The Antiquity of Landscapes on the North Australian Craton and the Implications for •Theories of Long-Term Landscape Evolution 47

Orr, Meredith. Feedback Effects of Regolith Development in Shaping the Dissection of the Great Divide 111/in Eastern Victoria 48

Pain, C.F. and C.D. Oilier. Some Misconceptions about Regolith Stratigraphy 49Pillans, Brad. The Brunhes/Matuyama Polarity Transition (0.78 ma) as a Chronostratigraphic Marker •

in Australian Regolith Studies 51

Ridley, W.F. Soil Genesis in Basaltic Upland Terrain of Tropical and Subtropical Regions basedupon Profile Chemistry and Mineralogy of Dominantly Australian Soils 52 •

Rivers, Clinton J. Ferricretes and Mesas of the Puzzler Walls, Charters Towers, North Queensland 53 •Robertson, I.D.M. Interpretation of Fabrics in Ferruginous Lag 54

Taylor, Graham. Regolith Research and Education 55 •

Tilley, D.B., C.M. Morgan and Tony Eggleton. The evolution of bauxitic pisoliths from Weipa,North Queensland 58

Wilford, J.R., C.F. Pain and J.C. Dobrenwend. High Resolution Airborne Gamma-Ray •

Spectrometry for Regolith Mapping 59 ID

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Morphology of Lags in the Cobar Area, NSW

S. Alipour, AC. Dunlop and D.R. Cohen Department of Applied Geology, University of New South Wales, NSW, 2052.

Deep weathering and varying amounts of transported overburden result in variation in the strength of geochemical relationships between surface materials and bedrock. The selection of the best combination of sampling media, processing and analytical methods to permit detection of mineralisation with subtle geochemical expression is dependent upon an understanding of the development of the weathered profile. In the arid and the semi-arid regions of Australia, the lag component of regolith is widely used as a sample media for regional geochemical exploration programs. In current investigation, lags from three exploration targets delineated by Dominion Mining in the Cabar area - Mrangelli (pb, As), Yarrawonga (Bi, As and Au) and Gowrie (Zn, Cu and Mn) - have been investigated. These targets display differing styles of mineralisation, geomorphic features and weathering characteristics.

Lags are widespread in the Cobar region and their characteristics have been related to landforms under a classification scheme developed by Senior (1992). The major landforms are categorised into areas of outcropping bed rock (DB), pediments with thin soil cover (DP) which merge with undulating pediments covered by thicker soil cover (D!), and fmally deposits of Quaternary alluvium on plains or in drainage channels (QA) where lags are buried. Sampling has concentrated mainly in areas of Dr and to a lesser extent DP and DB.

Lags in the - 11 +2mm fraction of 1000 samples have been divided into magnetic (ferrolithics and pisolites) and nonmagnetic (lithics, ferrolithics, pisolites and vein quartz) fractions. Lithics are weathered fragments or clasts in which primary fabrics are preserved. ferrolithics are similar to the lithics but with a significant proportion of Fe oxides within the clasts and pisolites are clasts composed mainly of iron oxides with no primary fabrics preserved. The boundary between these classifications is somewhat subjective. The relative proportions of each lag fraction are governed by landform. Non-magnetics and lithics are dominant in areas of higher (DB) relief, generally resemble the remnants of local lithologies and tend to be angular with rough surfaces. The magnetic clasts are generally very well rounded and display an intense, dark and highly reflective polish or varnish.

From examination of around 150 thin and polished sections two types of lag are recognised;

(a) Primary lithic clasts similar to, and directly derived from, out cropping bedrock in which the primary fabric of the parent material is largely preserved. Lag materials derived from the sandstones and siltstones are strongly ferruginised, except where intensely silicified. Goethitelhematite or maghemite coats the grains and progressively replaces the original cement and matrix. At :M:rangelli, where sparse sulphides in silicified sandstones are present, a significant proportion of the pyrite, galena and sphalerite has been preserved at surface, with the remaining sulphide grains converted to iron oxides. Similar textures can be observed in adjacent lags. No mineralisation has been observed in the angular quartz vein component of lags.

(b) Of the secondary remnants composed of clasts not directly related to the primary lithology, the three types of secondary lag recognised are (i) massive lags formed from aggregates of iron oxide grains, with a proportion of sand-sized quartz or other clasts, (li) simple lag pisoids consisting of well rounded, polished fragments with a core of primary or secondary particles and surrounded by single or multiple concentric bands up to 1 mm thickness, and (iii) composite lag pisoids which are well rounded, complexly rimmed, polished and well sorted. Internally they bear a complex texture and structure. Cores may be composite with cemented smaller clasts of iron oxides and silicates. A system of internal cracks is common in most lag types forming a radial or concentric configuration which is presumably related to episodic dehydration of iron oxides. These cracks may be sites for subsequent accumulation of laminated iron oxides. In secondary magnetic lags, maghemite is mostly present as rims up to 1 mm thick.

A significant proportion of the secondary lag appears to have a multi-stage origin, involving periodic solution and redeposition of Fe oxides, and interaction with fluvial and aeolian processes during previous cycles of weathering. The cores of both the composite and simple lag pisoids generally occupy more than 80% of their volume. The characteristics of the lags from the Cobar district suggest that, despite differences in the stability of metals incorporated in various oxide phases, geochemical patterns in lags from DB and DP landforms may be readily related to geological features in underlying and closely adjacent rocks.

Geochemical and lag patterns from or and QA landforms, where secondary lags make up significant proportions of samples, may be more subtle and enigmatic in nature. In these environments there is a greater contribution from more distant metal sources dispersed during previous cycles of weathering.

ABSTRACTS: Australian Regolith Conference '94 © Australian Geological Survey Organisation

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Classification and Origin of Laterites and Ferruginous Regolith Materials in the Yilgarn Craton, WesternAustralia

R. R. Anand, R. E. Smith and I.D.M. RobertsonCSIRO Division of Exploration and Mining, Private Bag, PO Wembley, WA 6014

Several types of ferruginous materials have been identified in the regolith. One group tends to be closely associatedwith the upper part of the lateritic profile and is referred to as lateritic materials. These include lateritic duricrust,lateritic nodules and pisoliths, mottles and ferruginous saprolite. A second group, iron segregation bodies andgossans, is more commonly found in the saprolite and is referred to as non-lateritic materials. A third group,lateritised sediments, consists of iron oxide impregnated and indurated sediments. Systematic differences betweenthese groups occur in morphology, mineralogy and geochemistry which suggest that they have formed under differentconditions.Lateritic materials: These form in upper parts of the lateritic profile and overlie saprolite. Rocks rich in iron,however may weather to thick ferruginous horizons without forming deep saprolites. Lateritic duricrusts can befurther sub-divided according to the nature of their secondary structures such as pisolitic, vermiform and massive(Anand et al.,1989). In some districts, lateritic duricrust and lateritic nodules and pisoliths form extensive blanketsover large areas whereas in others, it is limited in extent. These materials not only occur in the higher parts of thelandscape, but are also widespread in lower positions, beneath colluvium and alluvium. Colluvium and alluvium cancontain lateritic debris which is derived from the erosion of the laterite profile from the upland area.

Iron in lateritic materials is largely derived from the weathering of the parent rocks. During lateritic weathering,much of the ferrous iron, released by breakdown of the primary minerals, can particularly where the water table is nearthe ground surface, be oxidised and precipitated. As a result, a ferruginous horizon typical develops in the upper partof the deeply weathered profile. Local segregation of the iron in this horizon leads to the development of pisolitic andnodular structures. A variety of lateritic nodules and pisoliths in the laterite horizon can form from the same sourcerock. Nodules and pisoliths can be derived from both Ethic and non-lithic sources. Lithic nodules develop byfragmentation of ferruginous saprolite which show the preservation of relict rock textures. The non-lithic nodulesshow no relict textures but several generations of Fe-oxides, impregnated or desemminated through the clay, sand orgibbsite matrix. Most nodules and pisoliths have thin (1-2mm) cutans which are goethite- and kaolinite-rich anddeveloped by the deposition of Fe and Al around a core. Some pisoliths are strongly banded, which suggests a complexhistory of formation.

Lateritic materials are commonly yellowish brown to black and consist dominantly of kaolinite, Al-hematite, Al-goethite and quartz. In some types namely the Fe-rich duricrust, however, kaolinite is either absent or only present insmall amounts. Lateritic duricrust, nodules and pisoliths can be distinguished from ferruginous saprolite by the lossof primary fabric and by having abundant hematite and less kaolinite. The lateritic materials contain highly Al-substituted goethites, indicating ample availability of aluminium during its formation. Goethites in laterites overultramafic has low Al substitution than in laterites over mafic and felsic rocks.

Maghemite occurs only in magnetic nodules and pisoliths, formed by the heating of the goethite by bush fires.Amorphous Al-oxide has also been identified in pisoliths and nodules which can also form from heating of gibbsite.The composition of major and trace elements of lateritic materials is largely lithodependent at a landscape scale.Thus, laterites formed from granite are low in Fe and high in Si, Al and Zr whereas those derived from ultramaficrocks are low in Al, Si and Zr. The laterites formed from ultraxnafic rocks are however, higher in Fe, Cr and Ni thanthose formed from the weathering of mafic rocks. Quartz and resistant minerals such as talc, muscovite and chromitesurvive through much of the lateritic profile and are a further aid towards bedrock identification. Geochemicalsignatures in lateritic materials generally reflect the underlying bedrock mineralisation. These findings contrast thoseof 011ier(1994) who implies that laterites generally form in sediments that unconformably overlie either bedrock orsaprolite and do not show lithodependence. Nevertheless, some laterites are undoubtdly developed in colluvial andalluvial sediments whereas others, including some Fe-rich duricrusts, are the product of the lateral accumulation of Fein valley floors and now form low hills due to relief inversion. Such materials are, however, readily distinguished fromessentially residual laterites.

Non-lateritic materials: Non-lateritic materials include a wide variety of types whose development is not confined toa single unit of the lateritic profile. These materials include both stratabound Fe and discordant Fe-rich bodies andgossans. They occur as pods, ovoid bodies, coarse stratabound veins, lenses and large slabs and range in size from afew centimetres to several metres. As erosion proceeds, some of these materials are progressively exposed at thesurface, where they disintegrate and contribute to a coarse lag. Iron segregations are extensive in erosional regimes ofnorthern Yilgarn Craton.

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•2^ ABSTRACTS: Australian Regolith Conference '94

© Australian Geological Survey Organisation

Iron segregations are generally dense, dark brown to black and non-magnetic. They are invariably rich in Fe, Mn, Zn,Cu and Co and are dominated by low Al substituted goethite (<5mole%), with variable amounts of hematite andquartz. Maghemite and kaolinite are generally absent. The absence of maghemite here is compatible with the bushfire origin for other materials, since the iron segregations have been exposed at the landsurface only during the ariderosional stage and a forest cover would be lacking.

Iron segregations are the result of extreme ferruginisation, the Fe being derived from a variety of sources, includingweathering of Fe-rich and/ or sulphidic rocks, and by lateral enrichment by water. Very low Al substitution in thegoethite of iron segregations indicates that they must have formed in an environment almost free of soluble Al.

Lateritised sediments: These consist of iron oxide impregnated and indurated sediments of various ages and mayoverlie residual laterite or partly truncated profile. There is no genetic relationship between the ferruginous horizonand the underlying materials. Broad scale lateral movement of iron contributes to the formation of these materials.Iron derived from laterite moves in solution laterally across the landscape. Where iron is precipitated in favourablesites, considerable bodies of ferruginous materials accumulate. However, various secondary structures such as mega-mottles, nodules and pisoliths are also commonly developed in the sediments. These are largely formed by theredistribution and segregation of iron. For example, mega-mottles commonly occur in palaeo-channel clays.Megamottles are reddish brown, hematite-rich, irregular iron accumulations (upto several metres across) containingsmaller (<10 mm) sub-angular metallic granules. The mottles have a dull, earthy lustre and gradational to abruptboundaries with a surrounding smectite-rich clays. Root systems, surrounded by a sheath of bleached clays, arecommon and show a close spatial relationship to zones of Fe accumulation.Age relationships: Caution must be used in generalising the concept of laterite formation. Our data suggest that bothresidual and transported laterite occur in the landscape of the Yilgarn Craton. Distinction can be made on the basis offield relationships, mophology, mineralogy and chemistry. Archaean rocks and sediments in the Yilgarn Craton havebeen variously weathered and ferruginised. Thus, a complex and deeply weathered surface probably does not representa single weathering phase, but evolves to its present complex state through a series of superimposed weatheringevents. Initial weathering and erosional processes that lead to the formation of the lateritic mantle may have initiatedduring the Mesozoic times. This is suggested by the presence of transported pisolitic gravels which occur at the baseof the palaeo-channel infill. Transported pisoliths were presumably derived by erosion of an earlier, possibly pre-Eocene, laterite profile that predates incision of the paleodrainage. Eocene sediments have been further subjected toformation of megamottles and ferruginous granules. Recent sediments also show the development of mottles andnodules.

References

Anand, R.R., Smith, R.E., Innes, J., Churchward,H.M., Perdrix, J.L. and Grunsky, E..C., 1989. Laterite types andassociated ferruginous materials, Yilgarn Block, W.A. Terminology, Classification and Atlas. CSIRO Division ofExploration Geoscience Report 60R (unpaginated).

011ier,C., 1994. Exploration concepts in laterite terrains. The AusIMM Bulletin, 3,22-27.

ABSTRACTS: Australian Regolith Conference '94^ 3© Australian Geological Survey Organisation

Morphology and Genesis of Pedogenic Carbonates in some Areas of the YiIgarn Craton

R.R.Anand. C. Phang and M.J. Lintern CSIRO. Division of Exploration and Mining, Private Bag. PO Wembley, WA 6014

Carbonates are extensively distributed in the semi-arid and arid regions of Western Australia. The enrichment of Au in pedogenic carbonates provides an important sampling medium in the southern Yilgam Craton. Consequently, understanding their characteristics and origin is very relevant to geochemical exploration.

The morphology of carbonates varies significantly according to their position in the landscape. In depositional regimes, carbonates are present in a number of forms. including powdery and friable. nodular/platy calcrete, calcrete pods and calcrete sheets.

Calcareous clays occur extensively in the erosional regimes. dominated by mafic and ultramafic lithologies. Carbonates in calcareous clays generally occur as coatings or stringers on the soil matrix. Dolomite appears to be a common mineral on Mg-rich mafic and ultramafic bedrocks. Dolomite and calcite crystals are. in places. coated with fibrous palygorskite.

In relict regimes, nodular and pisolitic calcrete are common. The calcrete nodules are irregular and pinkish, varying from 1 to 5 cm in diameter that appear to have partially-to-completely replaced lateritic nodules. pisoliths and mottles.

Erosion and stripping of upper, more weathered, parts of the regolith. coupled with the distribution of mafic and ultramafic bedrocks, are important factors influencing the distribution of carbonates. Weathering of mafic and ultramafic bedrocks provides Ca and Mg-rich solutions that infuse the upper parts of the regolith. Carbonates in the depositional areas may have been derived by lateral transportation and deposition of weathered fragments of calcrete derived from the erosional areas which are then dissolved and reprecipitated at the top of the profIle. Other processes, such as mineralization of plant materials and transport by wind, appear to have also played important roles in the formation of pedogenic carbonates.

The morphology of these carbonates provides the main clue for mechanism of their formation. There are two distinct morphologies: (1) euhedral rhomohedral and (2) needle and micro-rods. The two distinct morphologies suggest two modes of formation. Euhedral rhomohedral of calcite and dolomite suggest their inorganic precipitation from solution. In contrast needle and micro-rod calcite suggest the role of biological agencies in the carbonate accumulation process.

Pisolitic and nodular calcrete resulted from replacement of Fe-oxides and clays by calcite and dolomite. Within a single hand specimen of calcified lateritic nodules and pisoliths. nodules at all stages of replacement occur. Replacement began at the borders of nodules, or along cracks within nodules, and proceeded inwards, to give calcrete nodules containing isolated massive cores of iron oxides and finally nodules consisting solely of carbonates.

4 ABSTRACTS: Australian Regolith Conference '94 © Australian Geological Survey Organisation

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•Quaternary Vadose Cakretes Revisited

Aro V Arakel• AWT EnS ight

PO Box 73, West Ryde NSW 2114•

• Soils with significant secondary accumulation of calcium carbonate (including calcretes) cover over 13% of the totalglobal land surface. Quaternary calcrete duricrusts commonly comprise an important component of regolith landscape

• in arid and semi-arid regions of Australia and elsewhere. Pedogenic carbonate deposits, petrologically andmorphologically similar to that of Quaternary vadose calcretes, are also abundant in the geological record, therefore, a

• clear understanding of their modes of formation and genesis is beneficial to mineral/petroleum exploration and palaeo-environmental reconstruction. This presentation reviews the information available on morphological features and

• mineralogical association of Quaternary vadose calcrete deposits from diverse landscape units in Australian arid zoneand discusses the controls on the type and rate of calcrete soil profile development.

• Vadose calcrete develops initially by downward percolation of carbonate-rich meteoric water primarily within the soilmoisture zone. Calretisation is initially through the processes of cementation and/or replacement of particles in the

• host material, with the thickness and lateral extension of the calcrete profile depending on local hydrology,physiographic setting and textural characteristics of the host material. Depending on local environmental and

• topographic features, calcrete soil profile development may proceed further through repeated episodes of vadosecementation, dissolution and breciation of the earlier formed soil components. The time involved and the degree of

• complexity of individual soil profiles is shown to depend largely on local topographic gradients and hydrologicconditions. A progressive redistribution of the soil calcrete components may result in a profile that exhibits complex

• micro and macro morphologies, mineralogical association and stratigraphic relationships. The examples provided inthis review exhibit the stages and time frame involved in the modification of calcrete soil profiles under different

• landscape and hydrological settings.

• The morphological similarities of commonly occurring calcrete duricrust in regolith terrains are used to provide aninventory of (i) stages involved in the modification of textural, compositional and morphological features of the

• Quaternary calcrete soil profiles, (ii) impacts of changing environmental conditions on calcrete soil characteristics,and (iii) processes that evidently act as a continuum within the upper few metres of calcrete soil profiles, producing a

• variety of mature landscape units within a comparatively short span of time. The time framework for process-productrelationships is assessed by considering the main phases of carbonate precipitation. Morphological features, such as

• relief inversion and multiple calcrete soil profile development, together with mineralogical associations are used toindicate the importance of ancient calcrete soil profiles to evaluation of natural resources and as a tool for palaeo-II^environmental reconstruction.

Whereas a major formative phase of vadose calcrete in Australian regolith landscapes appears to coincide with the• onset of aridity during Quaternary, the problems with establishing a reliable time frame for duricrust profiles represent

a challenge which require further research and probably new approaches to dating of arid zone duricrusts.•

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• ABSTRACTS: Australian Regolith Conference '94^ 5© Australian Geological Survey Organisation

•

The Genesis of Ferricretes in the Mount Lofty Ranges of South Australia

Dr R.P. Bourman University of South Australia

Iron-rich crusts, weathered, mottled and bleached bedrock in the Mount Lofty Ranges have long been regarded as the result of tropical 'lateritic' weathering, operating largely during the Tertiary on now uplifted 'peneplains'. Currently lying outside of the tropics these regolith materials have often been interpreted to imply continental drift and/or climatic change. The distribution and character of these materials have commonly been explained by the variable dissection of former contiguous and complete 'ideal laterite proflles'. Consequently the presence of bleached materials implied the former presence of a mottled zone and a ferruginous horizon ('laterite'). The uncritical application of this interpretation has resulted in simplistic explanations.

Within the Mount Lofty Ranges different types of ferricrete (vermiform, pisolitic, nodular vesicular to massive, slabby and ferruginised bedrock and ferruginised sediments) occur and are characterised by distinctive combinations of chemistry and mineralogy, which, in tum, may reflect the local environmental conditions under which they formed. The distribution of ferricretes, bleached and mottled zones displays great lateral variability and may reflect development in specific favourable environments rather than the variable erosion of former complete proflles. The topographic requirements for ferricrete formation and palaro-geographic reconstructions also suggest only sporadic formation in favourable local conditions. Thus former uniform blankets of 'laterite' may not have existed over large areas.

These studies question the viability of 'laterite' and weathered zones as palaeo-climatic indicators and morphostratigraphic markers in this area. Ferricrete formation has taken place by the absolute accumulation of iron and aluminium oxides during ongoing weathering and erosion of long exposed land surfaces, without the ferricrete, mottled and bleached zones being necessarily mono-genetically related. Stratigraphic evidence suggests that ferricrete development, mottling and bleaching was an on-going process throughout the Mesozoic and Cainozoic, with broadly equivalent types of weathering and ferruginous materials occurring on surfaces of different ages.

The following model has been proposed to explain the formation of ferricrete in the Mount Lofty Ranges. Weathering of a dissected land surface produced lateral variations in environmental conditions. In some higher parts of the landscape the underlying bedrock was bleached of iron, which accumulated in adjacent low-lying regions, such as organic-rich swamps (vesicular ferricrete), at breaks of slopes on valley sides and in valley bottoms (ferruginised bedrock and sediments). Slabby ferricrete formed at plateau margins where laterally moving near-surface ground water favoured the precipitation and oxidation of iron oxides in solution.

Within the zone of water table fluctuation, primary iron minerals in the basement rocks degraded by weathering under reducing conditions, forming ferrous iron that redistributed and segregated within the weathered and partly kaolinised rock to form ferric iron-rich mottles under oxidising conditions. These mottles, dominated by hematite, probably formed via the dehydration of ferrihydrite. Thin section observations suggest that the hematite mottles grew by -the coalescence of tiny hematite crystallites. As surface weathering and erosion proceeded, iron segregations were progressively exposed at the surface, hardened, disintegrated and formed lags. In favourable situations these iron-rich residua transformed to goethite by dissolution and reprecipitation, forming surface rinds on hematite-cored clasts including pisoliths and nodules. On steep slopes no thick residual tags formed, but only thin layers of pisoliths accumulated on and within soils overlying mottled zones. Pisoliths formed during transport frown higher parts of the landscape accumulating as precursors of pisolitic to nodular ferricretes.

These studies on particular varieties of 'laterite' may not be representative of all areas, but they provide alternatives to previous unsatisfactory and simplistic explanations in South Australia.


• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Towards Distinguishing Transported and in situ Ferricretes: Data from Southern AustraliaDr. R.P Bourman

University of South Australia.

The formation of ferruginous duricrusts by relative (in situ) and absolute (lateral) accumulation of iron and aluminiumoxides has been well documented, but not all workers have agreed on the significance of these processes and theirrecognition in the final weathering product. Some workers have regarded 'laterite' profiles as parts of stratigraphicsequences and have interpreted ferricretes on highland surfaces as remnants of iron-impregnated sediments of ancientvalleys or depressions, favourable locations for the physical accumulation of ferruginous materials and theprecipitation of iron oxides from soil or ground waters. This contrast with other approaches, which rest heavily on thetraditional notion of 'laterite' formation by vertical translocations of minerals under humid topical conditions onpeneplains close to ultimate base level, with incomplete profiles reflecting variable degrees of truncation by erosion.This paper presents criteria, which may assist in establishing the dominant mode of origin of the duricrust.Field observationsTopographic relationships to be considered include ferricretes on unweathered bedrock interfluves, ferricretes on steepslopes and completely mantling valleys of considerable relief with slopes of up to 200, ferricretes occurring in valleybottoms or in relative depressions, and reconstructed landscapes revealing former ferricreted valley floors nowcomprising high points in the landscape.

Strati graphic relationships In some situations ferricrete has developed in younger stratigraphic horizons overlyingolder weathered and mottled bedrock. A transported mode of origin is favoured by clear cut unconformable contactsbetween underlying zones with quartz veins, etc which are truncated and overlain by duricrusted stratified sediments.Where no clear unconfonnity is present, palaeontological data may demonstrate different ages of units in whichseparate parts of weathering profiles have developed. Detrital components in the crust, different from the underlyingmaterials. e.g. ferruginised alluvial deposits overlying weathered rock or saprolite reveal a transported origin, andtransported pisoliths typically are associated with stone lines. If ferricretes are in situ they should be formed in thesame materials as the underlying pallid and mottled zones. In such cases the absence of pisoliths may indicate in situformation.

Laboratory observationsMicromorphology

• Pisoliths with irregular shapes and diffuse external borders appear to have formed in situ, particularly as theydisplay none of the evidence for transportation.

• Pisoliths without rinds, pisoliths which are angular and those with the same framework grains within pisoliths aswithin the matrix materials are likely to have formed in situ.

• The formation of pisoliths by the physical break up of ferruginised bedrock and mottles, their transport andmodification in near surface environments is favoured by: incomplete or broken surface coatings on pisoliths,different particle size distributions of sand and silt grains in pisoliths and their matrix materials, different particlesize distributions of sand and silt grains in adjacent pisoliths, laminae of silt-sized quartz grains inside somepisoliths that are not continuous outside of the pisoliths, compound nodules cemented together by ferruginous,concretionary material whose structure has been truncated by abrasion, and the presence of complex combinationsof pisolith-within-pisolith structures.

• Multiple laminar goethite rinds on pisoliths, incorporating individual quartz grains or lenses between them may beevidence of an accretionary Origin. The deposition of the laminae and incorporating the quartz may have occurredin a succession of pedogenic environments, consistent with a long history of exposure, transportation andweathering.


Mineralogy and chemistry

• Dolerite dyke positions have been established by analysis of the overlying 'laterite' in the Darling Range.

• The simple mono-mineralogy of pisoliths appears to favour in situ formation.

• An indicator that favours a transported origin for pisoliths includes ferruginous pisoliths resting on materials, suchas calcareous clays and sediments, that could not have provided the materials for pisolith formation.

• In many pisolitic ferrieretes the inter-pisolith matrix is significantly different in mineralogical and chemicalcomposition to that of the pisoliths. In such cases, the pisoliths may be interpreted as elastic components differentfrom the materials in which they are found.

• Once in the soil environment pisoliths appear to undergo transformations that lead to higher total iron contentsand transformations in iron mineralogy from goethite to dominantly hematite and maghemite. Maghemite mayoccur both in the body of the pisolith and in outer concentric layers.

• Mass balance studies do not appear to hold great potential for distinguishing in situ and transported ferricretes.

Palaeomagnetism

• Studies of remanent magnetism preserved in individual pisoliths within pisolitic ferricrete have revealed scatteringof the data suggesting that individual pisoliths had undergone physical disturbance since their formation.

8^ ABSTRACTS: Australian Regolith Conference '94© Australian Geological Survey Organisation

••a^Duricrust and its relationship to the landscape at Johnson Creek, Northwestern Queensland

411Duricrust has formed over structurally complex dolomitic bedrock at Johnson Creek, Northwestern Queensland

• (20°15'S, 139°07'E). There is no fresh outcrop of dolomite, and the weathering profile ranges from approximately60m to over 150m thick, the thickest profiles tending to be developed over topographic lows. The weathering profile

• consists of the regolith units - soil, fine saprolite, coarse saprolite, and saprock. There are also three main weatheringzones - a surficial ferruginous/siliceous zone, an underlying pallid zone, and beneath that another ferruginous zone.

• Silcrete, ferricrete and indurated soils all occur at Johnson Creek. The silcrete is a reworked silcrete. It incorporatesrounded to angular silcrete clasts, chert clasts and quartz grains. Its reworked nature strongly suggests that it was

• formed in a valley, as do the rounded imbricated chert pebbles found immediately beneath the sikrete. The fact that itnow caps the highest hills in the area indicates an inversion of topography. The ferricretes and indurated soils are all

• found lower in the landscape, therefore the silcrete is the oldest duricrust at Johnson Creek.

• There are four main types of ferricrete at Johnson Creek - lag ferricrete, slabby ferricrete, hill slope ferricrete andvalley ferricrete. Lag ferricrete is found on hill tops almost as high as those capped by silcrete, and is believed to be an

• indurated soil. In one locality immediately below the lag ferricrete is slabby ferricrete. It is possible that the lag andslabby ferricrete formed at the flanks or edges of a broad hill or plateau after the bedrock hills had eroded to the

110^approximate level of the silcrete. There, laterally migrating water precipitated iron oxides under 'suitable redoxconditions. An analogy is seen in the modern landscape where ironstone (substantially ferruginised bedrock) is found

• only on the hill flanks. This topographic distribution of iron, and the fact that all the fenicretes have relatively lowamounts of aluminium compared to the saprolite, suggests that absolute accumulation of iron was the dominant

• process of formation.

On the hill slope immediately adjacent to the slabby and lag ferricrete are hill slope ferricretes, containing reworked• clasts of ferricrete which probably originated from the ferricrete on the hill top. The ferricrete composition reflects

topography, being hematitic on the high parts and becoming more goethitic down slope. It is quite likely that high• paleo water table levels resulted in the formation of the pallid zone, and during this time the ferricretes were also

formed. However field evidence indicates that the ferricrete and underlying pallid zone do not conform to the "classic• lateritic profile"; there is no distinct mottled zone, and no pisolithic formation.

• The modem landscape is one of broad low lying hills with broad valleys between. Found on the broad valley floors arethe remnants of indurated soils which are cemented by clays, with minor amounts of anatase and iron minerals.

• Johnson Creek is currently incising the valleys and has cut down 4m in places, exposing indurated gravel deposits inpaleo streams, and costeans dug across the modern drainage channels have exposed small pods of valley ferricrete.

• Also associated with the modern hydrological regime is the lower ferruginous zone in the weathering profile. It isfound at the level of the modern water table and its thickness, 35-60m, reflects seasonal and yearly fluctuations.

• The entire surface of the landscape at Johnson Creek is covered in a "silica skin". Drilling and creek exposures showthe silicification is usually to about 5m depth but can extend up to 10m depth. The stromatolite beds within the

• sedimentary sequence are particularly prone to silicification, often forming chert hills and ridges which, althoughprominent, are not as high as the duricrust capped hills. It is not clear when the surface silicification occurred, but it is

40^possible that the silicification was also the result of high paleo water tables, which erosion has since exhumed to forma reasonably resistant surface.

•

•••••• ABSTRACTS: Australian Regolith Conference '94^ 9

@ Australian Geological Survey Organisation

A.F.Britt* and Tony Eggleton• Centre for Australian Regolith Studies, Australian National University, Canberra ACT 0200

*Now at Department of Geography and Environmental Science, Monash University, Clayton VIC 3168.

a

••

Regolith Evolution at Mount Percy, Kalgoorlie, Western Australia

C.R.M. ButtDivision of Exploration and Mining, CSIRO, Private Bag, PO Wembley, Western Australia 6014^•

Mount Percy is about 2 km NE of the centre of Kalgoorlie, at the northern end of the Kalgoorlie-ICambalda greenstonesequence, about 8 km N. of the Golden Mile. Primary Au mineralization in the Mystery Zone at Mount Percy occursin fuchsite-carbonate alteration zones at the contact with porphyries intruding chloritic talc carbonate ultramaficrocks. The mineralized sequence has been deeply weathered and is concealed beneath an almost complete lateriticregolith over 60 m thick. The regofith consists of saprolite (50 m), which is clay-rich in the top 10 m, plasmic and^•mottled clays, and surficial horizons of lateritic gravels and duricrusts, the latter overlying the talc chlorite rocks. Thesurficial horizons contain pedogenic carbonates, and alunite is present in the upper saprolite. •The Au distribution in the regolith is typical for the region, with minor enrichment and wide lateral dispersion insurficial gravels and duricrust (in part associated with pedogenic carbonates), leaching and depletion in the underlying^•clay-rich horizons, and some secondary concentration and minor dispersion in the saprolite. Primary and saproliticAu mineralization is indicated by a broad superjacent Au anomaly (100-5000 ppb) in the calcareous soils and lateritichorizons, and by high concentrations of W (5-60 ppm), Sb (7-25 ppm) and As (10-400 ppm). High K contents,corresponding to resistant muscovite, give surface expression to the alteration zone. The porphyries and ultramafic^•rocks can be discriminated throughout much of the regolith by relative abundances of Ti, Zr, Ba and K. However, thelateritic horizons over the ultramafic rocks have abnormal geochemical signatures, with low Cr contents (<1000 ppm),^•and high contents of "immobile" elements (e.g. >200 ppm Zr) apparently derived from the porphyries. Thedistribution patterns of the elements are the result of the regolith developing and evolving under changing conditions.^•The lateritic regolith formed during an early, probably warm, humid period of strong leaching, and has been modifiedin more recent arid phases, during which the water-table declined, groundwaters have become saline and minerals^•such as alunite and calcite have precipitated in upper horizons of the profile. Gold and rare earth element mobilities,particularly during these later phases, have resulted in distinctive supergene distribution patterns.

The near-complete preservation of the regolith at Mount Percy implies that there has been little erosion. Nevertheless,some features of the distributions of the immobile elements suggest erosion may have been sufficient to cause localtopographic inversion. Compared to the talc chlorite rocks, the lateritic and clay-rich horizons on the porphyries and •fuchsitic rocks are thin, with silicified saprolite at or close to the surface in some places. This apparent truncation mayhave been due, in part, to these horizons being more friable and less indurated, thereby increasing their vulnerabilityto erosion. In addition, there is a shallow, clay-filled channel over part of the deposit. It is suggested that the talcchlorite rocks may once have occupied lower ground and the porphyries and fuchsitic ultramafic rocks higher ground. 11111The immobile elements, present in resistant minerals released from the porphyries, were transported downslope toaccumulate over the talc chlorite rocks, together with some chemically mobilized iron that subsequently hardened to 411/form a duricrust. Preferential erosion of the less indurated upper horizons on the porphyries and fuchsitic ultramaficrocks has led to topographic inversion.

Acknowledgements•This research was conducted as part of Project 241 of the Australian Mineral Industries Research Association. The

sponsors of the project are thanked for their financial support. The research was only possible with the assistance and 11111advice of Dr. P.C.C. Sauter of Kalgoorlie Consolidated Gold Mines Ltd.


•••

Terminology of Deeply Weathered Regoliths

C.R.M. Butt and R. R. Anand• Division of Exploration and Mining, CSlRO, Private Bag, PO Wembley, Western Australia 6014

The regolith comprises the entire altered, unconsolidated or secondarily recemented cover that overlies more coherent• bedrock and that has been formed by the weathering, erosion, transport and/or deposition of older material. There is,

unfortunately, no universally agreed system for the terminology of deeply weathered regoliths, whether for whole• profiles, individual horizons or for many distinctive secondary structures, despite being a subject of discussion for

many years. Many of the terms are poorly defined, and misapplied or used in different senses by different authors,• posing considerable problems for the identification and usage of regolith materials in geochemical exploration and

other applications.•

Since 1987, a simplified scheme for dominantly lateritic terrains has been developed in CSTRO-AMIRA projects,• principally for use in exploration. Initially, this has been applied in the Yilgarn Craton in Western Australia, but it has

more general use, both in Australia and overseas. This is compared in Table 1 with some of the principal systems of• nomenclature used elsewhere, to demonstrate the equivalence between different terminologies adopted in the

literature. In the Yilgarn Craton and, indeed, in much of Australia, the lateritic profile has been modified by physical• and chemical alterations induced by later tectonic and climatic changes (generally, uplift and a change to an arid

climate). Accordingly, the terminology and classification are based on the fundamental characteristics of this profile,• with modifications due to later events or features attributable to parent Ethology added as descriptors. Cementation is

one of the most recognizable modifications that occur in response to the change from a humid to an arid climate. The• most common cements are silica, Ca and Mg carbonates, iron oxides, aluminosilicates and gypsum.

Few new terms have been used. Saprock (Trescases, 1992) is the horizon representing the earliest stage of weathering;• a maximum of 20% of the weatherable minerals being altered is suggested as an arbitrary upper limit for this horizon.

Saprock may be absent or only very thin in some profiles. Saprolite may then be considered to be material with more• than 20% of weatherable minerals altered, and saprolith refers to all weathered materials having a lithic fabric, i.e.

saprock and saprolite. Pedolith is suggested to describe the (upper) parts of the regolith, above the pedoplasmation• front, in which the original lithic fabric, preserved in the saprolite, has been largely destroyed by soil formation and

other processes, and a new fabric has been developed. Plasmic and arenose are applied to clay- and sand-rich zones,• respectively, in which the lithic fabric has been destroyed by settling, compaction and consolidation. Not all pedolith

units may be present thus, lateritic residuum may be directly underlain by collapsed, brecciated or ferruginoussaprolite, rather than a mottled zone, especially over mafic and ultramafic rocks. As in all natural systems, manychanges are gradational, so that limits to some defined units (e.g., saprock) are arbitrary. Similarly, the placement of

• some units can be equivocal and may depend upon the purpose of the classification. For example, present or pastsilicification may be superimposed on different horizons of the pre-existing profile. Classification of the product,

• therefore, may be either according to the later event (i.e., presence of silica cement) or to the earlier event (i.e., thehorizon in which the silica has precipitated). The former recognizes a single siliceous unit (e.g. silcrete, hardpan) and

• the latter refers to the horizon that has been silicified (e.g., silicified saprolite, hardpanized colluvium). The latter ispreferred but, in some circumstances, identification of the cemented unit may be paramount, in which case the

• alternative classification could be adopted. Providing the descriptions of the material are adequate, classificationaccording to either scheme should be possible. Terms such as silcrete, calcrete and ferricrete may be used to refer to

• intensely cemented units that either preserve older fabrics or have developed new ones.

• References

Butt, C.R.M. and Zeegers, H. (Editors), 1992. Regolith Exploration Geochemistry in Tropical and Subtropical4111 Terrains. Handbook of Exploration Geochemistry 4. Elsevier, Amsterdam, 607pp.

• Nahon, D and Tardy, Y, 1992. The ferruginous laterites. In: C.R.M. Butt and H. Zeegers (Editors), RegolithExploration Geochemistry in Tropical and Subtropical Terrains. Handbook of Exploration Geochemistry 4.

• Elsevier, Amsterdam, 41-55.

Trescases, J-J, 1992. Chemical weathering. In: C.R.M. Butt and H. Zeegers (Editors), Regolith Exploration• Geochemistry in Tropical and Subtropical Terrains. Handbook of Exploration Geochemistry 4. Elsevier,

Amsterdam, 25-40.•

•••

ABSTRACTS: Australian Regolith Conference '94^ 11@ Australian Geological Survey Organisation

•

General terms Alternative Terminologies

CSIRO

SoilSolferrallitique

FrenchButt and Zeegers1992

Soil

Nahon and Tardy1992

Soil

Broadsubdivisions

Pedolith^Ferruginous soil[Latosol]

[Plinthite]

Cuirasse[Ferricrete]

Ferruginous zone[Laterite]

[Lateriticironstone]

Lateritic gravel

Lateritic duricrust(pisolitic, nodular,

massive,vermiform,fragmental)

Lateritic gravel

Cuirasse (pisolitic,nodular, massive)

Pebbly ferruginouslayer

Induratedconglomeratic iron

crust

Argilestachetdes

Mottled zone Mottled zoneMottled (clay)zone

Mottled (clay) zone

Plasmic/arenosehorizon

Plasmic/arenosehorizon

Collapsed and/orbrecciated saprolite

Bedrock Fresh rockRoche mereUnweathered/ freshbedrock

Protolith^BedrockUnweathered

rock

Table 1Terminology of deeply weathered regoliths with essentially complete lateritic profiles

Soft nodular crust^Carapacenodulaire

Saprolith^Saprolite[Pallid zone]

Saprolite

Saprock

Fine saprolite LithomargeArgiles

barioldes

Ferruginoussaprolite

Clay saproliteSaprolite

Saprock

Altdrationpistache

Coarse saprolite^Afene/grus

Informal or equivocal terms: [....]

ABSTRACTS: Australian Regolith Conference '94© Australian Geological Survey Organisation

Descriptive terms: (....)

12

•••

•Regolith Geochemistry and Mineralogy Over Two Mineralised Shear Zones Near Cobar, NSW

Chris Cairns', K.G. McQueen2 and Graham Taylor2

•' BHP Minerals, PO Box 425, Spring 11111 QLD 4004, 2 University of Canberra

The mineral deposits of the Cobar mining field are contained in a zone of en-echelon Ooverlapping shears which form• a major tecto-lineament on the western limb of a south plunging anticlinorial belt. The deposits are hosted in the

Cobar Supergroup which was intensely folded by east-west compressive forces predominantly in the Early to Middle-• Devonian Tabberabberan Orogeny. These structural zones provided foci for mineralisation by acting as fluid conduits

for the mineralising solutions. As a result, the orebodies are generally steeply dipping and, in plan, roughly ellipsoid• zones with much greater vertical than horizontal extent.

• The Cobar regolith consists of a pediplain of undulating rounded ridges (up to 10 m relief) and higher residuals (up to20 m relief) of Siluro-Ordovician sediments and metamorphics with overlying residual colluvival gravel, lithosols and

• red earth soils. This is punctuated by rounded to linear ridges of Siluro-Ordovician phyllite, slate, quartzite, quartzose-sandstone, schist and shale with a relief to 50 m and rounded ridges and ranges (up to 200 m relief) of Devonian

• quartzite and conglomerate and Siluro-Ordovician phyllite, schist, sandstone and conglomerate which are dominatedby lithosols. Zones of mineralisation correlate with areas of greater topographic relief due to silica and Fe-oxide

• induration.

Interpretation has been made of results of geochemical analysis of samples acquired from rotary air blast (RAB)• drilling conducted by CRA Exploration and surface samples acquired by the author. The two lines of nine holes each

are several kilometres apart and have been drilled to investigate two geochemical anomalies, one for Cu-Au (Wood• Duck) and the other for Au-Pb (Peak South). Samples were analysed for target and indicator elements by ICPOES and

for major elements by XRF. Quantitative mineralogical determinations of samples were achieved by interpreting XRD• traces with the SlROQUANT software package. Determination of target and pathfinder element concentrations in

secondary minerals was conducted by electron microprobe analysis and scanning electron microscopy.• Element/element and element/mineral correlations were used to interpret the distribution of target and pathfinder

elements in secondary minerals and the significance for exploration geochemistry.•

Microprobe results support the well documented abilities of Fe- and Mn-oxides to incorporate significantlabundances• of target and pathfinder elements. The secondary oxides are host to the majority of the anomalous target and

pathfuider elements. Microprobe results from colloform banded goethite clearly display an association Pb-P-As-Al• whose abundances varied antipathetically with the association Cu-Zn-Fe-Si. The association Pb-P-As-Al suggested the

possibility of a discrete second phase of transitional plumbogummite/philipsbornite composition existing with the• goethite but attempts to positively identify a second phase were not successful. The concentrations of Al and P

indicate that the goethite formed in low pH conditions.• Colloform banded Mn„oxide microprobe results display a dramatic antipathetic relationship between a

Pb„K„BIassociation and a Cu-Zn-Al association. This is due to Mn-oxide speciation and reflects the ability of the• chain structured coronadite-hollandite-cryptomelane group to incorporate elements of larger ionic radius (Pb-K-B),

and the layer structure of lithiophorite with its ability to incorporate smaller ionic radius elements (Cu-Zn). The• positive correlation of Al with Cu and Zn in lithiophorite suggests that substitution of A1 3+ for Mn3+ promotes

incorporation of Cu and Zn into the lattice. Bands of the Mn-oxides can have diffuse boundaries suggesting that the• Mn-oxides were originally precipitated as a relatively homogeneous gel and that during subsequent crystallisation,

solid state diffusion of elements has occurred in developing the banding of Mn-oxides species.• At the Peak South prospect, distribution of target and pathfinder elements can largely be attributed to the distribution

of the scavenging materials Mn- and Fe-oxides and clays. Element/mineral correlations show that Fe-oxides(particularly goethite) are associated with P and Zn and that kaolin is associated with Cu and Zn. The coincidence of

• anomalously high concentrations of Cu, Pb, Zn, Bi, Fe, As, Sb, Mn, and P reflects a primary mineralisationassociation perpetuated by the formation of secondary minerals essentially in situ. Au is not closely associated with

• any other mineral or element. The Wood Duck prospect displays a vertical zonation of target and pathfinder elementsin a quartz enriched shear zone from elevated Au/As at the top of the profile, down through Bi, Sb, Cu and Fe. A

• strong negative correlation between quartz and muscovite is a result of both quartz enrichment in the shear zone anddiffering abundances in lithologies either side of the shear zone. Surface samples contained elevated abundances for

• almost all elements and provide good contrast with the regional background.

The geomorphology of the Cobar area has implications for the surface expression of geochemical mineralisation• signatures. Much of the Cobar peneplain consists of colluvial and alluvial deposits punctuated by rises and ridges of

relatively low relief. Consequently, much of this material is difficult to interpret geochemically and drilling to bedrock• for sample acquisition is recommended. The residual material on the tops and upper slopes of ridges can be considered

effectively in situ and is an effective geochemical sample medium

•ABSTRACTS: Australian Regolith Conference '94^ 13© Australian Geological Survey Organisation•

Bauxite Genesis and Landscape Evolution at Gove, Northern Territory

lain D Campbell* & Graham M Taylor Centre for Australian Regolith Studies, Faculty of Applied Science,

University of Canberra, PO Box 1 Belconnen ACT 2616

* Present address: Division of Exploration and Mining, CSIRO, Floreat Park WA

The Bauxite at Gove, north eastern Amhem Land forms the upper horizons of lateritic weathering profiles that have developed on Mesowic sediments and cap plateaux in this region. Observations suggest that the plateaux system was once far more extensive than now with bauxite development prior to incision, but formation of economic bauxite may be a result of later enrichment

The typical profile through the bauxite from top to bottom is: topsoil; pisolithic bauxite; cemented pisolithic bauxite; tubular bauxite; nodular ironstone, mottled zone and saprolite.

The tubular bauxite does not indicate an unconformity as previously reported by various authors. It has a gradational boundary with both the overlying and underlying materials and is the result of a textural and mineralogical reorganisation of the original bauxite by a two stage process; flfStly the leaching of iron and the formation of kaolinite in the bauxitic matrix, followed by the removal of the silica resulting in an enrichment of the alumina.

Most of the bauxite in the Gave region is residual, Where transported loose pisoliths do occur they are locally derived sheetwash. Warping of the plateaux has caused the material on the hinge crests to be eroded and deposited on the surrounding profiles leading to over-thickened pisolith horizons on the fold limbs.

Most of the plateaus are residual, but there are local examples of relief inversion. Mount Saunders has a capping of transported bauxite and ironstone nodules unconformably overlying residual granitic sand. The sides of the mesa are cut into easily eroded granitic saprolite, with the base of erosion at the saprolite - granite boundary. This indicates etchplanation is the active erosive process in this region.

The regolith types occur as 3 broad associations which are used to map the area. While this method is not genetically based, there are correlations between genetic type and association. The Nhulumbuy Association are lateritic plateaux dominated by bauxites and ironstones; the Giddy Association are mainly erosional plains with colluvium at there boundaries and alluvium in there broad valleys. The Giddy and Nhulumbuy associations are generally much older than the Coastal Association. The relationships in the Eldo Valley show that the Giddy is being eroded much faster than the Nhulumbuy.

14 ABS1RACTS: Australian Regolith Conference '94 © Australian Geological Survey Organisation

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Geomorphic and Regolith Evolution of the Bathurst Region, NSW

Roslyn Chan Division of Regional Geology and Minerals, Australian Geological Survey Organisation

The Bathurst 1 :250 000 sheet area sits astride the Canobolas Divide which separates the headwaters of the Darling (Macquarie River system) and Murray (Lachlan River system) drainage basins, and covers an east-west transect from the Great Divide in the east across the tablelands and western slopes. The area has a complex geomorphic evolution dating back to the Carboniferous. This geomorphic history provides a framework for understanding the regolith materials and their distribution in the Bathurst region.

From the Carboniferous to the Late Cretaceous this area was tectonically relatively quiescent. Stripping of the Carboniferous regolith mantle provided sediments for deposition in the Sydney Basin from the Permian to Jurassic. Stability during the Cretaceous permitted the formation of an extensive deeply weathered palaeoplain interrupted by occasional inselbergs such as Mount Macquarie. This palaeoplain is still largely intact, though modified by later warping, volcanism and erosion, within an arcuate zone running from north-west of Molong to south-east of Blayney. Large dendritic river systems developed on this Gondwanaland palaeoplain. Drainage patterns and remnant alluvium on divides, as well as gravel terraces and underfit streams associated with some present drainage lines, indicate an ancient and more extensive proto-Macquarie River system that flowed north-west across the present Great Divide and Canobolas Divide. Its tributaries included segments of the present-day Belubula River and Mandagery Creek, which have since been reversed and progressively captured to flow west into the Lachlan River, and the Turon River whose headwaters included the Cox's River which has since been reversed and now flows to the east coast

Late Cretaceous rifting to the east of the present NSW coastline initiated the opening of the Tasman Sea. Downwarping associated with separation formed the Great Divide and caused river segments to the east of the divide to reverse. This beheaded northwest flowing rivers thus dramatically reduced sediment supply. Headward erosion of the shorter streams now flowing towards the new coastline caused some west flowing rivers to be captured. This is exemplified by the associations of the Turon/Cox's and Fish/Jenolan Rivers on the eastern edge of the Bathurst 1:250,000 map.

Differential erosion of the palaeoplain is reflected in the degree of preservation versus stripping of regolith across the map sheet. The northeast area is deeply incised with mountain and hill chains over metasediments and andesites, and more subdued relief on the more easily weathered Bathurst Granite. There are some palaeoplain remnants with shallow regolith in this area, indicating the former palaeoplain. The Oberon Plateau is part of the palaeoplain although some incision and,stripping has taken place. The low base level for erosion associated with the Bathurst Granite results in a progression from low relief highly weathered regolith high in the Macquarie River headwaters to greater relief and more stripping towards the granite.The area below the Abercrombie Escarpment has been largely stripped. Progressive phases of scarp retreat towards the north and east have produced inverted palaeodrainage preserved by lava flows, which help to date the phases of scarp retreat in this area. Along the western edge of the map palaeoplain remnants occur high on hilly terrain resulting from erosion of the palaeoplain towards a lower base level to the southwest.

The Murray Basin, southwest of the Bathurst mapping area, began subsiding early in the Tertiary and provides a mechanism for a lower base level and tilting towards the southwest. This lowering to the southwest probably formed the Canobolas Divide, and caused reversal of north flowing tributaries of the Macquarie River. There is evidence for a regional south-west migration of river channels as seen on Winburndale Creek, Macquarie River near Bathurst, west of Hill End and and in its headwaters, and in the lower reaches of the Lachlan River. The highly weathered palaeoplain declines in elevation to the west, and is another indication of tilting towards the Murray Basin.

Three episodes of volcanism in the Oligocene and Miocene have affected the warped, partIy tilted and eroded palaeoplain. The 34-42ma Airly Province in the northeast is preserved as inverted drainage on plateau remnants of the palaeoplain. The 16-19ma Abercrombie Province near Bathurst and in the central-south and southeast is preserved both as inverted drainage and within present valleys. Finally the 11-13ma Canobolas Province covers a large area radiating out from Mount Canobolas and is preserved as the eroded core, radiating lava flows and palaeodrainage lines, both inverted and within valleys, intact and dissected lava plains, and weathered sub-lava bedrock since stripped of lava.

Headward erosion of streams and differential denudation relating to geomorphic activity and resistance to weathering of bedrock lithologies continue and are contributing to surface lowering.


15

Soil Landscape Maps and Regolith Information

GAChapman Soil Survey Coordinator, Soil Conservation Service of NSW, PO Box 651 Penrith NSW 2751 Australia

The New South Wales Soil Conservation Service employs a team of fIfteen soil surveyors and is aiming to complete soil landscape mapping at 1:100,000 and 1:250,000 of Eastern and Central NSW by 2010.

Soil landscapes are areas of defInable combinations of parent material, terrain, vegetation and especially soils. In many respects soils are a shallow form of regolith. Soil and regolith distribution and properties are influenced by many similar historic and current site processes. Knowledge of previous and current regolith processes within the context of landscape evolution is valuable for understanding many soil properties and their distribution patterns. Consequentially soil landscape maps and reports may be of benefIt to regolith scientists

NSW Soil Conservation Service Soil Survey map legends group soil landscapes according to major land-forming processes which influence the mode of accumulation (or loss) of soil parent materials, and hence soil parameters. Many of the groups also reflect regolith processes. Soil landscape maps may be viewed 55 regolith maps on the basis of their colour groupings.

Soil Landscape Groups include:

• vestigial (discontinuous and shallow soils)

• residual (deep in-situ soil formation)

• karst (solutional weathering)

• colluvial (mass movement products)

• erosional (in-situ subsoils, with mobile topsoils)

• transferral (fluvial fan/piedmont deposited materials)

• alluvial (alluvial deposition)

• estuarine (delta deposition)

• aeolian (wind deposition)

• lacustrine (lake deposition)

• beach (wave deposition)

• swamp (organic mater accumulation)

• gilgai (shrink-swell processes)

• disturbed (human activity)

In soil landscape reports each soil landscape description includes a soil occurrence relationship schematic diagram. These diagrams illustrate the stratigraphy of individual soil materials in relation to position within the landscape. Soil materials are relatively morphologically homogeneous soil entities with significant lateral continuity. Soil materials often correspond with regolith materials.

Examples are provided In the poster paper.


• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

(Pedogenic) Gyperete Formation in Arid Central Australia•

X.Y. Chen• School of Resource, Environmental and Heritage Sciences, University of Canberra, P.O. Box 1, Bekonnen ACT 2616

•In arid Australia (<250 mm/yr rainfall), various gypsum sediments are widely distributed around salt lakes and along

• present or ancient drainage systems, which are primarily precipitated from saturated surface or underground brinesand may later he transported by water or wind. When these primary gypsum sediments are exposed to subaerial

• conditions and above the influence of ground water, they tend to be reformed by pedogenic processes and to betransformed to (pedogenic) gyperete. A gyperete consists of white, powdery-like masses of microcrystalline gypsum

• crystals and is usually cemented with a polygonal hardened "skin" on the exposed surface. These very fine (mostly<0.2mm) crystals are distinctively different from the parent gypsum sediments which mostly consist of larger

• (>0.5mm) crystals and have many lacustrine, aeolian or other primary sedimentary features. A gyperete usuallycontains >80% gypsum with minor quartz sands, clays, carbonate and heavy minerals. Various pedologic features

• have been developed such as channels, cutans, pedotubules, glaebules and secondary carbonates. The major processesof gyperete formation involve dissolution, leaching through profile and recrystallisation. Dehydration possibly occurs,

• helping to break larger crystals. However, dehydrated forms of sulphates tend to be rehydrated to gypsum when anywater available from rainfall or atmospheric moisture. The TL dating of co-occuring quartz sands in gypsum dune

• sequences indicates that a 0.5-1.0m gyperete horizon 'nay form in a period of several thousands years or even less. Atleast 3 episodes of gyperete formation have been dated as 80-100 ka, 35-50 ka and in Holocene. These gyperete

• formations followed major gypsum deposition episodes in salt lakes of central Australia and indicate landform stableintervals during which both gypsum deposition and reworking are negligible. The dates probably also indicate that any

• older (eg. >125 ka) gypsum/gyperete dune sequences may not be able to survive the dissolution and leaching under theclimatic conditions of central Australia.•

•••••••••••••••••• ABSTRACTS: Australian Regolith Conference '94^ 17

@ Australian Geological Survey Organisation

•

The Use of Oxygen- and Hydrogen-Isotopes in Weathering Profiles to Indicate Past Climates, Latitudes, Altitudes and Ages of Formation

Allan R. Chivas and Michael 1. Bird Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia

The oxygen- and hydrogen-isotope compositions (expressed as a180 and aD values) of a mineral are related to its temperature of formation and the isotopic composition of the water from which it crystallized. For minerals formed near the Earth's surface during weathering, the water is typically groundwater, or meteoric water (i.e. water in isotopic equilibrium with the atmosphere). As the isotopic composition of meteoric water varies principally as a function of the temperature of rainfall condensation at the time of recharge of groundwaters, both rainfall and unevaporated groundwaters have 5180 and 5D values related to global mean temperatures. Thus there is a regular relationship between latitude/altitude and isotopic values.

During weathering, this same variation in isotopic composition as a function of temperature is preserved in newlyformed minerals that are precipitated in equilibrium with groundwater. This relationship holds true for a variety of weathering minerals but is most easily demonstrated for the clay minerals, where it has been shown that the 5180 values for kaolinite are retained for up to 240 Ma. Thus gross distinction between cold-climate and warmer weathering is readily made.

Tectonic movement of weathered cratons either by translation or uplift can also be detected by comparison of the preserved 5180 values in kaolinite with those calculated for the present situation from the isotopic composition of modern rainfall. Conversely, if the rates of motion of such plates are known, the interpolated ages of weathering can be deduced. A broad conclusion of this work is that cold-climate weathering is a common event, but not so well preserved at current high latitudes owing to Quaternary glaciation. The presence of deep weathering profiles in the geological record should not be necessarily taken to indicate past "tropical" conditions or latitude.

The above methodology is ideally suited to the study of clays from both deep-weathering and reworked-sedimentary environments in the Gondwanaland continental fragments. For example, oxygen-isotope results from clays in Australia and India demonstrate the expected substantial climate shifts related to continental translations since the Permian. Isotopic results from South America, where continental drift was less during the Cretaceous and Tertiary periods show only minor changes with time. The existence of the Australian Eastern Highlands since the Cretaceous is also strongly supported by isotopic results on surficial clay minerals.


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•

•

• Stable-Isotope Indicators of the Origin and Age of the Weathering Zones on the Broken Hill OrebodyAllan R. Chivas' and Colin Wood2

• ' Research School of Earth Sciences, The Australian National University, Canberra, ACT 02002 CRA Exploration Pty Ltd, Orange NSW 28000

• The carbon- and oxygen-isotope composition of primary and secondary carbonates from the Broken Hill orebody aredistinctive. Primary calcite (81805m0w=+10700, 813CpDB=-22%o) appears to have uniform isotopic composition

• throughout the several orebodies. By contrast, in the overlying weathering profiles, cerussite (PbCO 3) has a 8180smowvalue of +16±1708 and a VC value of -16±2700 (23 samples); smithsonite (ZnCO 3) has 8180=+29±0.5%0 and 8 13C=-

• 8.5±1.5700 (5 samples); with the isotopic composition of a single sample of malachite between these two fields, butcloser to that of smithsonite. The 8 13C values of the secondary carbonates reflect the contribution of carbon from

III^contemporaneous soil-derived or atmospheric carbon dioxide.

• The 8180smow values of weathering-produced kaolinite in the upper parts of the gossans are +18.5±1.4700 and aretypical of weathering profiles of Late Tertiary age elsewhere in eastern and central Australia.

01

0••••••••••••••••••••• ABSTRACTS: Australian Regolith Conference '94^ 19

© Australian Geological Survey Organisation•

••

Pisolites all over: Classification, Genesis, and Evolution of Ferruginous Surface Grains^ •Jonathan D. A. Clarke

Western Mining Corporation, P. 0. Box 157, Preston, Victoria, 3072

Studies of ferruginous surface grains (FSGs) have shed considerable light into the classification, genesis, andevolution of these grains. Understanding FSGs is important for both general regolith interpretation and understandinggeochemical anomalies in mineral exploration.The classification system is derived from one proposed by CSLRO and has four levels. The first needs only superficialobservation to recognise surface lags. The second level separates FSGs out from other lag components and hencerequires more detailed observation. The third level of classification is based broad textural types observed frommicroscopically (homogeneous, lithorelic, pseudomorphic, vesicular, sandy, and oolitic). The fourth level relies onrecognition of modifying textures (concentric, cutanic, compound, unmixing, and syneresis).

FSGs form in three main environments, namely weathering profiles (mottled zone and saprolite), surficialenvironments (soils and surficial sediments), and subaqueous environments (lakes and rivers).FSGs form through four main processes, namely ferruginisation (most likely in the weathering profile, but also insurficial and subaqueous environments), concretion (surficial materials and in the weathering profile), accretion(subaqueous environments), and fragmentation of existing ferruginous duricrusts.

Diagenesis of FSGs results in replacement of clays by iron hydroxides. These minerals can also form through primaryprecipitation. Dehydration of hydroxides to form haematite occurs in arid environments. Strong heating by fires mayresult in the formation of maghaemite. Hydration of haematite to goethite also occurs in humid climates.Microfabric studies must be integrated with geochemical data, geomorphological mapping and regolith stratigraphyand petrology for maximum benefit.


•

•

• Regolith Landform Features of the Northeast Yilgarn, WAMA. Craig' and H.M. Churchward2

• I Austalian Geological Survey Organisation, Division of Regional Geology and Minerals PO Box 378 Canberra 26012 Formerly AGSO/CS1RO. Present address: Box 159 Balingup. WA. 6253.

• Regolith landform mapping under the NGMA by AGSO now extends throughout the northeastern part of the EasternGoldfields of WA to include Menzies, Edjudina, Leonora, Laverton, Duketon, Sir Samuel and Wiluna 1:250k sheets.

• The compilation scale is 1:100k.

The geomorphic history of this part of the Yilgarn is complex, and contains a wide variety of regolith landform• features. Vital among them are sandplains and sheet flood fan systems. They are pivotal in any account of the regolith-

landform evolution of the region, and are the focus of this discussion.•

The sandplains are placed into one of the following two categories:• 1. Those sandplains above breakaways that are related to preserved elements of an older landscape

• 2. Those sandplains below breakaways that are related to younger sheet flood fan systems within the landscape.

•^Sheet flood fan systems can be assigned to:

1. Those relict features derived mainly from greenstone sources (with some from granitic sources), in some instances• now dissected and positioned in lower parts of the landscape and, representing older phases of activity.

2. Those systems derived from either greenstone or granitic sources or mixtures, and now active or have been active• in the recent geomorphic past.

• The variations occurring within the sandplains and sheetflood fan systems are not always so easily recognized becauserelict and more recent landscape elements often merge imperceptibly from one to another. The importance of 3D

• visualization is paramount in these situations. Especially valuable is the vertical exaggeration afforded by stereoscopicaerial photo pairs where the merging of the two landscape elements can be immediately obvious. However this alone

• this does not reveal the full complexity. Regolith attributes including geobotanical indicators collected on a regionalbasis allows a clearer definition and understanding of the variety and complexity present.•

••••••••

•••••••


Evolution of some Landforms and Regolith in Eastern Arnhem Land

IHCrick Division of Regional Geology and Minerals, Australian Geological Survey Organisation

Regolith-landform mapping of eastern Arnhem Land was undertaken by the Australian Geological Survey Organisation during 1992 and 1993 as part of the National Geological Mapping Accord. Regolith-landform units and associations have been developed for the region and maps and reports are nearing completion.

Regolith and landforms have been strongly influenced by the presence of Cretaceous sediments which unconformably overlie Proterozoic and Cambrian rocks throughout much of the area. Erosion since these sediments were exposed has dissected what was originally an almost continuous cover over large areas. A deep regolith has developed on the erosional remnants of the Cretaceous cover which, on the Gove Peninsula, includes bauxite. Parts of the Mitchell and Parsons Range may have remained above sea level during sedimentation in the Cretaceous.

Near the summit of these ranges deep, highly weathered sediments forming mesas unconformably overlie the Proterozoic rocks. Remnants of river meander channels are also present high in the ranges. The age of these features is uncertain but they are possibly older than Cretaceous and are clearly relicts of an ancient landsurface. To the west of the Mitchell Range, remnants of the pre-Cretaceous land surface, found on sediments of the Cambrian Wessell Group, are preserved.

Proterozoic sandstones in this area are relatively resistant rocks and form many of the higher landforms. Regolith development over them has been limited and generally only a thin cover has formed. Sand eroded from these sediments has, in places, formed extensive deposits which are mostly alluvial in origin. However, some sand deposits appear, from their position on interfluves, to be aeolian. The presence of an inland relict dune field to the west of the Mitchell Range suggests that this region has undergone a period of aridity.


• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

•Episodic Weathering: Evidence from 40Ar/39Ar Laser Microprobe and Two-Stage K-Ar Dating of Weathering-

Related Cryptomelane-Hollandite and Alunite/Jarosite in Western and Northern Australian Regolith

• D. Dammer, I. McDougall and A. R. ChivasResearch school of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia•

• K-bearing Mn oxides from the cryptomelane-hollandite (C-H) group and sulphates from the alunite and jarositegroups are formed during terrestrial chemical weathering of rocks. These minerals are amenable to isotopic dating by

• K-Ar and 40Ar/39Ar methods. In ideal circumstances, such ages will reflect the time since formation of these regolithminerals. However, the use of these minerals for determining the age of weathering by 40Ar/39Ar and K-Ar dating

• methods can he complicated by: (1) admixture of mica inherited from bedrock and (2) mixing of severalchronologically different generations of weathering-related minerals. Both problems can cause the K-Ar age measured

• on an impure weathering-related mineral to not accurately reflect the time of its formation. To correct for admixture ofinherited mica a two-stage K-Ar dating technique was used. This method uses a material-balance approach to correct

• for radiogenic 40Ar contributed from inherited mica.

Some samples of C-H have finely banded and colloform textures, and therefore may represent several generations,• prompting the need to investigate the age differences on a microscale. The combination of microsampling and laser

microprobe 40Ar/39Ar dating allowed us to date thin overgrowths, groups of adjacent growth bands and thin veinlets• that traverse the matrix.

• Total fusion age determinations of several groups of growth bands of densely layered C-H from the Woodie WoodieDeposit (NW Australia) range hetween 24.6 (outer rim) and 30.0 Ma (core), suggesting an averaged growth rate of

• 0.8 ± 0.3 mm/Ma. This is a much smaller value than a previously reported example of 6.4 ± 1.2 mm/Ma measured onbotryoidal growth bands of cryptomelane from Brazil (Vasconcelos et al., 1992). The growth rate reported here might

• be the result of very slow growth of C-H, or the presence of hiatuses in C-H precipitation, perhaps indicative ofalternating periods of humid and dry climate in the area. The fad that the relative probability graph of total fusion ages

• from the dated banded C-H shows at least three distinct peaks (25,26.5 and 28-30 Ma) supports the latter idea.

•The relative probability graph of total fusion ages measured on void filling banded and colloform C-H in samples froma vertical profile through a mesa-like deposit near Horseshoe (NW Australia) shows peaks at 48-52, 43-44, 39-40, 35,

•and 29-30 Ma. In addition banded overgrowths of C-H were deposited at about 7.5 Ma ago in the lowermost part ofthe profile. This evidence is interpreted as reflecting an episodic history of weathering-related product formation andrelated to alternation of more humid and drier climatic periods during the Tertiary in Australia.

Reference• Vasconcelos, P. M., Becker, T A., Renne, P. R. and Brimhall, G. H., 1992, Age and duration of weathering by 4°K-

•40Ar and 40Ar/39Ar analysis of potassium-manganese oxides: Science, v.258, p.451-155.

••••• ABSTRACTS: Australian Regolith Conference '94^ 23

© Australian Geological Survey Organisation•

Landscape Evolution on the Barkly Tableland

CJ. Edgoose and K.T. WinstanleyConservation Commission of the NT, Alice Springs, NT.

The Barkly Tableland occupies an area of about 100,000 km2 in the central east of the Northern Territory andextending into Queensland. It is characterised by extensive flat to gently undulating 'black soil plains' or 'downscountry'. At first appearance the Tableland appears to he uniform- certainly in contrast to the dramatic landscapes ofcentral Australia it lacks variation. However this perception is purely one of scale and comparison. The Tablelandlandscape is diverse and continunlly subject to change, but via some quite unusual mechanisms which are a function ofa unique geological history, soil type, climatic fluctuation and most importantly lack of slope.

The Tableland is predominantly formed over middle Cambrian (-520 Ma) dominantly carbonate sedimentary rocks ofthe northern Georgina Basin (Smith, 1972). Parts of the northern region have a thin cover of Cretaceous marinesediments over the Cambrian rocks. The dominant exposures of the sediments of the Georgina Basin occur to thesouth and south-east of the Barkly Tableland, where they form extensive low ranges which outline the broad foldsformed during the Alice Springs Orogeny (-350 Ma). The northern part of the basin was very shallow and quicklyfilled with the first influx of sediments, suggesting a subdued hinterland existed at the time. The Alice SpringsOrogeny did not affect the northern part of the basin- here the sequence has remained largely flat lying (Smith, 1972).This absence of folding and uplift indicates that very little or no topographic variation subsequently formed. Theyoungest hills or mountains that may have existed in the region would have developed during the Neoproterozoic(-1100 Ma) or Palaeoproterozoic (-1800 Ma) (the age of orogenic events in the surrounding and underlyingbasement). The landscape of the Barkly Tableland has probably been an area of subdued to flat topography for at least500 million years and probably much longer.

During the Tertiary widespread laterite profiles and detrital deposits developed in the largely peneplained landscape,probably strongly associated with the drainage and ground water regimes of the time. Thin deposits of limestoneaccumulated in scattered swamps and drainage. These deposits largely coincide with the present day endoreisicdrainage system, and extensive remnants of the lateritic palaeoplains show only very slight elevation above the blacksoil plains. These observations indicate that the topographic configuration of the Bandy Tableland has changed little.

The very weak incision by present day drainage and the slowly receding lateritic palaeoplain provide evidence thatminor rejuvenation of the landscape has occurred sometime since the Tertiary. The Barkly Tableland probablycontains as great a topographic variation now than it has for the past 25 million years.

Historically the 'downs' have been described as Tertiary swamp deposits (Christian et al, 19544 Randal and Nicholls,1963) formed concurrently with the lateritic plain, or soils derived at least in part directly from the carbonate substrate(Randal, 1966; Grant, 1989). We suggest that the soils are largely derived through transformation and stripping of thelateritic palaeoplain.

The Barkly Tableland comprises considerable expanses of gilgaied surfaces and observations over large areas showthat their distribution is strongly landscape correlated. Generally they lie in the areas of greatest slope, on both thelateritic palaeoplains and the 'downs' where they abut, and therefore in the most dynamic part of the landscape. Webelieve that rather than gilgai forming in this part of the landscape, gilgai are one of the major processes in formingthis part of the landscape.

The lateritic palaeoplains are dominated by kaolinitic clays, typical of surfaces formed during the Tertiary. The gilgaiarid 'downs' arc dominated by smectite clays- the formation of which is favoured by post-Tertiary drier climaticconditions. We suggest a conversion of clays is taking place, the localised expression of which is the development ofgilgai. This conversion is strongly associated with the stripping of the older surfaces initiated by the weak post-Tertiary rejuvenation, and together are the major processes In the development of the 'downs'. Veen (1973) describesa process of transformation of day soils in the Brigalow lands of SE Queensland.The Barkly Tableland could easily be considered a monotonous and unhanging environment A study of its geological,geornorphological and climatic history demonstrates that even areas that on a large scale have changed little overimmense periods of time can be dynamic. The mechanisms of change are however substantially different.


References

Grant, A.R. (1989). Landform, soil and pasture associations of a 'downs' landscape, western Barkly Tableland. in'Proceedings of the Animal Industry tri-partite Conference', Katherine, 1989.

Randn11, M.A. (1966). Alroy, NT. 1:250 000 Geological Series Explanatory Notes. Bureau of Mineral Resources.

Randall, M A and Nichols, R.A.H. (1963). The geology of the Alroy and Brunette Downs 1:250 000 sheet areas,Northern Territory. Bureau of Mineral Resources, Record 72.

Smith, K.O. (1972). Stratigraphy of the Georgina Basin. Bureau of Mineral Resources, Bulletin 111.

Veen, A.W.L. (1973). Evaluation of clay mineral equilibria in some clay soils (Usterts) of the Brigalow lands. Aust. J.Soil Res., 11:167-184.


Stable Isotope Constraints on Silcrete Formation

Suzanne D. Golding1 and John A Webbz. 1 Dept of Earth Sciences, University of Queensland, 4072

Z School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083

Silcretes occur in Australia in two distinct geographical areas. The inland silcretes are considered to be the product of either soil forming or groundwater processes, i.e. pedogenetic and groundwater silcretes of Milnes and Thiry (1986). In contrast, silcretes of the humid parts of eastern Australia are frequently spatially associated with basalts, leading many workers to postulate a genetic relationship (e.g. Langford-Smith, 1978; Ollier, 1978; Collins, 1985). The 'subbasaltic' silcretes of eastern Australia as a group are distinguished from the inland silcretes by lower TiOz contents (young, 1985), in contradiction to a proposition of Summerfield (1983) that lower TiOz contents in silcretes reflect formation under arid to semi-arid climatic conditions. Apart from their association with overlying or adjacent basalts, the 'sub-basaltic' silcretes resemble the inland groundwater silcretes petrographically and appear to be related to groundwater silicification of bedrock and/or regolith.

The oxygen isotope composition of a mineral such as quartz reflects the temperature and oxygen isotope composition of the environment in which the mineral crystallised. In general, detrital quartz grains in sediments retain the oxygen isotope composition that they had in the parent rock. However, quartz cements may have distinctive oxygen isotope compositions which reflect both the isotope composition and temperature of the pore fluids (Savin, 1980). Silcretes may form by different silicification processes in arid inland and humid coastal environments (evaporative and groundwater processes respectively), so oxygen isotope analyses may constrain the relative importance of these processes in the formation of particular silcrete occurrences.

Oxygen isotope analyses of Victorian 'sub-basaltic' silcretes are systematically enriched in 180 relative to adjacent bedrock or regolith, and this can be related to the extent of low-temperature silicification by local groundwaters. The 0180 values of individual silcretes and their precursors exhibit a linear correlation with the proportion of introduced silica, as estimated from composition-volume calculations. In contrast, the 0180 values of pedogenic silcretes from northern South Australia appear to reflect the position of the silcrete samples within the specified soil profile rather than the extent of silicification. Thus, pedogenetic massive silcretes from the top of a stratigraphic section through a soil profile are enriched relative to the underlying columnar silcrete, suggesting that evaporative processes have been particularly important during the development of the upper massive silcrete zone.

These relationships suggest that oxygen isotope analyses in combination with macroscopic, petrographic and compositional studies can be used to constrain the processes and environment of silcrete formation.

References

Collins, N.G.S., 1985. Unpubl. BSc Hons, Univ. Melbourne Langford-Smith, T., 1979. Silcrete in Australia, Dept Geog., Univ. New England, 1-11.

Milnes, A.R. & Thiry, M., 1986. 12th/nt. Sed. Congress Abs., 213.

Ollier, C.D., 1978. Silcrete in Australia, Dept. Geog. Univ. New England, 13-17.

Savin, S.M., 1980. Handbook of Environmental Isotope Geochemistry, Elsevier, Amsterdam, 283-327.

Summerfield, M.A., 1983. J Sediment Petrol., 53: 89-90.

Young, R.W., 1986. Zeitschriftfur Geomorphologie, 29: 21-36.


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Stream Morphology and Neogene Landscape Evolution in the Lachlan Valley, NSWGeoff Goldrick

Department of Geography and Environmental Sciences, Monash University

Streams occupy a special position in the study of landscape change over geological time in that they not only facilitatechange through the detachment, transport and deposition of materials, but are themselves shaped by other forces suchas tectonics (Seeber and Gornitz, 1983; Keller and Rockwell, 1984; Bishop et al., 1983). Studies such as Merrits andVincent (1989) have shown that streams are important recorders of landscape evolution processes such as tectonics. Ifthis is generally the case, a good understanding of the factors that determine stream morphology over geological timemay aid the elucidation of unknown landscape evolution histories. Particularly important questions include: the inter-relationships between stream longitudinal and plan form and factors such as geology, tectonic history and climaticchange; and, the relevance of concepts such as (dynamic) equilibrium. These questions are explored in a study ofstream long profiles and landscape evolution in the middle and upper Lachlan Valley in central NSW.

References

Bishop, P., Young, R. W., and McDougall, 1. (1983) Stream profile change and long term landscape evolution: EarlyMiocene and modern rivers of the east Australian highland crest, Journal of Geology, 93,433-474.

Keller, E. A., and Rockwell, T. K. (1984) Tectonic geomorphology, Quaternary chronology and paleoseismicity, inCosta, J. E., and Fleisher, P. J., eds., Development and Applications of Geomorphology: Berlin, Springer-Verlag, p.203-239.Merrits, D., and Vincent, K. R. (1 989) Geomorphic response of coastal streams to low, intermediate, and high rates ofuplift, Mendocino triple junction region, northern California, Geological Society of America Bulletin, 101, 1373-1388.

Seeber, L., and Gornitz, V. (1983) River profiles along the Himalayan arc as indicator of active tectonics,Tectonophysics, 92, 333-367.

Young, R. W. (1983) Waterfalls: form and process, Zeitschrift fur Geomorphologie Supplementband, 33, 81-93.


Landscape Processes in the Upper Todd River Catchment, Central Australia, and Implications for AssessingLand Use Impacts

A R GrantConservation Commission of the Northern Territory, Alice Springs, NT

The landscape of the upper Todd River catchment in Central Australia is complex, reflecting a vigorous but stagedevolution since possibly the early Tertiary. At that time, bedrock and sediments in a prior valley were subjected tointense weathering which resulted in the development of a deep mantle of saprolite. The valley floor sediments weresilicified then dissected, and detrital laterites later developed in drainage floor environments. Rapidly aggradingpiedmont deposits flanking adjacent uplands subsequently buried this prior valley and culminated in the developmentof an extensive palaeoplain which is now drained by 16 Mile Creek. The Todd River drainage system has dissectedthis palaeoplain to form the present catchment, partly exhuming the landforms of the early Tertiary valley floor in theprocess.In a contemporary context, the main geomorphic activity within the catchment involves sheet flow processes on slopesand episodic floodplain development. Where soil surface conditions are favourable, sheet flow processes result in theformation of microterraces on slopes. These microterraces are stabilised by vegetation and evident as mulga groves.Sediment shed from microterraces eventually aggrades in broad drainage floors. The upper Todd River floodplain hasevolved through the episodic vertical aggradation and subsequent catastrophic erosion of sediment prior to theHolocene and subsequent events have been of insufficient competence to effect its reconstruction.The assessment of the geomorphic impact of pastoral land use in arid environments is often hindered by landscapecomplexity. Geomorphologists are also faced with the dilemma of partitioning accelerated erosion from active butapparently natural processes of sediment movement. In the case of the upper Todd River catchment, only gully erosioncan be readily attributed to land use as it closely correlates with prior access track alignments. Floodplain erosionseems primarily a consequence of the impact of enormous palaeofloods rather than occurring solely as a result ofgrazing use. Other evidence of erosion is difficult to assign to a specific cause on the basis of field geomorphicassessments as a range of interacting site and event specific variables may be involved.


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Soil Genesis in a Longitudinal Dune·Swale Landscape, NSW, Australia

R S B Greenel,2 and W D. Nettleton3

•4

I CSIRO Division of Wildlife and Ecology, PO Box 84, Lyneham, ACT 2602 2 Current address: Department of Geography, School of Resource and Environmental Management, Australian

National University, Canberra, ACT, 0200 3 CSIRO Division of Soils, GPO Box 639, Canberra, ACT 2601

4 Current address: USDA Soil Conservation Service, National Soil Survey Laboratory, NSSC, Lincoln, Nebraska.

Xerollic Haplargids in fme-Ioamy, siliceous, thennic families are paralleled by fine, mixed thennic ones in a longitudinal dune-swale landscape in the Kulwin Dunefield east of the Darling River in New South Wales. Our objectives were to identity the parent materials and explain the genesis of the soils. The very fine sands in the two soils are mostly quartz. Weatherable minerals make up < 109 kg-I. Clays have cation exchange capacities at pH 7.0 of 500 to 600 cmol (p+)kg-l and clay mica amounts to about 150 g ktl. The textural difference in the two soils is in large part explained by differences in their aeolian parent materials. The fine material making up the swale is thought to be derived by winnowing of the simultaneously fonning dune by eastward-moving, converging helicoidal air currents and redeposited by slower moving, diagonal air currents across the swale. The initial aeolian sediment source appears to have been the Darling River flood plain. The dune-swale landscape has been reported to have formed about 16 to 20 ka. The paleosol 175 cm below the dune surface, like that of the lower solum of the swale soil, has a higher silt content than the dune ground soil. A calcic horizon with 220 g kg- l of carbonate has fonned in the swale soil below a depth of 40 cm. In the dune the ground soil has only traces of carbonate, but the upper part of the paleosol apparently has been enriched with carbonate from the dune. Silt- and clay-size clay bodies (parna) occur in less dense pans of the swale soil and throughout the Bt horizons of the dune ground soil. In denser parts of the swale soil skelsepic fabric occurs. Illuviation argillans occur in voids and channels in the swale soil argillic horizon and on sands and bridging between them in both the swale soil and the ground soil on the dune. Pedogenic processes of carbonate removal, pedoplasmation, and clay translocation thus have occurred in the soils during and since deposition of the parna.


29

High Resolution Microscopy of Synthetic Ferrihydrites and their Transformation into Goethites

Greffie, C.1-2

, Amouric, M.3, Benedetti, M.4 and Parron, C.l

1 URA CNRS 132, Laboratoire de Geosciences de l'Environnement, Universire Aix-Marseille ill, 13397, Marseille cedex 20, France; 2 Soil Science and Plant Nutrition, University of Western Australia, Nedlands WA 6009, Australia, 3 CRMCC, Campus de Luminy, 13288, Marseille, France, 4 Soil Science and Plant Nutrition, Staringgebow, Marijkeweg II, Wageningen, Netherlands.

Freshly precipitated hydrous ferric oxide gels were prepared by adding KOH potassium hydroxide to O.IM ferric nitrate solution while stirring vigorously until achieving a pH of 7.0-7.5 at room temperature. The resulting suspensions were then freeze-dried or for some of them, included in a Nanoplast FB 101-hydrophilic resin. Such hydrophilic resins are usually used for biological materials and only recently for colloidal materials because they do not disturb very delicate structures and do not contract during drying (about 2%).

Freeze-dried iron phases formed during the conventional synthesis of 2-line ferrihydrites and characterised effectively by X-ray powder diffraction as 2-line ferrihydrites, are in fact, by high resolution transmission electron microscopy (HRTEM), mixed phases with various degrees of structural organization and composed of 2-line to 7-line ferrihydrites and "amoeba-like" goethites. Ferrihydrites consist of aggregation of pseudo-spherical domains of colloidal dimensions (30 to 50A in diameter). A complete structural evolution sequence has been revealed from 2-line ferrihydrites to amoeba-like goethites. The transformation between 2-line and 7-line ferrihydrites consist of individualisation of elementary volumes with increasing internal organisation until lattice fringes appear inside microdomains (4.50 and 2.23A which are respectively (002) and (112) fringes of 6-line ferrihydrites as described by Eggleton and Fitzpatrick, 1988). A significant relationship between 7-line ferrihydrites and "amoeba-like" goethites has been observed. The boundary ferrihydrite / goethite is not clear, the outlines of microdomains becoming less and less visible with development of the goethite phase. Without freeze-drying, iron phases appear to be very poorly aggregated and consist of ferrihydrites microdomains ranging from 20 to 50A in diameter. No lattice fringes have been detected inside microdomains and their electron diffraction patterns are always very poor with only 1 broad ring at 1.50A (typical (30) scattering band offerrihydrite) indicated that this product is very poorly ordered.

In conclusion, it seems that freeze-drying should not be used for such poorly organised phases because it probably disturbs elementary colloidal structures by causing irreversible aggregation and intense dehydration. The study of such delicate and poorly organised phases requires HRTEM observations, in less damaging preparations such as hydrophilic resin inclusions.

Direct high resolution observations reveal a complete structural evolution sequence of these iron phases from 2-line ferrihydrite to amoeba-like goethite and confirm that the dissolution/crystallisation process seems to be a necessary step in the formation of goethites from ferrihydrite particles (Fischer, 1971; Schwertmann and Murad, 1983; Manceau and Drits, 1993).

References

Eggleton R.A. and Fitzpatrick R.W. (1988). New data and a revised structural model for ferrihydrite. Clays and Clay Miner. 36, 111-124.

Fischer W.R. (1971). Modellversuche zur Bildung und Auflosung von Goethit und amorphen Einsenoxiden im Boden. Diss. P U. Munchen.

Manceau A. and Drits V.A. (1993). Local structure of ferrihydrite and feroxyhite by EXAFS spectroscopy. Clay Miner. 28,165-184.

Schwertmann U. and Murad E. (1983). Effect of pH on the formation of goethite and hematite from ferrihydrite. Clays and Clay Miner. 31, 277-284.


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RTMAP • AGSO's Regolith Mapping Database

Murray Hazell Division of Regional Geology and Minerals, Australian Geological Survey Organisation

AGSO has developed a number of field and laboratory databases for digitally recording data collected by AGSO's National Geoscience Mapping Accord projects. The databases operate on AGSO's corporate mainframe Oracle Relational Database System, allowing full interrogation, analysis and integration of the databases. Data can be analysed using the Oracle SQL language or in the spatial domain via a GIS.

The RTMAP Database is one component of AGSO's field and laboratory databases. It is designed to record descriptions of regolith field mapping sites and regolith landform mapping unit polygons. RTMAP shares a common Sites table with the other AGSO field databases which standardises the way geographical data is recorded in AGSO. Locational method and accuracy are recorded with the geographical data giving the Sites table a degree of scale independence. R TMAP has been designed to be as flexible as possible within the necessary confines of database design, to allow for variations in mapping emphasis. Users are able to select and describe a wide variety of attributes as required by their mapping needs and the scale at which they are working. Besides the standard descriptions of regolith and landforms some of the other possible attributes that can be described are: sedimentological descriptons, environmental hazards and gamma ray spectrometric readings.

The second part of RTMAP is the mapping unit description database which records descriptions of mapping unit polygons. Describing regolith landform mapping units digitally enables the full gamut of variations within, and between regolith landform mapping units to be easily interrogated and integrated with other datasets such as Digital Elevation Models, various remote sensing methods, surficial geochemistry and age determination methods. Using RTMAP and AGSO's other field and laboratory databases AGSO can produce standard series maps based on standard attributes or maps based on particular themes directed towards certain client end uses such as mineral exploration or studies of environmental degradation.


31

The Differential Weathering of Granitic Rocks in Victoria, Australia

S.M. HillCentre for Australian Regolith Studies, The Australian National University, Canberra, ACT, Australia, 0200

Granitic rocks are a major component of the Victorian basement geology, representing approximately one quarter ofthe exposed Palaeozoic rocks in the state. In the process of being exposed these rocks have been weathered to productsthat are more stable in the near surface physico-chemical conditions. Various factors interact and combine to influencethe development of weathering of granitic rocks, including: parent material, time, climate, topography and livingorganisms. This poster examines the influence of some of the factors of weathering pertaining to the nature of thegranitic parent material and their potential to contribute to differential weathering of these rocks.An examination of the geomorphic features and landscape settings of granitic rocks in Victoria reveals manydifferences. Some plutons are prominent landscape features, rising above the surrounding landscape (eg. MountBuffalo, Pilot Ranges, Strathbogie Ranges and the Baw Baw Plateau, Mount Cole, Ben Nevis and Langi Ghiran),while others are more subdued, occurring as low lying areas usually covered by fluvial sediments and deep weatheringprofiles (eg. (Glenlogie, Beverley and Amphitheatre, Harcourt and Ararat plutons and plutons in the Suggan Bugganarea). The development of these "positive" and "negative" relief plutons is largely accounted for by differentialweathering of the granitic parent material. Topographic "steps" corresponding to structural, textural andcompositional variations within the granites at Wilsons Promontory and Mount Buller also relate to differentialweathering and erosion. The outcrop morphology of different granites also varies markedly, particularly in the shapeand size of tors.Difference in joint frequency is a well documented influence on differential weathering of granitic rocks, and in somecases accounts for these differential weathering features. Other features of the parent material such as crystal size andtexture, mineral composition and hydrothermal preconditioning can also be significant. The medal biotite percentageof the granitic rock (and to a lesser extent the medal plagioclase) is a major factor to be considered; where rockscontaining a greater proportion of these more labile minerals are more susceptible to weathering. Crystal grain sizeand textures have a variable influence on differential weathering, where fine grained granites may be both more andless resistant to weathering than coarser grained granites in different circumstances. This variable weatheringbehaviour may be accounted for by the presence of micro-fractures within mineral grains and by the greatersignificance of other factors other than the grain size. By considering the factors influencing the development oflandscape features associated with different granitic rocks within Victoria, models for the differential weathering ofthese rocks can be proposed.Differential weathering of granitic rocks within Victoria has a major influence on the landscape and regolith features.An understanding of these influences has implications in geological sampling and mapping, as well as inunderstanding the landscape features of these areas.


Mobility of Base Metals through Regolith at Broken Hill, NSW, based on Detailed Regolith Mapping and• Chemical Analyses

• E. B. Joyce and D. Lulofs *School of Earth Sciences, University of Melbourne, Parkville VIC 3052

• *now Western Mining Corporation, P.O. Box 114, Daw Park SA 5041

•Regolith profiles developed in zones of Pb-Zn mineralisation have been investigated at Maybe11 and Stirling Vale,

• north and west of Broken Hill. Outcropping metasediments and metavolcanics of the Proterozoic WillyamaSupergroup include quartz-gahnite horizons hosting the mineralisation.

•In the two selected study areas, approximately 200m by 300m, regolith has been mapped at a scale of 1: 500. Regolith

• profiles were studied in exploration trenches and in soil pits prepared as part of the study.

Rocky outcrops at the top of each ridge are buried downslope by increasingly thick slope mantles. Foot slopes are• gullied, and lead down to major drainage lines with alluvial deposits. Quartz-gahnite horizons occur as topographic

highs in both areas, and contain up to 13.5% Zn. Gossanous material is associated with the quartz-gahnite rocks, and• iron oxides contain substantial levels of base metals, although secondary minerals with base metals are absent. On mid

and lower slopes relatively fresh Proterozoic bedrock is overlain by a relict aeolian deposit or parna sheet, which has a• desert loam soil. Locally transported sheetwash deposits occur at the foot of slopes.

• Soils on the parna were found to be anomalous in Zn, Cu and also their pathfinders Cd and As, and indicatedmobilisation of base metals into the windblown deposit and soil. Similar anomalies were found in the young stream

• sediments. However calcrete in subsoils contained no anomalous levels of Zn, presumably due to the low solubility ofZn at high pH.

• Base metal occurrences in the soil were associated with amorphous iron oxides and also with silicates (presumablygahnites). These metal bonding sites indicate the dispersion haloes in the regolith are due to a combination of physical

• and chemical dispersion.

• In this example of regolith profiles developed on wind-transported materials in an arid terrain, Zn and Cu havephysically and hydromorphically dispersed from weathering quartz-gahnite, exposed upslope and underlying lower

ID^slopes, and the base metal signatures now occur in the relatively youthful added aeolian deposits.Reference• Lulofs, Damien, 1993. Mobility of Base Metals through Regolith, Broken Hill, N.S.W. Unpublished Honours report,

• School of Earth Sciences, University of Melbourne.

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The Geochemistry of Sub-Basaltic Silcretes in Central Victoria

Joyce, E BI, Webb,J.A.2 and Collins, N. G. I

1 School of Earth Sciences, The University of Melboume, Parkville, Victoria, 3052. 2 School of Earth Sciences, La Trobe University, Bundoora, Victoria, 3083

Silcretes in central Victoria are massive in appearance and lack pedogenic features, and almost invariably underlie or are adjacent to basalt flows. The material silicified represents surface or shallow subsurface lithologies, ranging from Ordovician shales and sandstones to the regolith developed on this bedrock, to Tertiary quartz-riCh sands and gravels.

Chemically these silcretes are all very similar, with a very high silica content (>90%), up to 5% F~03 and less than 2% Ti02• A number of silcretes analysed were compared with the immediately underlying unsilicified material using isocon diagrams. These showed that during silicification there is, of course, a major input of silica, accompanied by a substantial loss of alumina (presumably due to clay breakdown) and minor loss of other elements. There is little, if any, loss of Ti02, indicating that although it may be mobilised during silicification to form anatase colloform bands, it does not move around on a large scale.

Opaline silicified wood embedded within a silcrete horizon at one locality contains high levels of Ni, Cr and V absent from quartz silcretes. This implies that the silicifying solution was enriched in these elements.

It is most likely that the silicification proceeded initially as precipitation of a gel, which transformed to amorphous opal-A and then microcrystalline chalcedony with time. The silica may have been adsorbed on clays, causing the clay lattice to rupture and releasing alumina. Co-precipitation with iron hydroxides may also have been important, as these will neutralise the negative surface charge on the silica groups in solution. Evaporation is likely to have played a minor role, if any, in silica precipitation within the central Victorian silcretes, in contrast to its importance in the formation of the pedogenic silcretes of inland Australia (see companion paper on oxygen isotopes).

Given the close spatial relationship between the silcretes and basalt in central Victoria, it is probable that the basalts are the source of the silica-rich solutions. Weathering of basalt releases abundant silica into groundwaters, as well as elements like Cr, V and Ni (all abundant in basalts). The latter elements would have been incorporated in the initial amorphous silica precipitate, and then excluded as the opal recrystallised to chalcedony.


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Radiometric Pattern Indicate Prior Alluvial Material•Julienne Kamprad

• Australian Geological Survey Organisation, GPO Box 378 Canberra ACT 2601

•As part of AGSOs regolith mapping for the Lachlan Fold Belt National Geoscience Mapping Accord Project, a dark

• blue-black feature on a radiometric image K U Th (RBG) in the vicinity of Killonbutta State Forest approximately 10kms west of Molong on the Bathurst 1:250 000 Sheet was investigated to see if the forest flora had affected the

• radiometric signal. Field examinations showed the dark feature or area of low radiometric signal was in fact a result ofalluvial, for the most part unconsolidated, material. The alluvium was deposited in a prior time, is possibly of Tertiary

• age and in the present time is subjected to an erosional regime.

Soil samples of the alluvium from the dark radiometric pattern and adjoining patterns of blue (also Tertiary• alluvium), red (Silurian sedimentary bedrock) and white(granite bedrock) allowed a preliminary comparison of the

•alluvial units.

Selected geochemistry data is tabulated below.

Targeted Radiometric Pattern Blue Blue BlackZone 1 regolith Orange brown sand with

subrounded and rounded pebblesOrange brown sand with subrounded

and rounded pebbles.K20 % 0.33 0.2U ppb 3.17 2.17Th ppb 15.6 15.3Al203 % 5.3 10.5Fe203 % 4.1 1.2As ppb 4.5 2.8Ce ppb 37.0 63.0Zr ppb 343.0 512.0

• These results indicate:

1. An apparently small elevation in U and K have been sufficient to produce the visually different radiometric• features;

• 2. The U does not appear to be associated with the clays or rare earth minerals but could be associated with the Fe 203 ;3. The results are sufficiently encouraging to warrant a more detailed investigation.

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The Karoonda Pedoderm: Plio-Pleistocene Lateritic Profile of the Western Murray Basin, SoutheasternAustralia.

Andrew KotsonisSchool of Earth Sciences, University of Melbourne

Landforms and soils preserved within the western Murray Basin record the history of Late Cainozoic climate changefrom wetter to more arid conditions. Underlying the Quaternary dune fields of the Mallee country, subdued NNWtrending curvilinear ridges of the Loxton-Parilla Sands (Ludbrook, 1957; Finnan, 1965, 1966) represent shorelinepositions of a regressive Pliocene sea. Following the retreat of the sea, the Loxton-Parilla Sands were subject to deepweathering, with the resultant lateritic profile termed the ICaroonda Surface or Pedoderm (Firman, 1973).Development of the ICaroonda Pedoderm is synchronous with the exposure and cessation of coastal deposition, and alateral succession of increasing pedogenesis from west to east can be recognised. In the southwestern sector of thebasin, the Karoonda Surface was largely flooded by Lake Bungunnia, with consequential burial of the pedodermbeneath lacustrine Blanchetown Clay. The Karoonda Surface has been affected by extensive Quaternary modificationincluding erosional and post-depositional processes.The lateritic profile is characterised by pisolitic ferricrete, which typically shows multiple rinds and cements thatindicate a complex history of development and modification. The cores of the pisolites preserve features inherited fromtheir source (ie. the underlying Loxton-Parilla Sands), but the rinds coating the pisolites are distinctly finer grained.Silica mobilisation and dissolution are characteristic. In the older easternmost profiles, larstic' pipes are infilled withpisolites set in finer quartz sands. These older profiles typically show multiple development of pisolites with successiverind accretion. These rest on Loxton-Parilla sandstone, and indicate down wasting of the weathered profile.

The younger profiles are better preserved, and comprise of the pisolitic ferricrete layer overlying clayey mottled zone,and friable, jointed Loxton-Parilla Sandstone at base. Within the upper surface, pisolites are typically encased withinfiner grained sands and silts that form the present surface, from which limonite-cemented rinds have developed. Thedepth of this zone is less than 3-4 m. The total depth to weathering extends beyond the underlying sandstone to over10 m. The sands are cemented by kaolinitic clays and secondary iron cements.The development of the ICaroonda Surface is consistent with evidence of wetter climates during the Pliocene insoutheastern Australia, and contrasts sharply with the Late Quaternary soils which are typically calcareous, andassociated with aeolian landforms across the basin. The change from iron and sesquicaide mobilisation to carbonatemobilisation marks the onset of arid climates in Australia.ReferencesFirman, J. B. (1965). Q Geol Notes Geol. Surv. S. Aust., 16:1-2.Firman, J. B. (1966). Q Geol Notes Geol. Surv. S. Aust, 20: 33-7.Firman, J. B. (1973). Geol. Surv. S. Aust, Rep. of Invest., 39.

Ludbrook, N.H. (1957). J. & Proc. Roy. Soc. NSW, 90:174-180.


Surficial Geochemical Expression of Mineralised Systems in the Olary Block, South Australia•

David C. Lawie and Paul M. Ashley• Department of Geology and Geophysics, University of New England, Armidale NSW 2351

Introduction•

Geochemical data have been collected from iron-rich regolith, ranging from gossanous ironstone outcrop to pisolitic1111^lag, developed throughout the Proterozoic Olary Block. Prominent among ironstone forming rocks in the area is the

Bimba Formation, a thin (<50 m) relatively continuous unit which may be up to 250km in strike length and which• has the potential to host stratiform base-metal mineralisation. The majority of the outcropping Bimba occurs in the

central and northern parts of the area, with considerable strike length inferred by drilling and magnetics to occur to• the north along the Benagerie Ridge, where the Olary Block extends under Cainozoic and older rocks. The aims of

this study are to characterise element associations in iron rich material of different genetic type and geomorphic• maturity and determine the parentage of samples of unknown origin.

Sampling Techniques•The sampling program involves two types of iron-rich media, ironstones and lags. ironstones range from gossanous

• material which crops out in geomorphically immature areas above iron and base metal beating, sulphide-rich BimbaFormation, to their more degraded equivalents, where outcrop occurs as float and mbble. Lags were sampled initially

• where their genetic type was clearly evident, for example from down-slope of prominently outcropping ironstone.Legs of this type are sourced from Bimba Formation derived gossanous ironstone and quarts-magnetite iron

• formations. Leg sampling was then extended to areas of little or no outcrop in the north of the field area whereparentage is unknown. Possible sources of these lags are degraded Bimba Formation, iron formation, pisolitic laterite

• or Adelaidean and Cainozoic sediments. Leg deposits range in extent from linear features of a few meters in width tohundreds of meters in length, to blanket deposits of many hundreds of hectares in extent.

• Analysis Techniques

• Samples were analysed for 20 elements mainly using ICP-MS for improved detection limits. Data were thentransformed using both log 10 and Box-Cox optimised power transformations. To extract trends and associations in the

• structure of the data matrix, multivariate methods were used including RQ-principal components analysis (RQ-PCA)and dynamic cluster analysis. RQ-PCA proves very useful as both samples and variables (elements) may be shown on

• the same graph, and related groups of samples together with their associated element behaviour determined.Results

•Sixteen separate Bimba occurrences of varying maturity were sampled. The Bimba consistently contains anomalous

• concentrations of Cu, Zn, As and in places Pb, Se, Sb, U, Mo, Co, Bi, Au and Ba. The Bimba occurrences may becharacterised by varying associations of these elements. Examples include Cu-Co, Cu-As-Mo-Ag, Zn-Pb-Mn-Ba-As

• and Mn-Cu-As-Zn. Analysis indicates the most important factor influencing element concentrations and associationsin the surface material is the primary composition of the parent rocks. No obvious correlation exists between element

• concentration and level of geomorphic maturity. Samples taken from extremely degraded outcrop, or lag, may containmulti-element base-metal concentrations in the thousands of ppm, with little or no differential element leaching

• evident. Leg samples generally reflect the chemistry of their parent rocks, albeit with a somewhat subdued response.A notable feature of the entire dataset is the generally high levels of As throughout the area. For example, apart from

• the Bimba derived iron-rich material, lags commonly contain 100-200 ppm As. Other problematic ironstoneoccurrences include pisolitic laterite with anomalous Cu-Zn-(As) and a Bimba occurrence with anomalously low As-

• Cu-Pb-Zn. This occurrence may result from the weathering of Fe-rich calcsilicate and carbonate rocks but with little orno sulphides. The least anomalous samples from the area are those derived from quarts-magnetite iron formations.

• Acknowledgments: Financial support from the ARC and North Exploration is gratefully acknowledged.

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Signposts Old and New: Prediction of Present and Palaeo-Supergene Geochemical Environments using IronRedox Chemistry

L.M. LawranceKey Centre for Teaching and Research in Strategic Mineral Deposits, Geology Department, The University of Western

Australia Perth, Western Australia

Iron redox products are important indicators of pest and present geochemical environments in the regolith. Iron is inmost rocks and, on weathering of primary iron-bearing minerals under reducing conditions, iron is mobile as anunassociated ferrous ion (Fe2+). These reduced matrices are generally pale grey-green to dark grey. With access ofsufficient oxygen, Fe2+ is unstable and is oxidised to ferric ion (Fe 3+). The Fe3+ is insoluble, except at very low pH, andis hydrolysed and precipitated as visually distinct yellow-brown-red iron oxyhydroxides. Typically, Eh and pH increasetowards the surface and the oxyhydroxides are precipitated at a sub-horizontal front in the regolith referred to as theredox front Only a small percentage (1-2%) of iron is sufficient to cause strong colouring of the profile. The iron oxideprecipitated depends on the solution chemistry. Commonly, a mixture of haematite, which imparts red-brown colours,and goethite, which imparts yellow-brown colours, is produced, in which the ratio Hm/(Hm + Gt) depends largely onpH. The formation of haematite from ferrihydrite is promoted by alkaline conditions whereas under more acidconditions goethite predominates. This precipitation of iron oxyhydroxides releases hydrogen ions, which result inacid conditions.

2Fe2+ + 1/202 + 3 H20 => 2Fe0OH + 411 +

mobile ferrous ion ==> goethiteThis reaction is rapid and of limited reversibility, except by specific organic processes. Therefore, iron oxides arelargely stable to further weathering and are immobile unless transported as a colloidal sot. As such, they are anindication of past weathering intensities and of Eh-pH conditions within the profile. Migration of Fe2+ to the redoxfront and the transport of colloidal iron oxyhydroxides at the watertable result in the enrichment of iron just above theredox front over time to form a redox zone. Redox zones are best developed over ferruginous rocks and in topographiclows. Active redox zones are characterised by yellow-brown goethite.Aquic conditions of the regolith, and therefore the depth of the redox front, are constantly changing with changes inclimate and topography. The redox front usually occurs at the watertable. The laterite formed during lateritisation canbe viewed as the first formed redox zone. The associated underlying mottled zone is a redox transition zone resultingfrom variable access of water and oxygen with seasonal changes in the watertable. Weathering under drier climates isless intense as the volume and activity of water is reduced. Subsequent weathering processes generally overprint theoriginal lateritic profile. Predominantly goethite is precipitated within voids in the saprolite with the descent of theredox front and redox zones are formed with standstills in the watertable. As the upper profile dehydrates and the pHincreases towards the surface under dry conditions, haematite predominates and generally characterises palace-redoxzones. With a reversal to wetter conditions, goethite, precipitated at an active redox front, may overprint haematiticmaterials, with iron depletion evident as paler colouring.Truncation of profiles by erosion may expose weathered zones which become overprinted by mottling caused byvariable access of water and oxygen. This feature may also develop in transported coven Palaeo-redox products in theform of haematitic saprolite, lateritic nodules and pisoliths remain. The course components commonly have goethiticoxidation selvages. The redoximorphic features produced are distinctive from mottling formed during lateritisation.

The local production of acid conditions above a descending redox front not only promote weathering but enhanceelement dispersion in the regolith. This process indirectly results in the supergene enrichment of metals, such as goldand platinum. In addition, the structure of goethite is ideal for element adsorption and, away from the active redoxfront, trace element anomalies develop. The subsequent conversion of goethite in the drier upper parts of the profilecause element expulsion as the haematite structure is less capable of retaining trace elements.This paper will discuss the identification of present (active) and palace-geochemical environments under variousweathering regimes as indicated by iron redoximorphic features.


•Evidence for a Buried Paleosol in the Weipa Region,North Queensland

Mate Le Gleuher l , lain D. Campbe112*, Tony Eggleton l and Graham Tayloi-2

• 1 Centre for Australian Regolith Studies, Australian National University, Canberra ACT 02002 Centre for Australian Regolith Studies, University of Canberra, P.O. Box 1, Belconnen, ACT 2616.

• *Present address: CSlRO Division of Exploration Geoscience, Private Bag, PO Wembley, WA 6014.•Laterites and underlying sediments acquired by drilling in the Andoom and Weipa areas, Cape York Peninsula, North

• Queensland have been studied. In the Andoom area, the bauxite overlays the marine Cretaceous sediments of theRolling Downs Group which consist of claystones and sandy clays with various proportions of smectite, kaolinite,mica, feldspars and quartz. In the Weipa area, these marine sediments are found at a depth of about 18 metres, and arecovered by fluvial/deltaic sediments which are considered either as the upper part of the Rolling Downs Group or asTertiary sediments of the Bulimba Formation. The formation consists of 6 to 10 metres of coarse sands with about 10% of kaolinite which are covered by 8 to 12 metres of laterites (pallid and mottled zones, iron stones, pisolitic bauxite

• and soil).

• A detailed micromorphological, mineralogical and chemical study of the material of the drill holes has shown theexistence of a truncated paleosol 0.70 meter thick developed at the top of the marine sediments, in the Weipa area.

• The material is composed of a white sandy clay (kaolinite, quartz, with minor mica and smectite) with goethiticpinkish mottling and nodules. Cutans are often observed at the surface of the quartz grains. These features obscure the

• sedimentary fabric of the sediments.

The chemical trends shown by the major and trace elements contents determined by X-ray fluorescence are typical of aweathered material:

• increase of the Fe203, M203, TiO2 contents;

slight decrease of the Si02 content;- increase of the V, As, Cr, Zr, Ce, Ba, Pb contents.

• Goethite and heavy minerals (zircon) concentration is mainly responsible for the observed trends.

The presence of a paleosol indicates that the sediments of the Rolling Downs Group have been exposed long enough toenable a significant weathering. The paleosol has been subsequently buried under fluvial/deltaic sediments. The coarsesands beds which directly cover the peleosol suggest a high energy environment which probably led to the erosion ofthe upper part of the weathering profile. Finally, the fluvial/deltaic sediments do not represent the upper part of theRolling Downs Group but are likely to be part of Tertiary Bulimba Formation.


Structural Characteristics of Kaolin Minerals from Eastern Australian Regolith

Chi Ma and Tony EggletonDepartment of Geology, The Australian National University, Canberra, ACT 0200

Kaolin samples recovered from a wide range of regolith along eastern Australia (i.e., Pittong, Lal La!, Meredith, Vic.;Woodside, SA; Swan Bay, Bexhill, NSW; Weipa, Tarong, Cooyar, Nanango, Pierce's Creek, Mount Morgan, Qld.)have been studied mainly using XRD, scanning electron microscopic (SEM) and transmission electron microscopic(I EM) techniques. Kaolinite occurs in most samples from weathering, hydrothermal and secondary transportedorigins, whereas halloysite appears mainly in the samples from weathered basalts.Kaolinites exhibit structural difference that range from perfectly in-periodic minerals in most samples fromweathering and hydrothermal profiles of granite (e.g., Pittong, La! Lal) to highly disordered materials in sedimentarykaolin deposits (e.g., Swan Bay).Electron microscopic studies indicate some close relationships between texture, morphology and genesis of thekaolins. In primary kaolins the kaolinites show no orientation and no particle size fractionation. Book-shaped andvermiform kaollnites are found in nearly all primary kaolins but are uncommon in transported kaolins. In sedimentarykaolins the texture seems to be tight, which might be controlled by the effects of particle size fractionation andsedimentation. Halloysite tubes with an internal tunnel show great morphological variations in the weathered basalticsamples. Parallel halloysite tubes occurring in samples from Tarong, together with kaolinite and smectite, are believedto form from broken kaolinite plates along b-axis.High-resolution TEM (HRTEM)) study reveals the structural details of kaolin. Although HRTEM imaging of kaolinare very difficult due to rapid electron beam damage, few lattice fringes showing 7.1- A periodicity and even 3.6- Aand 3.5- A sub-periodicities have been obtained. The 7- A lattice fringes of kaolinite crystals always show mottledcontrast. Possible one or two smectite layers were found as a cover layer on some kaolinite particles in the samplesfrom Weipa kaolinite deposit. Such kind of disordered mixed-layer kaolinite/smectite clays (only few smectite layers)are not detectable by xRD. Some properties of industrial grade 'pure' kaolinite may arise because of single non-kaolinite layers at the surface. Transformation from mica to kaolinite by topotactic alteration was demonstrated insamples from the Weipa pallid zone. In addition, the selected area electron diffraction (SAED) pattern of the [001]orientation indicates the structure of kaolinite from all sites is c-centered.

All halloysite HRTEM images show a 7- A fringe spacing, which may suggest that halloysite has lost its all interlayerwater and has collapsed in the high-vacuum condition of TEM. Distinction between kaolinite and halloysite (7- A or10- A) by the different spacing of the lattice fringes is impossible under the TEM. Based on SAED analysis, all tubularhalloysite crystals are elongated along the b-axis and show a two-layer periodicity. The distinctive two-layer structuremay be used as a key characteristic for the identification of tubular kaolin particles, particularly, when halloysitemakes up a very low amount in a mixture with kaolinite.

Quantitative EDAX examinations (AEM) indicate that composition varies among individual kaolin particles withdifferent and even same morphology. Kaolinite from weathering granites has lowest structural Fe content (Fe20 3 <0.3wt%). Tubular halloysite in the in-situ weathered basalt samples from Bexhill and Tarong seems to have a higher Fecontent (F'e203, 3 to 6 wt%), whereas halloysite tubes at Woodside and Mount Morgan contain only 0.6 wt% Fe20 3 .


•

•

•Chronicling the Australian Duricrusts: Relevance of the Marine Succession

Brian McGowran• Department of Geology & Geophysics, The University of Adelaide, Adelaide SA 5005

110The classical stratigraphic methods of superposition and correlation come to duricrust history with three burdens. Thefirst is the doubt that always accompanies one's attempts to sort out the succession of ferricretes, silcretes, andbleached profiles--is there really a recoverable succession, or instead a repeated reworking of earlier profiles? The

• second burden is related: all too infrequently can we extrapolate from well-dated unconformities in a succession ofstrata to land surfaces with compounded chemical carapace s. And even where we think we can, the significance of a

• fossil or a numerical observation will have a less simple and less clear meaning. The third burden is a cultural one.Geology is a synergy of two traditions—the tradition of natural history which deals in patterns in rocks, fossils, and

• events of diverse kinds, and the tradition of natural philosophy which deals in mostly physico-chemical processes. Ingeological age determination and correlation, for example, we have the modem Cainozoic-scale geochronologies

• which are an amalgam of biochronology, radiochronology, and geomagnetochronology, plus various more recentchemochronologies based on isotopes, particularly strontium, oxygen and carbon.

• This interaction between the two great traditions has not been so clear in the disciplines enquiring into the regolith.The geologically-oriented observer will be conscious of major reorganisations in the global environment, and he/she41^tends to be impressed by pronounced changes in rates leaving a chronologically discontinuous record. The chemically-oriented observer will be more aware of the processes and more inclined to down play rate changes and

•

• four chronological packets of strata:(iv) Latest Miocene to Quaternary

• Late Oligocene to middle Miocene.• (ii) Late-middle Eocene to early Oligocene.

• (i) Late Paleocene to early Eocene.

The global cooling and global fall in sea level from coeval highs in the early Eocene has occurred largely in four major• steps: in the early middle Eocene, earliest Oligocene, middle Miocene and late Pliocene. Those steps interpolate

between the four packets of strata which are times of climatic and sea level reversal: in the later Eocene, early• Miocene, and early Pliocene. Meanwhile, Australia has been drifting into lower latitudes, encouraging a view of

Australian Cainozoic biogeohistory now entrenched in sedimentology, palaeobotany, geomorphology, neritic• palaeontology—that this drift northwards somehow has generated a climatic trajectory special to this continent.

However, the Australian scenario fits the global pattern of change in climate and sea level too well to need ad hoc• buttressing by auxiliary hypotheses. Likewise, the deep weathering at high palaeolatitudes in the Eocene is bipolar and

due to high rainfall--a topic central to nonuniformitarian explanations of poleward heat transfer (but not necessarily• warm and certainly not tropical climates).

• Times of enhanced stratal accumulation are times of higher sea level. They are generally warmer and wetter, shiftingthe terrestrial vegetation spectrum away from the desert end and toward the rainforest end. Deep chemical weathering

• is stimulated and the released silica reaches the ocean, relieving one of the major factors limiting oceanic productivity.The reverse holds for these generalisations for the intervening slices of Cainozoic time. They are well supported as

IIII global generalisations, and such chronological relationships as we have for deep weathering are at least consistent.Earlier conclusions that deep weathering centred on the Eocene and Miocene have been supported by recent methods

• of dating.

The neritic strati graphic succession on the Australian margins is a link between the oceanic and the global, on the• one hand, and the continental and the terrestrial, on the other. It supplies a framework of chronology, a local

manifestation of the putative global sea level at the second and third order scales, and a local scenario of global• palaeoceanographic and environmental change. Any geohistorical appreciation of the development of terrestrial biotas,

landforms, and ferricretes and sikretes surely must be in that context. The marginal stratigraphic record comprises

411^ABSTRACTS: Australian Regolith Conference '94^ 41© Australian Geological Survey Organisation

Some Observations on the Mirackina Paleochannel

G.M. McNally 1 and LR. WiIson2

1 Department of Applied Geology, UNSW 2 Roads and Traffic Authority, Parkes NSW

The Mirackina Paleochannel (MPC) is a chain of silcrete-capped mesas extending for about 200 km through the far north of South Australia, north of Coober Pedy. it was first recognised as an exhumed Late Tertiary drainage feature by Barnes and Pitt (1976), who also defined the Mirackina Conglomerate to include the channel fill sandstones and their ferruginous/siliceous caprocks. This formation overlies, and is partly enclosed by, the Bulldog Shale of Lower Cretaceous age. The latter has two very distinct forms: an upper, intensely kaolinised and weakly silicified zone 10-30m thick, and an underlying expansive clay-shale.

This paper presents geological observations and preliminary findings from an ARC-funded research project on the location and evaluation of duricrusts for road making materials; however the geotechnical results of this work will be reported elsewhere. The study concentrated on an area west and northwest of Arckaringa homestead, about 150 km north of Coober Pedy. The aspects to be discussed here include:

• The geomorphic expression of the MPC as a sinuous parallel chain of mesas dipping inwards towards a nonindurated channel fIll, which has been largely removed by subsequent erosion;

• The stratigraphic and topographic relations between the adjacent, but more extensive, Stuart Range plateau and Arckaringa Hills land surfaces;

• The influence of groundwater discharge on the form and degree of cementation of the MPC sediments; and on the ferruginisation which makes the MPC so distinctive on satellite imagery; and

• Mass movements caused by expansive Bulldog Shale soils on the lower slopes of the mesas and its displacing effects on the duricrust cappings.

The main conclusion from the work to date is that though the MPC presents a striking example of reversed topography and exhumed paleodrainage on airphotos and LANDSAT imagery, its Internal structure Is much less conspicuous at ground level. its sedimentary fill is thin, usually only a few metres thick, and mostly only preserved along the channel margins. Beds have been extensively tilted and displaced by landslides or obscured by talus. Much of the non-indurated core of the paleochannel has been scoured out by erosion, leaving a glacier-like trough.

The ferruginous silcrete capping of the MPC is typically about 30m lower than the adjacent Stuart Range and Arckaringa land surfaces, thought to be equivalent to the Cordillo Surface of Early Tertiary age, and has a gradient of about 1:2800 towards the south and southwest. The Mirackina Conglomerate is therefore younger and is presumed to be an upstream correlative of the lacustrine Etadunna Formation, and hence of Miocene age The silicification occurred later; possibly during the Pliocene.

Barnes, L C and Pitt, G M (1976) The Mirackina Conglomerate. Quarterly Notes, Geological Survey of South Australia 59, July 1976, p2-6.


• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

S•O

Geochemical Exploration of Retrograde Schist Zones:Could this Approach Detect Blind Broken Hill Type Orebodies?

• K.G. McQueenCentre for Australian Regolith Studies and Faculty of Applied Science, University of Canberra

0^Retrograde schist zones are a feature of the Broken Hill Block. These are planar or curviplanar zones with a well

•developed schistosity defined by retrograde and hydrous minerals, including micas and chlorite. The zones typicallydisplace units which they intersect by a combination of folding, attenuation and transposition. They appear to have

411/^fulfilled the function of faults under conditions of ductile deformation and some also contain major faults whichappear to post-date the initial formation of the schist zones (Vernon and Ransom, 1971; Stevens, 1986). The zones are

Sup to 2 km wide and up to 60 km long and several major zones have intersected Broken Hill type mineralization, forexample the British, De Bavay, Globe-Vauxhall and Thackaringa-Pinnacles shear zones. Field relationships,

0 structural studies and K-Ar and Rb-Sr dating indicate that at least some retrograde schist zones existed at about 1600-1570 Mn, soon after prograde metamorphism and possibly even following the regional 1)2 deformation, and thatfurther activity extended up to 520 Ma (Stevens, 1986; Barnes, 1987).

•Extensive surface exploration of the Broken Hill Block over a period of more than 100 years has discovered numerous

41111^small occurrences of Broken Hill type mineralization, but has failed to detect any large or medium sized orebodies,other than the Broken Hill deposit itself. This suggests that any large deposits that might exist are deeply buried or

40^blind and are thus unlikely to show detectable geophysical or secondary geochemical signatures at the surface. Oneapproach to detect such blind deposits would be to look for leakage geochemical anomalies along suitable channel

5^ways and then follow these back to the source (e.g. Koksoy, 1978). The retrograde schist zones and associated faultsmay be useful for this purpose.

411^It is clear that there has been a number of stages of retrogression in the Broken Hill area and also in the retrogradeschist zones. These zones generally contain a lower amphibolite facies mineral assemblage, dominated by quartz-

• biotite-muscovite, and some show faulting and brecciation with low greenschist grade rocks containing chlorite andclay minerals (Vernon and Ransom, 1971). In metamorphic environments fluid flow is commonly guided by active1110^deformation zones and it is therefore likely that the retrograde schist zones were important in focussing fluids. Underthe higher grade conditions fluid movement accompanying ductile deformation was most likely via grain boundaries

• and microfractures (cf Bell and Cuff, 1989). At lower grade conditions brittle deformation would have allowed fluidflow via through-going, interconnected fractures. The nature of the permeability in these zones would thus have varied

• throughout their history.

O The activity of fluids and concentration of U and B in hydrous alteration phases during early retrogrademetamorphism at Broken Hill has been demonstrated by Ahmad and Wilson (1981). They suggested that fluids may

O have originated from dehydration reactions accompanying granulite facies metamorphism or from granites andpegmatites which post-date the high grade metamorphism. There is also evidence to confirm element and metal

• mobility along the retrograde schist zones themselves. Lawrence (1968) described vein type galena and sideritemineralization in the British retrograde schist zone where it cuts the main Broken Hill lode. The lead in this vein has

• isotopic characteristics similar to those in Thackaringa-type mineralization (Cooper, 1970). Both (1978), on the basisof Pb and S isotopic data and the close spatial relationship between Thackaringa type galena-siderite rich veins and

• retrograde schist zones, suggested that this type of mineralization formed by partial remobilization from earlierBroken Hill type mineralization during retrograde metamorphism. Many of the Thackaringa type veins are developed

O along faults which cut the schistosity of the retrograde schist zones and some veins cut later granitoids, pegmatites anddolerite dykes, indicating that any remobilization probably occurred late (between 520 and 280 Ma; Stevens, 1986).

• Barnes (1987) has also suggested that hydrothermal activity associated with retrogression and confined to theretrograde schist zones, at least in the later stages, was responsible for a range of vein type mineralization including

411^the Thacicaringa-type deposits.

Retrograde schist zones and associated faults thus represent potential leakage channels for elements remobilized from• Broken Hill type orebodies that they may have intersected or interacted with. The concept for using these zones in

exploration would involve surface sampling of regolith and rock materials within retrograde schist zones which are9

^

^known to have intersected lode horizon rocks. From this it may be possible to detect secondary dispersion fromprimary leakage anomalies derived from major mineralization in these lode horizon rocks. This could be followed up

• with percussion chip sampling of fresh rock from these anomalous areas to establish the primary dispersion pattern

411^related to leakage.The concept could be tested by:

O 1. Underground sampling of retrograde schist zones around the Broken Hill orebody to determine the nature anddegree of ore element remobilization along the zones in the immediate vicinity of the deposit.

•


411

2. Further examination of element remobilization utilizing samples from drill core through retrograde schist zones atgreater distance from the Broken Hill orebody.

3. Surface sampling of major retrograde schist zones which have intersected the Broken Hill orebody to determine ifany evidence of remobilization is preserved in weathered material.

Likely problems and complications would include:

1. Difficulties in interpretation due to the initial, probably low level, dispersion patterns being overprinted bychemical weathering effects in the regolith. This could be partly overcome by RAB sampling of bedrock or byutilizing elements or element and isotope ratios that have not been significantly affected by secondary processes(e.g. Pb isotope ratios).

2. Complications related to the development of Pb-Zn veins of Thackaringa type within some retrograde schist zones.This may be overcome by examining trace element associations (e.g. Bi, Cu and Zn and mobile elements such asAs, F and Sb, associated with or more abundant in the Broken Hill type sulphides) and possibly using Pb isotopediscrimination techniques. Existing data indicate that the different styles of mineralization in the Broken HillBlock have significant differences in their lead isotope characteristics (Russell et al., 1961; Russell and Farquhar,1970; Cooper, 1970; Reynolds 1971; Gulson et al., 1985). Lead remobilized from Broken Hill type mineralizationwould have an additional radiogenic component. In fact, Both (1978) has suggested that addition of radiogeniclead to lead remobilized from Broken Hill type lead might explain the isotopic ratios of the Thackaringa deposit.These ratios could also be explained by mixing of disseminated stratiform lead and radiogenic lead.

3. Complexities due to the likely prolonged history of fluid movement through retrograde schist zones. It would benecessary to establish a clearer understanding of the structural sequence and history of fluid activity in the variousretrograde schist zones. This would be needed to constrain isotope modelling.

4. The difficulty of determining fluid and element flow directions within the retrograde schist zones, necessary totrace the source of any leakage anomalies.

Ahmad, R. and Wilson, C.J.L., 1981. Uranium and boron distribution related to metamorphic microstructure -evidence for metamorphic fluid activity. Contributions to Mineralogy and Petrology, 76,24-32.

Barnes, R.G., 1987. Multi-stage mobilization and remobilization in the Broken Hill Block, Australia. Ore GeologyReviews, 2, 247-267.

Bell, T.H. and Cuff C., 1989. Dissolution, solution transfer, diffusion versus fluid flow and volume loss duringdeformation metamorphism. Journal of Metamorphic Geology, 7,425-448.

Both, R., 1978. Remobilization of mineralization during retrograde metamorphism, Broken Hill, New South Wales,Australia. In Verwoerd, W.J. (ed) Mineralization in Metamorphic Terranes. Geological Society of South Africa,Spec. Pub. 4,481-489.

Cooper, J.A., 1970. Lead isotope classification of the A.B.H. Consols and Brown's Shaft veins at Broken Hill, N.S.W..Proceedings. Australasian institute of Mining and Metallurgy, 234, 67-69.

Gulson, B.L., Porritt, P.M, Mizon, K.J. and Barnes, R.G., 1985. Lead isotope signature of stratiform and strataboundmineralization in the Broken Hill Block, New South Wales, Australia. Economic Geology, 80.488-496.

Koksoy, M, 1978. Relationship between geological, geophysical and geochemical data obtained from blind lead-zincmineralization at Keban, Turkey. Journal of Geochemical Exploration, 9, 39-52.

Lawrence, L.J., 1968. The mineralogy and genetic significance of a Consols-type vein in the main lode horizon,Broken Hill, N.S.W.. Proceedings Australasian Institute of Mining and Metallurgy, 226,47-57.

Reynolds. P.H., 1971. A U-Th-Pb isotope study of rocks and ores from Broken Hill, Australia. Earth and PlanetaryScience Letters, 12, 215-223.

Russell, R.D. and Farquhar, R.M., 1970. Lead isotopes in geology. Interscience Publications, New York, 243 pp.

Russell, R.D., Ulrych, T.J. and Kollar, F., 1961. Anomalous leads from Broken Hill, Australia. Journal of GeophysicalResearch, 66, 1495-1498.

Stevens, B. P. J., 1986. Post-depositional history of the Willyama Supergroup in the Broken Hill Block, N.S.W..Australian Journal of Earth Sciences, 33,73-98.

Vernon, R.H. and Ransom, D.M., 1971. Retrograde schists of amphibolite facies at Broken Hill, New South Wales.Journal of the Geological Society of Australia, 18, 267-277.


••• Genesis of Laterite Deposits in Western India

Surendra Mora and Aro Arakel• AWT EnSight

51 Hermitage Road, West Ryde, NSW 2114

• Laterites and major bauxite deposits of Western India occur on different types of parent rock, including basalt,sandstone, shale, granite, granophyre and charnockite. In the present work, twelve lateritic profiles have been studied

• in detail to assess the geological setting, mineralogy and geochemistry of major and trace elements. After construingthe data the genesis of these deposits have been attempted with reference to their palaeo-environmental setting.

• The study indicates that the Western India laterites and bauxites are derived via the following chemical weatheringpathways: parent rocks: parent reck H altered rock H saprolite H laterite H bauxite H ferricrust. The presence orabsence of saprolite horizon may be attributed to the palaeo-drainage conditions. However in Kutch Region of GujaratState, tectonic activity resulted in formation of small, isolated land locked basins (Biswas 1971) in which limestones,

• siderites, lignites, and minor quantities of pyrite and some bentonite were associated with laterites and bauxites,illustrating the influence of local pH and Eh conditions. The mineralogy based on XRD and lR analysis reveals the

• following minerals; gibbsite and goethite as the dominant minerals (where as hematite is predominant in laterites),and boehemite, diaspore and anatase as minor minerals. In saprolite Imofinite is the dominant mineral and

• montmorillonite, gypsum, illite, halite, anatase, hematite and allophane occur as minor/trace mineral constituents.

• Twenty six elements viz. Si, Al, Ti, Fe, Mn, Mg, Ca, K, Na, Sr, Rb, V, Co, Zr, Ni, Cu, Zn, Nb, La, Ce, Y, P, S, C. Pb,U, were also determined for selected samples. The statistical analysis of geochemical data reveals a distinction of

• bauxite formed in continental and in terrestrial marine environments. The processes like sorption, adsorption andcation exchange are evident from the presence of Mn and Ca; further, the positive trends of Zr, V, Ga and Cr and

• negative trends of Na, Ca, Rb, La, Zn and Ba suggests geochemical control of migration of elements duringlateritisation. In marine environment bauxites show elevated concentration of- Ca, Mg and Fe as major elements. The

• influx of sea and contemporaneously high ground water levels are shown by elevated concentration of rare earthelements in parent rock which have been subsequently depleted during formation of laterites and bauxites. A

• geochemical model depicting the stages in genesis of laterite and bauxite deposits of Western India is presented anddiscussed.


Getting the most out of Geophysical Data Sets for Regolith Mapping Purposes - the Application of ForwardModelling and Residual Analysis

Tim Munday, Stewart Rodrigues & Andy Gabe11CSIRO Exploration and Mining, Private Bag, PO Wembley, WA 6014

Recently, increased emphasis has been placed on the role of airborne geophysical, particularly gamma-ray, data asbasis for improved regolith characterisation. However, the value of these data, notably for regolith mapping, has beenlargely determined by the skill and experience of the regolith geologist and the products generated for analysis andinterpretation. Im provements in regolith mapping resulting from the use of airborne geophysical data sets, in additionto air photos and/or remote sensing data, can be evaluated by using an existing map (i.e. the geological model for agiven area) to predict the response in the geophysical data and comparing that prediction with the measured survey.The difference between the two (i.e. the residual), will show where the modelling is inadequate or where the mappedgeology does not fit the data. If the model adequately predicts the response for each mapped unit, then the residualsshould be explained by the noise alone. This forward modelling approach is used routinely in the analyses of magneticand gravity geophysical data sets, but has seen limited application with others, particularly those commonly used forregolith mapping purposes.Forward modelling and residual analysis provides a means for identifying subtle variations contained in geosciencedata sets - variations that may be masked by major differences in those data due to changes in bedrock geology and/orregolith materials. These subtleties could indicate small but significant changes in the composition of regolithmaterials which may warrant investigation in the field and incorporation into a map.

The application of a forward modelling approach to improved regolith mapping for an area in the Yilgarn Craton isdescribed. The procedure is demonstrated through the analyses of an aerial gamma-ray survey against a regolith-lanclform map derived from the interpretation of air-photos and Landsat TM data. A prototype system (MODRA) forthe interactive modelling and analysis of multiple (2D) data sets was used.The airborne radiometric data were modelled using a simple equation to take account of the relative exposure ofbedrock geology (an important factor in determining the radiometric response) along with the nature of the materialpresent at the surface. Results of this modelling are displayed for the three radioelement combined and presented as atenary image (K = red, Th = green and U = Blue). The predicted response for each mapped unit is displayed as aparticular colour or hue (the 'mean' colour indicative of a mean radiometric response), while variations in colourintensity argueably representing variations in 'exposure'. The exposure term might also be taken to indicate variationsresulting from the degree of weathering and/or relative position in the weathered profile. The residuals were thengenerated by subtracting the modelled response from the actual, or measured values. Residuals that are higher or lowerthan the predi cted response are of prime importance. The results indicate that changes in the mapped regolith may bewarranted. Iterating the model, having made such modifications, until only the noise remains encourages full use to bemade of the data for mapping purposes.Further research is required to develop better models for use in a forward modelling process. To some extent these willbe data dependent. When using airborne gamma-ray data, a critical component to the successful application of thisprocedure is a better understanding how the various radioelements are distributed in deeply weathered terrain. Otherissues that require attention include the problem of pixels that overlap onto more than one regolith unit.


• The Antiquity of Landscapes on the North Australian Craton and the Implications for Theories of Long-TermLandscape Evolution

• Jonathan NottDepartment of Geography, Australian National University, Canberra, Australia, 0200

•

• The relative age of land surfaces across the north Australian craton has traditionally been determined by their relativeheight in the landscape and the morphological characteristics of the deeply weathered regolith on each surface. These

IIII^land surfaces have, for over half a century, been argued to he largely the product of cycles of erosion throughout theCainozoic. Detailed inspection over a 300,000 km2 area, however, revealed that much of this region is composed of

• ranges, valleys and plains which pre-date the Cretaceous; valleys incised into upland plateaux, and graded to lowlandplains, are in numerous places filled with Cretaceous sediments. Many of the lowland plains are exhumed Proterozoic

• and pre-Late Jurassic features. Clear facies changes within the Cretaceous strata suggest that the lowland plains maywell be older than neighbouring areas of upland plateaux which were likely graded to the Cretaceous sea. The

• antiquity of these landscapes and the disparity in age between higher and lower elevation planation surfaces highlightsthe inadequacy of global applications of existing models of long-term landscape evolution and theories concerned with

• the preservation of paleoforms in cratonic settings.

•••••••••••••••••••••• ABSTRACTS: Australian Regolith Conference '94^ 47

@ Australian Geological Survey Organisation•

••

Feedback Effects of Regolith Development in Shaping the Dissection of the Great Divide in Eastern Victoria^•Meredith Orr

School of Earth Sciences, University of Melbourne, Parkville. Vic 3052^ •

•The high plains region of eastern Victoria, which includes the Bogong, Dargo and Cobungra High Plains, consists ofplateau remnants covered in places by Oligocene basaltic lava flows. Incision prior to the flows had been minimal in^•comparison to post-basaltic dissection of the region. This dissection is the continuation of base level adjustmentswhich had already been taking place at the southern margin of the highlands before extrusion of Oligocene basalts^•(Bishop & Britt, in prep.).

As rivers incise into the highlands the controls on their directions include superimposition from the earlier landscape,lithology, structure and the direction to base level after tectonic movements. However the history of these river

•systems is not as simple as these controls would predict. Incision itself changes the parameters by which subaerialprocesses operate. It creates a greater surface area of the highlands exposed to subaerial processes, it changes the

•distribution and rate of flow of groundwater, and it changes the nature of how regolith is produced and redistributedwithin the highlands. The backfilling of basins and valleys alter the base level conditions to which rivers in the

•highlands are adjusting. All of these factors, at various stages of incision, can create feedback effects which alter theway in which the highlands are being dissected.

•An example of this is the way in which the high plains themselves may have been formed. At the Dargo High Plains,which has a basement of folded Ordovician sediments, the extent of the high plains is largely controlled by the^•distribution of Oligocene basaltic lavas on the surface. Formation here may be a relatively simple case of inversion ofrelief resulting from lava flows. However on the Bogong and Cobungra High Plains, which together with other^•associated high plains make up the broadest area of the remnant plateaus, lava flows cover only a minority of thesurfaces. There is evidence of some removal of basaltic material along the tops of the high plains, both by retreat of^•weathered basalt in a bench-like fashion and removal of fresher rock to periglacial block streams, yet there is littleevidence that all the high plains surfaces were once covered by lavas. It may be that incision itself has caused a^0feedback effect which has preserved these areas of low relief within the highlands.

The Bogong and Cobungra High Plains have a basement consisting predominantly of gneisses and granites. The high^•plains are made up of a number of land surface types which are manifest in sequence at High Plains Creek at the

•southern side of the Bogong High Plains. From lowest to highest these land surface types are: fresh gneissic rockexposures, gneissic tors, in situ weathered gneiss, sub-basaltic lake and swamp sediments, weathered basalts (in

•benches) to fresher basalt columns forming the higher surfaces of the plains. The fresh gneissic rock exposures mark asharp change in gradient from the low relief of the high plains to the high relief of the incised rivers.

The rivers draining the high plains maintain this sharp break in relief both in the areas where basaltic material hasapparently been removed from the plains surface and in the broad areas where basalts are non-existent on the surfaceof the high plains. The preservation of the plateau remnants may not be directly the result of basalt flows causinginversion of relief. Wahrhaftig (1965) observed that granitic rocks buried in gruss weather more rapidly than exposed^•rocks because they are kept in contact for most of the year with reactive groundwater solutions, whereas exposedrocks are wet only with each rainfall event and are dry for most of the year. The implications of this is that where^•weathering keeps pace with incision, streams can remove the weathered material and incise river courses relativelyeasily, but where fresh granitic rock is accidently exposed by the rivers, a local long-persisting base level is created in^•which the rate of weathering of the exposed rock is greatly reduced, and the resistance to erosion of the fresh rock ismuch greater than the weathering material of the streams in which fresh rock had not been exposed. Wahrhaftig usedthese observations to explain the stepped topography of the southern Sierra Nevada region in California, in whichareas of low relief have been left above the level of steeper incised streams.^ •The presence of fresh rock at the boundaries between low and high relief within valleys of streams draining the high

•plains surface may be the parallel of the process described by Wahrhaftig. If this is the case then basalt flows wouldbe complementary, but not essential, to this process by shifting the initial streams to the valley sides in which

•weathering profiles would be shallower than the valley base. The preservation of the high plains would be a feedbackeffect of incision, where paradoxically rapid incision of the older landscape has caused the preservation of low relief

•within the highlands, as well as creating the steep dissected terrain characteristic of eastern Victoria.Reference^ •Wahrhaftig, C. 1965: Stepped topography of the southern Sierra Nevada, California. Geol. Soc. Am. Bull., 76: 1165-

1190.

•48^ ABSTRACTS: Australian Regolith Conference '94

© Australian Geological Survey Organisation

•••

Some Misconceptions about Regolith Stratigraphy•

C F Pain l and C D 011ier2

• 1 Division of Regional Geology and Minerals, Australian Geological Survey Organisation2 Centre for Resource and Environmentl Studies, Australian National University

•

• Several misconceptions about the origin and stratigraphy of regolith arise from lack of understanding ofgeomorphology and weathering. Inappropriate application of stratigraphic principles learned in another context can

• lead to error if applied unthinkingly to regolith profiles. In this paper we consider a few of these misconceptions andstress the need for care in both pure and applied regolith research.

• 1. The term stratigraphy has come to mean, amongst other things, the placing of geological events or materials in theorder in which they occur.Some people talk of the stratigraphy of regolith materials as if horizons in a regolith

• profile could be treated like strata in a sedimentary succession.. Regolith studies are not as simple as the study ofsuccessive sedimentary strata.

For layers of sedimentary materials, the simple rules of stratigraphy apply.• a) A weathering profile must be younger than the original material that is weathered.

• b) A material overlying a weathering profile must be younger than the weathering profile.

c) If a weathering profile cuts across several sedimentary layers, the profile is younger than the youngest materialit crosses.

• Within the weathering profile, simple stratigraphic laws do not apply.

•a) In a profile with two or more parts (e.g. above and below a water table), all portions may form simultaneously.

b) The same applies to catenas. Profiles with different morphologies can develop in different parts of the• landscape at the same time.

c) Soil horizons that formed parallel to the present land surface are generally younger than the formation of that• slope.

• d) Soil horizons that parallel the present land surface are younger than any deep weathering of the parentmaterial.

• e) A deep profile may form progressively over a long time as weathering processes work their way down into theunweathered parent material.. The upper part is therefore older than the lower part.

I) Water table complications may result in repetition of regolith units within single profiles. For instance the• Mornay Profile of Queensland has several silcrete layers all formed at about the same time at the end of the

Cretaceous.

• Conclusion: Weathering effects do not follow simple stratigraphic principles.

• 2. Regolith profiles are often treated as if they are a result of vertical differentiation processes. However, verticalsections along are inadequate for interpreting regolith profiles and two or three dimensions should be considered.

• A section is not as good as a catena, and a catena is not as good as three-dimensions(regolith maps)

Conclusion: Lateral movement and accumulation of the products of weathering in low parts of the landscape is the• rule rather than the exception

• Stratigraphic significance: Treating regolith profiles as stacks of "strata" is even more dubious when it realisedthat some materials come in laterally. There will also be lateral variations in materials of the same age, akin to

• facies variations in a sedimentary layer.

3. To explain regolith profiles, landscape evolution by total landscape lowering is often invoked.• a) Many landscapes have remnants of very old landforms and deposits (e.g. Permian glacial pavements, lava

flows), incompatible with surface lowering of more than a few metres.Conclusion:. Massive landscape lowering is very improbable.

Stratigraphic significance. Evidence for general landscape lowering is usually based on interpretation of regolithprofiles as pure vertical sequences. This can do violence to concepts of landscape evolution. It is usually safer to re-


interpret the stratigraphy and genesis of the profiles rather than invent novel (and sometimes demonstrably untrue) models of landscape evolution.

4. A frequent error is to relate regolith to present-day climatic and biological conditions. This is usually incorrect.

a) Even in apparently simple regions, the regolith often has a complex history that stretches back to times unlike the present.

b) The effects of climate on the kind of weathering extend only as far down as biological influences, about 1 - 2 m .. Below that, climate ( mainly temperature), affects only the rate of weathering. The degree of weathering depends more on time and geomorphic stability.

c) Geothermal heat becomes more important for groundwater temperatures than surface temperatures at surprisingly shallow depths - lOs rather than laOs of metres. Climate at the ground surface is irrelevant for processes at the weathering front once it is more than a few lOs of metres deep.

Conclusion: Present climate has very little to do with regolith distribution and characteristics.

Stratigraphic significance. Some parts of regolith profile may be genuinely old, like old strata, and it is vital to interpret inherited features of the profile for what they are, rather than to assume that they are all in equilibrium with present day conditions.

5. Many researchers believe that the regolith is in some fonn of long-tenn equilibrium between rates of weathering, erosion, and uplift. In Australia, where landsurfaces can be dated, it is clear that this is seldom true, and there is a great deal of inheritance in both landforms and regolith.

50

Conclusion. Regolith is seldom in equilibrium, but has inherited characteristics.

Stratigraphic significance: If a landscape is in equilibrium, then all parts of the regolith are modern and there is no place for inherited, ancient components. If inherited features can be demonstrated (like the bauxites in Hack's classical area) then equilibrium has been disproved. Equilibrium is most prevalent on active all-slope (angular, feral or Aa landscapes), but even here there are commonly some inherited features on parts of the slope, like the earthquake stripped slopes of New Guinea.


• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

••

The Brunhes/Matuyama Polarity Transition (0.78 ma) as a Chronostratigraphie Marker in Australian Regolith• Studies

• Brad PillansResearch School of Pacific & Asian Studies, Australian National University, Canberra, ACT, 0200

•

• The polarity of the Earth's magnetic field has been generally similar to the present (i.e. "normal polarity") over the last0.78 Ma, except for perhaps several brief intervals of less than 2 x 10 3 years. Paleomagnetists refer to the last 0.78 Ma

• as the Brunhes Normal Chron. From 2.60 to 0.78 Ma the Earth's magnetic field was dominantly reverse polarity ,andthis time interval is called the Matuyama Reversed Chron, containing two well documented intervals of normal

• polarity: the Jaramillo Subchron (0.95-1.07 Ma) and the Olduvai Subchron (1.77-1.95 Ma). In this talk I will giveexamples of the application of the application of magnetic reversal stratigraphy to regolith studies. In particular I will

• illustrate how the last major polarity reversal - the Brunhes/Matuyama polarity transition, which occurred about 0.78Ma (Spell & McDougall 1992, Geophysical Research Letters, 19, 1181-1184) - can be used as a valuable

• chronostratigraphic marker in regolith studies.

Most Australian surficial deposits are strongly weathered. Consequently, the Natural Remanent Magnetisation (NRM)• of such deposits is a Chemical Remanent Magnetisation (CRM), generally acquired, during weathering, by low

temperature formation of secondary iron oxides. Thus, whereas Thermal Remanent Magnetisation (TRM) is acquired• during cooling of igneous and metamorphic rocks, and Detrital Remanent Magnetisation (DRM) is acquired during

sediment deposition, a CRM may be acquired long after deposition or cooling of the host sediment(rock. An important• feature of CRM's is the long period of time over which the magnetisation may be acquired, typically more than 10 3

years, which means that secular variations are probably averaged out, and short polarity reversals (such as those• during the Brunhes Chron) are unlikely to be recorded. On the other hand, if weathering continues to occur over time

periods of >106 years then the CRM may contain overlapping reverse and normal polarity signals which are difficult to• separate even with careful sequential demagnetisation in the laboratory.

• The Brunhes/Matuyama transition has previously been located in several Quaternary sequences in Australia, includinglake sediments (e.g. Lake George, Lake Buchanan and Lake Amadeus), glacial sediments in western Tasmania

• (Barbetti & Colhoun 1988, Search, 19, 151-153) and beach ridge sediments in South Australia (Idnurm & Cook 1980,Nature, 286, 699-702). Current research by the author has shown that the B/M transition can be identified in

• weathered alluvial fan gravels at Sellicks Beach and Hallet Cove near Adelaide. At both sites the B/M transitionoccurs within a strongly mottled unit named the Ochre Cove Formation. At Canberra, strongly weathered fan gravels

• on Black Mountain have reversed polarity, and near Charters Towers in north Queensland a soil formed on a 2.5 Mabasalt flow contains a reverse polarity in the lower B horizon. These latter three examples illustrate that reverse

• polarity magnetisation can be preserved in a wide range of regolith materials, particularly in oxidising environments. Iconclude that regolith materials with reverse polarity CRM almost certainly pre-date the Brunhes/Matuyama polarity

• transition at 0.78 Ma.

•••••••••••

ABSTRACTS: Australian Regolith Conference '94^ 51• @ Australian Geological Survey Organisation

•

Soil Genesis in Basaltic Upland Terrain of Tropical and SUbtropical Regions based upon Profile Chemistry and Mineralogy of Dominantly Australian Soils

WFRidley School of Geology, Queensland University of Technology, Brisbane

Because of the uniqueness of calcium and, to a lesser degree, magnesium as macronutrient accumulators in vegetation, the status of these elements, as exchangeable cations (Ca2+ + Mi"1 and as pedogenetic carbonate, was assessed in relation to climatic factors; particularly the influence of mean annual rainfall (MAR) upon soil moisture regime as caused by seasonal incidence and intensity.

Weighted profile average values of (Ca2+ + Mi"1 reach a maximum at the boundary between udic and ustic moisture regimes (Yaalon, 1983). This boundary also separates pedocals from pedalfers of Marbut (1951). At, and about, this udic - ustic boundary, bisiallitization may occur and become the dominant clay forming process. As the soil moisture regime becomes progressively wetter, on the one hand, and drier, on the other, monosiallitization followed by allitization occur to give an overall wethering sequence, from desert to very wet regimes, of allitization, monosiallitization, bisiallitization, monosiallitization, and allitization (Trecases, 1992). Soils corresponding to these respective processes, when at or near equilibrium with prevailing climate, are: Red Desert Soils, Saturated Euchrozems, Black Clay Soils, Unsaturated Euchrozems, and Icrasnozems.

Recognition of soils approaching equilibrium with prevailing climate is subjective, and is based upon expected properties of high base saturation and presence of carbonate within the udic zone and the recorded maximum values of (Ca2+ + Mij in the ustic zone. Soils at or near equilibrium appear to be rare. Most soils exhibit properties of inheritance and polygensis.

The magnitude of the zone of bisiallitization as determined by seasonal incidence of MAR may be inferred from the following Table:

District MAT

N.Aust S.E. Qld. NE&N Riv,NSW

Oc

20-24 17-20 13-19

Summer/winter % 100/0 70/30 SO/50

Bisiallitization Rainfall

Range mm

670-710 <500-1100 <500-1300

Growing Max,recorded Season (Ca2++Mij

(Months) m-equiv/100g soil Width mm 40 3 36

>600 7 74 >800 12 91

In Northern Australia there is an abrupt change in soil types over an interval of no more than about 40mm of MAR. By contrast in the New England Region and the Northern Rivers District on New South Wales the corresponding change in soil types extends over an interval of more than 800 mm MAR.

References:

Marbut, C.F. 1951: "Soils: Their Genesis and Classification. A memorial volume of lectures given in the Graduate School of the United States Deparetment of Agriculture in 1928." 134pp. (Soil Science Society of America: Madison)

Trescases, J.J. (1992), Chemical Weathering. Chap. 1.2. Vol.4. Regolith Exploration Geochemistry in Tropical and Subtropical Terrains. Eds C.R.M. Butt and H. Zeegers. In Handbook of Exploration Geochemistry. G.J.S. Govett (Ed). Elsevier.

Yaalon, D.H. (1983), Climate,Time, and Soil Development. Chap. 8. In Pedogenesis and Soil Taxonomy. L.P.

Welding, N.B. Smeck and G.F. Hall (Eds.). Elsevier.


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Ferricretes and Mesas of the Puzzler Walls, Charters Towers, North Queensland•

Clinton J.Rivers* and Tony Eggleton• Centre for Australian Regolith Studies, ANU. PO BOX 4 Canberra ACT 2601

*Present address: Terra Search Pty Ltd. PO Box 981, Castletown, Hyde Park QLD 4812•

• Deep weathering profiles capped by ferricrete comprise mesas of the Puzzler Walls group, located 13km east of

• Charters Towers. The mesas are capped by siliceous and ferruginous duricrusts developed on thick units of deeplyweathered Tertiary sediments and Paleozoic granitoids. The granitoids of the area host mesothermal quartz vein gold

• mineralisation, commonly with associated sulphides particularly galena.

Analyses of lateritic material from me mesas reveal two types of ferricrete, an older ferricrete developed from the• weathering of the granodiorite and a younger nodular ferricrete overlying Tertiary/Quaternary sediments. The

ferricretes are distinguished on the basis of their landscape position, micromorphology (petrology), geochemistry and• mineralogy.

• The older (Tertiary?) ferricretes have developed in situ on the granodiorite. They comprise ferruginous aggregates ofthe underlying mottled saprolite, having formed by the relative accumulation of iron as other, more soluble elements,

•have been leached out. The younger (Tertiary-Quaternary?) nodular ferricretes consist of re-worked/transported ironnodules coated by several different generations of goethite rinds. These nodular ferricretes are interpreted to haveformed by the accumulation of iron, precipitated from ground water on valley sides or oxidation fronts.

•Geochemistry of both these ferricrete types reveal enrichment of transition metals, some rare earth elements and

• elements commonly associated with gold mineralisation in the Chartres Towers district (Pb, Sb, As). The latterelements may be useful pathfinders in exploration as the results of the study show that they are enriched in the

• ferricrete. It still needs to be established whether this enrichment results from proximity to underlying mineralisation.

•••••••••••

••

•ABSTRACTS: Australian Regolith Conference '94^ 53© Australian Geological Survey Organisation

• The Chartres Towers district in semi-arid tropical North Queensland has large areas of transported and residual cover,It is host to several major gold and base metal deposits anti has a long history of mineral exploration, but few studies

• of the regolith. As exploration targets mineralisation under cover, an understanding of the regolith is essential forinterpretation of geochemical and geophysical data. Transported regolith presents further complications.

•

Interpretation of Fabrics in Ferruginous LagI. D.M. Robertson

Division of Exploration and Mining, CSIRO, Private Bag, P.O. Wembley, Western Australia 6014

Lag is a surface veneer of diverse fragments that includes, in deeply weathered terrain, very ferruginous concretions.Ferruginous lag is common in the Yilgarn Block and is particularly abundant over mafic and ultramafic rocks, whereit concentrates by residual accumulation, churning or bioturbation and is dispersed laterally, largely by sheet wash.Thus, lag may represent either the underlying material or a largely-removed laterite or other cover. It has been usedextensively and successfully in the search for Au and base metals in relict and erosional regimes having complete andpartly truncated profiles respectively. Its ability to retain pathfmder elements and its generally limited dispersion leadsto stronger and slightly larger anomalies than those of corresponding soils; however, its petrographic potential for rocktype identification has been neither fully recognised nor exploited.Lag gravels are generally related to the regolith substrate and to processes of erosion and deposition. The outwardappearance of the lag gives clues to the nature of the regolith; the interior fabrics of the lag can contain evidence of theunderlying rocks and their weathering history. Ferruginous lag fragments are lithorelics or lateritic nodules andpisoliths. Iron oxides may pseudomorph the original minerals and fabrics, but identification of these precursors can bedifficult, particularly where there has been intermediate stages comprising alteration to clays. However, ifrecognisable elements of the original fabric remain intact, despite these mineralogical changes, vital informationsurvives. Where femiginisation has occurred during early weathering, a wide variety of primary rock fabrics may bepreserved. If it has taken place during later weathering stages, secondary fabrics, related to authigenic minerals mayalso be preserved. If it has occurred after pedoplasmation, primary fabric elements are lost, although pockets of earlierfabrics may still be preserved and can be found by careful search.Primary fabrics found in lag are illustrated by a phyllitic schist of the ore host at Beasley Creek, in which a schistosefabric is preserved by mica relics and fine-textured pseudomorphs after kaolinite formed in the saprolite. The fabricsof mafic and ultramafic schists from the same site are largely overprinted by very fine-grained, matted pseudomorphsafter layer silicates, although clustering of these silicates into equant groups might reflect a granoblastic or decussateprimary fabric. Lag overlying deeply-weathered Permian tillites consists of diverse, polymictic fragments, with faintfabrics and a breccia-like structure, set in a relatively fine grained, matrix-supported, arenaceous matrix.

Igneous fabrics of peridotites of the Ora Banda Sill are preserved as hematitic pseudomorphs after olivine, a fewgoethitic pseudomorphs after talc-altered pyroxene and abundant, drop-like crystals of weathered chromite. Thechromite crystals have a reticulate pattern of goethite-filled cracks around their rims. Primary fabrics on thepyroxenite are not nearly as well preserved; a fine-scale, parallel, goethitic structure indicates amphibole, talc or,rarely, pyroxene and there are a few indistinct pseudomorphs probably after smectites and vermiculites. The lag issignificantly poorer in hematite and richer in goethite than that of the peridotite; the few chromite grains are smallerand poorly formed.

Pseudomorphed mineral fabrics may also be destroyed by secondary iron oxides. In places, there have been multiplecycles of iron dissolution and precipitation. After dissolution to form voids, goethite forms colloform, delicatelybanded structures lining, and even filling, vesicles. In others, the fabric is replaced progressively by spongy ormassive goethite. In many places, fabric destruction is incomplete, leaving partly erased fabrics. Hematite occurs bothovertly, as small lozenge-shaped crystals and covertly, with goethite, as an ultra fine-grained mixture; this appears tobe a product of dehydration, leading to a variety of colours and reflectances in the goethite.Vermicular, accordion-like pseudomorphs are common on the periphery of silicate relicts, close to, and intimatelyassociated with, secondary goethite structures. It is suggested that these pseudomorphs are Fe replacements ofauthigenic recrystallisation of kaolinite in the saprolite. In some places, wide-spread development of accordionstructures form fabric-destroying blasts in the upper saprolite. Despite these depredations, careful search for usefulprimary fabrics in lag is a powerful and non-invasive aid to identification of underlying rocks, particularly in areas ofpartial profile truncation.The work reported here was carried out as parts of CSIRO/AMIRA projects 240, 241 and 252. The sponsors arethanked for their support.


Regolith Research and Education

Graham TaylorCentre for Australian Regolith Studies, University of Canberra

Introduction

Twenty years ago, regolith (Merrill 1897), was an almost unknown word in Australia. When we formed the Centrefor Australian Regolith Studies five or six years ago colleagues, one assumes meaning well, advised "I wouldn't usethat word if I were you, no one knows what it means". Well ! six years and look at how things have changed.

To date most regolith research has been done by earth scientists, including both geologists and soil scientists, butrarely do they communicate or read the literature from each discipline. Some regolith work is being done bygeomorphologists, but again their contribution is by-and-large separate from that of soils or geology. This paper is, inpart, a plea for better and more frequent cross-disciplinary research. The other part is a plea for open minds andiconoclasm in regolith research. Many gospels exist in our area of interest, many, if not most, are based on very oldliterature which has become stuck in our thinking, or on no research at all, simply that someone read it somewhereonce.

Regolith: an interdisciplinary subject

In early October I received the latest copy of the Australian Collaborative Land Evaluation Program Newsletter (vol.3/3); in the context of land resource assessment the editor writes "Understanding the nature and behaviour of theregolith, groundwater and the underlying substrate are crucial to the sustainable management of natural resources. Our next issue is devoted to the role of environmental geoscience in land resource assessment.

The concept of regolith studies being important in land management is obviously clear, the importance of it in mineralexploration has been known to those of us in the game for 20 years or so, but I think it fair to say that over the last fewyears it has become clear to almost all the industry that regolith knowledge is one of the critical factors in the pathforward. The water industry is beginning to see relationships between the nature of the regolith and catchment watermanagement and quality. Agricultural scientists have for a long time seen the value of understanding certain aspectsof the regolith (the part they call soil) and have over the last 100 years or so collected vast masses of knowledge anddata about the regolith.

Having said that, there are those who see things from the other way round, and the sooner we change their attitudes toa more co-operative one, the better for regolith science and theirs. One example, and no disrespect intended, isecology, particularly vegetation ecology. CSIRO Division of Land Use used to work in this area for a while correlatinggeology, soils, vegetation etc. prior to the day of GIS even. There is an obvious connection between regolith andvegetation which is deeper than the regolith skin (soil) and it related primarily to geochemical turbation in theregolith. Allow me the luxury of totally uninformed discussion. Deep rooted plants suck nutrient from deeper thanshallow rooted ones. They recycle these to the soil when they drop leaves or die. If this did not happen shallow rootedplant communities would go hungry; they need their larger neighbours to bringing goodies from the deepersupermarket. Whether this is right or wrong matters little, the point is vegetation is part of the regolith, and anunderstanding of it and its communities is critical if we are to fully appreciate the regolith as a dynamic system.The study of the regolith clearly must be interdisciplinary if real progress is to be made. Sure we all have our ownparticular interests, and they are very important, but just for a moment or two take off the blinkers and chat to ourneighbours the agricultural chemist on one side and the sewerage engineer on the other, their problems are related toyours in a real way.

Some directions for regolith research

Broken Hill '94 is addressing some problems relating to the Earth science side of regolith studies. We will come awaywith new ideas to try, criticism of our peers, additional knowledge, something! There are many ways we can learnmore, one of the easiest is to talk to colleagues while we are at this conference. From necessity, discussion here will benarrowed to mainly those aspects of regolith addressed by the conference. I'd like to put together a few ideas for futureresearch outside this and in keeping with the interdisciplinary nature of the subject.Recent work by Tony Eggleton and his students has clearly shown that almost all minerals undergo a solution phase atsome stage in their weathering. A gel or amorphous phase is then formed from which minerals ultimately crystallise.As part of this research in CARS it has also been discovered that in some materials (bauxite as one example) up to75% of the material is amorphous. Soils scientists have known that soils contain reasonable proportions of amorphousmaterial but geologists have not associated it with mineral based regolith materials. The implications of these twofindings are huge. The chemical activity of gels or amorphous phases is probably 10 times greater than crystallinemineral phases, and this follows directly into exploration strategies, sampling programs and so on.


It also demonstrates that few if any mineral components are stable during weathering. They may not move far, but if elements go into solution they have the potential to move. How does this fit with many preconceived notions many of us have. Aluminium is immobile during weathering, after all that's why we get bauxite. How then do we make clay minerals from weathering olivine?

Once mobilised how do elements move through the regolith? Do they diffuse through saturated materials, concentrate at watertables and what are the processes that determine which element precipitates where. Is it simple phase chemistry related to Eh and pH or does it depend on water activity and void size as suggested by Tardy and Nahon. Or is the activity of water simply controlled by the size of the largest interconnected pore? Is stress important in mineral solubility, movement and precipitation as it is in deformed and metamorphic rocks.

Do we understand enough about nutrient movement in plants, where the nutrients concentrate (fruit, leaf, stem?) and whether concentrations are highest during periods of non-growth or in summer? What sort of volumes of trace elements and major nutrients are cycled through plants? Can plant nutrient cycling cause chemical turbation in a chemically differentiated regolith? And so on.

What are the roles of algae and bacteria in weathering? We know they occur, can be seen to metabolise mineral matter and concentrate certain elements, but just how important are they in the overall process and do they contribute to the overall nature of weathering proftles; does algal/bacterial ecology change with depth in the regolith? Can bacteria be used to mine for particular elements or groups of elements?

What are the effects of larger infauna and flora in the regolith. David Tilly has recently shown that some of the complex pisolites at Weipa are biogenic. Rhizomorphic features are common and can be seen to cause mixing in otherwise differentiated proftles. Termites, as described from near Charters Towers can control mottling in the weathering profile, and other similar bugs do the same. Do we understand enough about these? No.

Research is also required to develop robust models for the formation of many regolith features so regolith mapping can be approached with the same confidence as geological or soils mapping are done. We don't, for example have adequate models for the formation of 'lateritic' weathering proftles. Imagine trying to map geology without adequate models for granite intrusion or for the folding of rocks; it would be difficult if not impossible on a routine basis. We need to develop similar models for regolith geology. To continue briefly, what about the concepts of unconformities as they apply in regolith materials? With many duricrusts there is a textural and fabric conformity of the host material but a temporal and chemical unconformity with the hardening agent. What about landsurfaces and how they form; is landscape inversion the gospel some would have us believe? I certainly don't have answers, only opinions and they are yet to be tested.

The solution to these problems and many of the others lies in working with colleagues from different backgrounds with differing paradigms and knowledge.

Tertiary Education and Regolith Studies

Given the interdisciplinary nature of regolith studies how do we go about training regolith scientists. Regolith studies is based in Earth Science, so its appropriate to educate professionals in the area through Earth Science. The problem is that few Earth science departments give students the opportunity to learn in the areas necessary to get an appreciation of the subject breadth? Currently the only formal course available in this country is at CARS.

In undergraduate programs how many students get the opportunity to study weathered rocks, clay mineralogy, geomorphology, soil science, geohydrology and remote sensing/GIS? As these are all necessary prerequisites for a practicing regolith scientist so some major changes are required in our undergraduate courses. Currently most departments have strengths in the conventional areas of Earth science; petrology, geochemistry, sedimentology and stratigraphy, structural geology, economic geology and palaeontology. How do we increase the teaching in regolith areas?

Perhaps one of the problems is that these types of undergraduate courses could have been taught in 1945 and we need to change. Change is not just required for students to get a regolith background but also so students can avail themselves of all the other employment opportunities for Earth science graduates. Environmental geology, land management, land appraisal, water studies and so on. The opportunities in these areas are increasing rapidly with huge injections of government cash (e.g. Landcare, Total Catchment Management).

The answer is complex and difficult. There is no doubt that students must have a solid grounding in the fundamental sciences and Earth science at the same time as regolith and other areas. The only possible solution is in more integrated curricula where students study the fundamentals with examples from these other areas. This requires a whole new generation of tertiary teachers who are prepared to address these difficult problems.


• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Some examples may be to study:

• weathered rocks while studying their fresh equivalents;

• plastic material deformation while studying brittle deformation in structural geology;

• weathered ore deposits in economic geology;

• industrial minerals as fully as sulphides;

• a course in geomorphology in fIrst year along with the normal course in physical geology; and,

• electives in areas like soils, remote sensing/GIS, ecology and hydrology.

Equally it is important to have some texts and practical class manuals which teachers can use as models for courses. No such book suitable for undergraduate study exists.

Equally there is currently a need to develop a series of short-course programs for currently practicing geology professionals who have little or no understanding of surface and surface related processes, much less the ability to work with them. It is from the presently practicing professionals that much of our future knowledge will come, so this is a very important aspect of regolith education if the discipline is to progress.

Conclusion

We need not to be afraid to enter areas with which we are unfamiliar, to try things which we are not sure will work, to question existing 'knowledge' and to follow our convictions. In this way our understanding will progress more rapidly, eventually leading to the desired outcomes of studying the regolith; fInding new ore bodies, improving our environment and assisting in developing sustainable land and water use practices.

I began mentioning the gospels in the second paragraph, I would like to finish in a similar vein: many of us seem to be seeking the holy grail rather than simply trying to understand the now and then of a particular region. Generalisation follows understanding, and real understanding follows detailed work.


57

The evolution of hauxitic pisoliths from Weipa, North Queensland

D. B. Tilleyl, C.M. Morgan2 and Tony Eggleton1

1 Centre for Australian Regolith Studies, AND, Canberra, ACT 0200 2 Comalco Mineral Products, Weipa, QLD 4874

Three different processes appear to be responsible for the development of bauxitic pisoliths; namely spherulisation, agglomeration and cortication.

Spherulisation is the development of aluminous spherules in a kaolinitic plasma. in response to the desilicification of kaolinite during bauxitisation. Spherules may be composed of either gibbsite (AI(OHh), boehmite (yAlO(OH) or

tohdite (5Al20 3 • H20), depending on the thermodynamic activity of water at the time of spherulisation.

Agglomeration is associated with the epigenetic replacement of kaolinite with hematite, resulting in the formation of ferruginous-kaolinite nodules. Under such conditions, the silicification of bauxite may also occur, resulting in the formation kaolinite and its adhesion onto the outer surface of spherules and older generation pisoliths. Ferruginouskaolinite nodules themselves, may evolve into bauxitic pisoliths during a subsequent phase of bauxitisation.

The process of cortication, which is intrinsically linked to bauxitisation, leads to the formation of concentricaIlybanded cortices around spherules and nodules. Chemical reactions which contribute to cortication include hydration, dehydration, silicification, desilicification, ferruginisation and deferruginisation.

A maximum of three distinct packets of conical layering are recognised within the pisoliths of Weipa. A relative concentration of quartz often marks the boundary between two packets of conical layering. When quartz is absent, there is invariably evidence for a hiatus in the form of truncated conical banding and/or radial cracks.

Bauxitic piSOliths from Weipa appear to have undergone a maximum of three separate phases of bauxitisation during their evolution. In the intervening periods, kaolinisation and ferruginisation were the dominant processes, leading to mottle and nodule formation.


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••• High Resolution Airborne Gamma-Ray Spectrometry for Regolith Mapping

J R Wilford', C F Pain' and J C Dolu-enwend2

• Division of Regional Geology and Minerals, Australian Geological Survey Organisation2 United States Geological Survey

Gamma-ray spectrometric data (radiometrics) are an invaluable remote sensing tool for differentiating regolith typesbased on their potassium, thorium and uranium signatures. The radiometric signal effectively "sees through" the

• vegetation cover and can record information up to 40 cm below the surface if conditions are optimal. On Cape YorkPeninsula, AGSO mapping teams have used radiometric imagery for maping both bedrock and regolith, as part of the

• NGMA North Queensland Project. The interpretation was based on single channel colour-sliced images, false colourcomposite images with red assigned to potassium, green to thorium and blue to uranium, and as stack-line profiles.

• The profiles are important as considerably more information is recorded along flight-lines than between them.

•Bedrock radiometric responses are well known, and reflect their mineralogy and geochemistry. On Cape YorkPeninsula granite and some metamorphic rocks have high potassium values, while other metamorphic rocks are low in

4110^potassium and relatively high in thorium and uranium. Mesozoic sediments consisting of mudstones and sandstonesare dominated by thorium. However, these responses are modified by the nature of the regolith. On old landsurfaces in

•the Coen Inlier differrent granite bodies show up because of different patterns on the radiometric image. Thesepatterns are related to the nature of weathering on the different granites. For example, one weathers to give a deep

•residual quartz sand cover, which shows up as dark areas on the image. Another granite weathers to saprolite, withmany corestones; this granite shows up as being brighter and particularly redder on the image.

• Degree of weathering and relative landscape stability can be assessed. On gently undulating granitic terrain, residualsands, where the more soluble constituents have been removed in solution leaving behind mostly quartz, are reflected

• in the imagery by low potassium, thorium and uranium values. In contrast, steep actively eroding slopes on the samegranite are shown in the imagery by high potassium values. Moderately high potassium values over the north west

• corner of the Ebagoola 1:250K sheet correspond to cracking clay soils and reflect the presence of potassic clays (egillite) and/or absorption of potassium ions within the lattice of swelling clays (eg montmorillonite). Deeply weathered

• bauxitic and iron pisolitic residual plateaus are recognised by their high thorium signatures.Provenence of sediments can often be distinguished. The distribution and origin of sediments in beach ridges and

• chenier plains on the eastern coastal plain is recorded by high K and Th signatures, reflecting granitic andmetamorphic source rocks respectively. Similarly, in the west rivers are clearly seen in the imagery because of the high

• feldspathic content of the channel and recent overbank deposits, derived from granites and metamorphics in the CoenInlier.

•To date very few of these observations have been related to the mineralogy and geochemistry of the regolith. This is

• the subject of on-going work.

ConclusionsID

• It is possible to discriminate different basement lithologies based on their potassium, thorium and uranium• signatures.

• Although broad lithological divisions can be identified in the imagery most of the variation within these4111^Ethological groups relate to the regolith cover and to geomorphic processes in the landscape.

• • Subtle variation in Ethology or changes due to rock alteration can only be effectively understood when theresponses due to the regolith cover are known.

• • Different weathering styles, which largely reflect underlying Ethology, time and geomorphic processes, can be

•readily discriminated on the imagery. The imagery therefore has considerable potential for mapping regolith,including soils.

• • As well as helping to map the distribution of regolith materials the imagery can also be interpreted as an 'activitymap' identifying stable landforms with deep weathering as opposed to younger landforms with active erosion andstripping of the surficial cover. The imagery thus has considerable potential for environmental and soil studies as itcan delineate areas of erosion and land degradation.

• The surface expression of buried mineralized zones is tied to the geomorphic and weathering history of an area. Whenthe radiometric response and weathering relationships of the regolith are understood, anomalies relating tomineralized alteration may be more effectively resolved.


brokenhii/l, 14-17 n 1994 - … 14-17 n 1994 abstracts by c f pain, ... kprd lnn dtr ttrn indt rr...

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