relative sea level control of deposition in the late permian newcastle coal measures of the sydney...

Sedimentary Geology

ELSEVIER Sedimentary Geology 107 ( 1997) 167- 187

Relative sea level control of deposition in the Late Permian Newcastle Coal Measures of the Sydney Basin, Australia

Chris Herbert* School of Earth Sciences, Macquarie University, NSW 2109, Australia

Received 23 March 1994; accepted 11 April 1996

Abstract

Accumulation of the 400 m-thick Late Permian Newcastle Coal Measures of the Sydney Basin was controlled by changes in relative sea level. Three 3rd-order sequences, which constitute the coal measures, consist of 4th-order depositional sequences of fluvial conglomerate, coal, and small paralic/lacustrine deltas/crevasse splays, deposited on a coastal plain landward of a marine shoreline. Each 4th-order sequence was deposited durin g a single 4th-order relative sea level cycle. Following falls in relative sea level alluvial conglomerates derived from the New England Orogen filled incised valleys above sequence boundaries forming lowstand systems tracts. Sigmoidal conglomerates with ‘giant crossbeds’ were deposited as alluvial fill in compactional moats formed at the toes of abandoned paralic deltas. Alluvial sediments passed through the coastal plain directly to the marine shoreline causing the shoreface to prograde. Rising relative sea level, caused siliciclastic sedimentation to wane. During these hiatuses, in the transgressive systems tract, a rising water table stimulated peat mire growth blanketing the entire non-marine area. When the vertical accumulation of peat was outpaced by increasing rates of rising relative sea level, transgressing lagoons, interdistributary bays and lakes inundated the mires above maximum flooding surfaces. Continuing relative sea level rise in the highstand systems tracts caused paralic/lacustrine crevasse splays, crevasse subdeltas, and small deltas to prograde westwards into the Newcastle half-graben from a volcanic source on the Offshore Uplift. At this time of rising base-levels sediments were trapped on the coastal plain while the marine shoreface was starved. Falling relative sea level terminated paralic/lacustrine delta progradation and initiated exposure and erosion to repeat the cycle and initiate another 4th-order sequence.

Two source areas supplied sediment into the Newcastle half-graben. An easterly source on the Offshore Uplift shed volcanic detritus into the Newcastle Coalfield via paralic/lacustrine deltas, and the New England Orogen shed volcano-lithic detritus via braided streams. The supply from the New England Orogen was switched on or increased by a fall in relative sea level while supply from the Offshore Uplift was switched off, reduced, or diverted, and vice versa during a rise in relative sea level. Increasing rates of 2nd-order falling relative sea level resulted in an upward change from predominantly marine shoreface and coastal plain sedimentation to predominantly fluvial sedimentation in the upper Newcastle Coal Measures.

Keywords: sequence stratigraphy; Late Permian; coal measures; Sydney Basin

* Present address: 17 The Quarterdeck, Carey Bay, NSW 2283, Australia

0037-0738/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SOO37-0738(96)00027-9

168 C. Herbert/Sedimentary Geology 107 (1997j 167-187

1. Introduction

The Late Permian Coal Measures of the Sydney Basin, Australia, form a southwest-thinning silici- elastic wedge of marine, deltaic and alluvial sediments that prograded in a southwest direction from the erogenic New England Fold Belt, across the Sydney Basin, to the cratonic Lachlan Fold Belt (Herbert, 1980) (Fig. 1). The New England Fold Belt, including its offshore extension to the east (Offshore Uplift), provided volcano-lithic detritus to the subsiding foredeep or retroarc basin (Her- bert, 1980). In the extreme northeastern part of the basin, the upper part of the coal measures above the Waratah Sandstone is referred to as the Newcas- tle Coal Measures (about 400 m thick, Fig. 2). In this area, these units and the underlying 40-m-thick Dempsey Formation, were deposited in what may originally have been a ‘piggy back’ basin located between the Offshore Uplift (Bradley, 1993a,b) and

the eastern flank of the Lochinvar Anticline (Fig. 1): essentially a north-south-oriented half-graben. The broadly regressive sediments deposited in the half- graben grade upwards from off-shore marine shale of the Dempsey Formation, through shoreface and barrier-island sandstone of the Waratah Sandstone, to coastal plain sandstone/shale, and fluvial conglomerate in the lower Newcastle Coal Measures, to dominantly fluvial conglomerate in the upper New- castle Coal Measures. Commercially important coals are intercalated throughout.

Herbert (1995) defined three 3rd-order depositional sequences within this interval: Sequences F, G and H (Fig. 2). It is proposed that these sequences consist of thinner, higher-frequency, 4th-order sequences controlled by 4th-order relative sea level changes. Mitchum and Van Wagoner (1991) suggested that 3rd-order sequences which are internally composed of higher-frequency sequences should be termed composite sequences. However, until the 4th-

- (c)

-E

-i/-l 1

Broken’Bay /

Fig. 1, Late Permian Coal Measures were deposited across the entire area of the Sydey Basin shown here. The topmost one-third of the coal measures in the proximal northeastern part of the basin are known as the Newcastle Coal Measures (outcrop in black). The coal measures were deposited in a north-south-oriented half-graben between the Lochinvar Anticline and the faulted Offshore Uplift in the Lake Macquarie and Offshore Synclines (large inset). Cross-sections: (a) Susan Gilmore Beach to Shepherds Hill, Fig, 5; (b) Redhead to Dudley, Fig. 6; (c) cross-section in Fig. 8; (d) cross-section in Fig. 9.

C. Herbert/Sedimentary Geology 107 (1997) 167-187

FORMATIONS MEMBERS

Narrabeen Group Om ales Point Coal-

Wallarah Coal

Catherine Hill Bay

Mannering Park Tuff ToukleylBuff Point Coal

Great Northern Coal

Eleebana

Fassifern Coal loo m

Chain Valley Coal Bolton Point Congl

Croudace Bay

Upper Pilot Coal Reids Mistake Seahampton Sandston

Lower Pilot Coal

Australasian Coal

Montrose Coal .

Wave Hill Coal _

Highfields 300 m

Fern Valley Coal ’ \

Kotara

Victoria Tunnel Coal She herds Hill Nob ys Coal , g Bar Beach Dudley Coal e

Tingira Congl

Edgeworth Tuff Upper Fern Valley Coal

LoweXrZZliey Coal

Merewether Congl

N.Qbby.s. Tuft Signal Hill Congl

Bogey Hole Yard Coal 400 m Tighes Hill Borehole Coal Waratah Sandstone

Dempsey (Tomago Coal Measures)

Cockle Creek Congl

Ferndale Congl

-t

HST

nfs

‘ST

ts

I ST

‘SB

t HS-

mfs -ts-

LST .SB

HST

.mfs - ts

TST

LST

SB-

169

SB r Fig. 2. Composite stratigraphic column of the regressive nonmarine/paralic 400-m-thick Newcastle Coal Measures underlain by the marine Waratah Sandstone and Dempsey Formation, modified from McKenzie and Britten (1969). 3rd-order sequences from Herbert (1995). Abbreviations in Fig. 3 caption.

C. Herbert/Sedimentav Geolog>a 107 (1997) 167-187

Fig. 3. Conceptual cross-section illustrating strata1 geometry within an unconformity-bounded depositional sequence. Moditied from Christie-Blick (1991) for a ramp-margin basin appropriate to the Sydney Basin. Strata1 lines depict flooding surfaces which bound parasequences, the basic building blocks of sequences. Stipple represents coastal plain to fluvial sediments, blank represents marine offshore and shoreface sediments, circles represent coarse estuarine/alluvial sediments, SB = sequence boundary; ts = transgressive surface; mfs = maximum flooding surface; ivf = incised-valley-fill; LST = lowstand systems tract; TST = transgressive systems tract: HST = highstand systems tract.

order sequences in the Newcastle Coal Measures are more clearly defined and regionally recognized it is considered premature to apply that terminology here.

Environmental interpretations for the Newcas- tle Coal Measures occurring above the shoreface Waratah Sandstone include: fluvial (Branagan and Johnson, 1970; Diessel, 1992a), fluvio-lacustrine (Conaghan, 1982; Jones et al., 1987), delta plain to alluvial fan (Hunt and Hobday, 1984) and lower to upper delta plain (Conolly and Ferm, 1971; Herbert, 1980; Warbrooke, 1981; Diessel, 1992a). Branagan and Johnson (1970) stressed “the unusual charac- ter of this coal basin, in particular the very coarse units, . despite the presence of many ‘normal’ coal basin features.” Interpretations assumed continuous deposition without explaining the juxtaposition, with sharp contacts, of fluvial gravel, finer-grained deltaic sediments, and peat mires. I attempt to resolve this difficulty by referring the contrasting depositional environments to changing relative sea level (result- ing from a combination of eustatic sea level and subsidence/uplift).

Correlation of coal seams and interseam sediments in the Newcastle Coal Measures is compli- cated by lateral changes in facies and thickness as reflected in the complex stratigraphic nomenclature. Sequence stratigraphic principles (Posamentier et al., 1988; Posamentier and Vail, 1988; Van Wagoner et al., 1990) provide insights into the combined effect of eustatic sea level change and subsidence (re-

ferred to herein as relative sea level). The resultant changes in relative sea level interact with siliciclastic deposition to produce recognisable strata1 dis- continuities such as transgressive surfaces, flooding surfaces, and sequence boundaries (Fig. 3). Parase- quences, the strata1 building blocks of depositional sequences, are conformable successions of temporally equivalent beds and bedsets bounded by marine flooding surfaces or their correlative surfaces. They are generally progradational and characterised by upward-shallowing facies.

Two types of upward-coarsening parasequences are defined herein: marine and paralic. Marine shoreface parasequences occur seawards of the shoreline and can be identified in the Waratah Sand- stone/Dempsey Formation, whereas paralic parasequences occur landwards of the marine shoreline and can be identified in the overlying Newcastle Coal Measures. A typical paralic parasequence is shown in Fig. 4. All parasequences in the following discussion of Sequences F, G and H are considered examples of paralic (or lacustrine) deltas, subdeltas and crevasse splays with varying amounts of top delta alluvium.

2. Sequence F

Sequence F constitutes the lower part of the New- castle Coal Measures below the Charlestown Con- glomerate (Fig. 2). It consists dominantly of shale to medium-grained sandstone with minor coarse-

C. Herberi/Sedimentary Geology 107 (1997) 167-187 171

Fig. 4. (a) Coastal outcrop of part of the nonmarine/paralic New- castle Coal Measures at Redhead, from Nobbys Tuff to the Fern Valley Coal. Location in Fig. 6. (b) Interpretive sketch showing a typical 4th-order paralic delta parasequence (Ps), 15-20 m thick, overlying the Victoria Tunnel Coal (4 m). Triangle de- picts upward-coarsening trend. Ps is bounded by par&c flooding surfaces; the lower one, interpreted as a 4th-order maximum flooding surface, overlies the Victoria Tunnel Coal (mfi), the other, beneath the Fern Valley Coal and associated tuff, is interpreted as a landward correlative of a 4th-order transgressive surface (ts). Note that in this area where the lowstand conglomerate is not present, the 4th-order sequence includes strata from the base of the Victoria Tunnel Coal to the base of the Fern Valley Coal and the sequence boundary (SB) is coplanar with the transgressive surface (tdS3). Note in situ tree stumps above and below the Victoria Tunnel Coal indicating subaerial conditions.

grained sandstone, conglomerate, coal, and tuff. Two coastal outcrops (Fig. l), (a) the l-km-long outcrop from Susan Gilmore Beach to Shepherds Hill and (b) the 4-km-long outcrop from Redhead to Dudley, reveal the depositional style of Sequence F.

(b) Fern Valley Coa

P%.&C delta-top

E u. 5 m

P-3EGllC

- tir delta-front I

z PS

Par& prodelta

E Victoria Tunnel -3 Coal (peat mire)

E LL

I :: $ 5 Nobbys Tuff _c (ash-fall & f.0 base-surge)

Fig. 4. Continued.

2.1. Susan Gilmore Beach/Shepherds Hill

In cliffs north of Susan Gilmore Beach to Shep- herds Hill, the Dudley Coal splits around two parasequences (Psl and Ps2), and a third (Ps3) occurs above (Fig. 5). The Dudley Coal is split by a metre of shale which thickens northwards to a lo-m-thick, upward-coarsening parasequence (Psl). Ps2 appears in the north as a shale separating the main coal from a thin ‘rider’ coal. Ps2 thickens southwards to about 5 m as the rider coal diverges from the Dudley Coal to encase the parasequence (the rider coal is interpreted to have draped the abandoned delta-front slope of a northerly prograding, crevasse-subdelta). Another upward-coarsening parasequence (Ps3) dis- plays northerly inclined bedding which downlaps onto the top of the Dudley rider coal (indicating northerly progradation). Ps3 is capped by the Nob- bys Coal and Tuff. Psl-Ps3 form an imbricate stack of parasequences delineated by coal seams. Sand- stone above the Yard Coal and in Ps3 contains faint, locally abundant burrows which resemble the ichnofossil Mucaronichnus (MacEachern and Pemberton, 1992; Male, 1992).

172 C. Herbert/Sedimentary Geology 107 11997) 167-187

S N

SUSAN GILMORE BEACH SHEPHERDS HILL

Merewether Congl 00 00 0

o o”o

Fig. 5. Sketch of coastal outcrops of the nonmarine/paralic lower part of the Newcastle Coal Measures from Susan Gilmore Beach to Shepherds Hill (a in Fig. 1). Northerly prograding, offlapping parasequences (Ps l-3) comprise upward-coarsening (triangles), paralic crevasse splays and small deltas encased in coal. Lithology legend in Fig. 6.

2.2. RedheacUDudley

Cliffs between Redhead and Dudley (Fig. 6) expose an imbricate set of 3 parasequences (Psl-Ps3) and the Merewether Conglomerate Member which constitute the Kotara Formation. PSI is a parasequence that coarsens upward from shale to sandstone (Fig. 4) to form a mound-shaped body with inclined bedding that downlaps southwards and northwards onto the top of the Victoria Tunnel Coal (Fig. 6). The southerly inclined upper surface is draped by the Fern Valley Coal. Fossil tree stumps, some with attached branches, project several metres above the underlying coal and are encased in the overlying shale.

The Merewether Conglomerate, about 12 m thick,

S

REDHEAD

occurs above an erosional surface that cuts through Psl down to the top of the Victoria Tunnel Coal, but is not significantly erosive into the coal. The inclination of the erosive surface is subparallel to the slope of bedding in Ps 1. Similar bodies of conglomerate at Little Redhead (3 km to the north) and at Shepherds Hill, also in the Kotara Formation, are exposures of a single channel (Diessel, 1992a) or are different channels separated by deltaic parasequences. Inclined bedding, interpreted as giant crossbeds (Conaghan, 1982) or lateral accretion beds (Diessel, 1992a), imparts a characteristic sigmoidal shape (Fig. 7). Fossil tree stumps that project several metres into the conglomerate from radial roots in sandy sediments immediately above the basal erosion surface indicate subaerial exposure before gravel deposition.

N

DUDLEY

Fern Valley Coal Kotara Fm

Conglomera,e Sandstone Shale T”ff coa,nree stump

bk$ria Tunnel

Fig. 6. Sketch of coastal outcrops of the nonmarine/paralic sediments in the Newcastle Coal Measures from Redhead to Dudley (b in Fig. 1). Offlapping paralic delta parasequences (Psl-Ps3) in the Kotara Formation intercalate between sigmoidal fluvial conglomerate bodies (dashed lines depict principal bedding planes, Sp, see Fig. 11). Triangles depict upward-coarsening trends. Note in situ tree stumps on top of Victoria Tunnel Coal. In the subsurface, if the Merewether Conglomerate was not intersected or incorrectly interpreted, Psl would be correlated incorrectly with Ps2. However, in this continuous coastal exposure the two parasequences are seen to be separated by a 4th-order sequence boundary at the base of an incised-valley-filling conglomerate (Merewether Conglomerate). All these sediments were deposited on the coastal plain landward of the marine shoreline.

C. Herbert/Sedimentary Geology 107 (1997) 167-187 173

Fig. 7. Coastal outcrop of the Merewether Conglomerate between Redhead and Dudley as represented in Fig. 6. Note the overall sigmoidal shape and inclined bedding planes which resemble giant crossbeds. See Fig. 11 for explanation.

Parasequence Ps2 drapes the Merewether Con- glomerate and thickens northwards as the conglomerate thins. Lateral facies changes from sandstone to shale are rapid in places. Fossil tree stumps as much as 6 m high, with their roots in the top of the Victo- ria Tunnel Coal have been buried by Ps2. Sand-filled scours around the stumps indicate rapid sediment accumulation.

Parasequence Ps3, abruptly overlies Ps2. Tuffa- ceous sediments and the Fern Valley Coal drape Psl and Ps3 and the Merewether Conglomerate. Fossil tree stumps that project from the top of the inclined, mound-like surface of Psl into the tuff beneath the Fern Valley Coal indicate subaerial exposure of Psl before the deposition of tuff and peat.

2.3. Subsuflace

The complex splitting and coalescing of coal seams between the coastal outcrops in the Lake Macquarie Syncline and the flank of the Lochinvar

Anticline defines the geometry of elastic sedimentation in Sequence F. This added dimension can be observed in an east-west cross-section shown in Fig. 8. The Borehole, Yard, Dudley and Nobbys Coals encase parasequences stacked in a retrogradational, backstepping-to-the-east, pattern. Erosion- ally based sigmoidal bodies of conglomerate define the western pinchout of each parasequence. Each parasequence/conglomerate couplet is draped by a coal seam which converges and coalesces with previously deposited coal seams to the west to form the West Borehole Coal. Isopach maps (Branagan and Johnson, 1970) indicate that the conglomerates fill channels which outline the convex-to-the-west terminus of each parasequence. Two or more bodies of conglomerate may occur between major coals, e.g., two Cockle Creek Conglomerates in the Bogey Hole Formation, and two or three Merewether Con- glomerates in the Kotara Formation, as mentioned previously. The thick pyroclastic-surge Nobbys Tuff caps the retrogradational parasequence set which can

174 C. Herbert/Sedimentary Geolq~ 107 (1997) 167-187

Lochinvar Anticline Lake Macquarie Syncline

E Fern Valley Coal

Victoria Tunnel Coal Nobbys Coal & Tuff Dudley Coal

Yard Coal

. ‘. West Boreho,e Coa, . , ‘, ‘_ ‘. ‘, ‘. Borehole Coal Walatah Sandstone

Fig. 8. Subsurface cross-section of the lower part of Sequence F, from the flank of the Lochinvar Anticline (west) to Redhead (east) (c in Fig. 1). Modified from Diessel and Warbrooke (1987) by reducing vertical exaggeration to 50: 1, levelling on the Borehole Coal, and smoothing thickness variations. Note the: (1) backstepping or retrogradational pattern of paralic delta parasequences from the Borehole to the Nobbys Coal (3rd-order transgressive systems tract); (2) Nobbys Tuff in position of 3rd-order maximum flooding surface/condensed section at change of parasequence stacking from retrogradational to progradational; (3) progradation and downlap of paralic delta parasequences onto Nobbys Tuff (3rd-order highstand systems tract); (4) location of conglomerate members at the distal terminus of each parasequence (high-frequency, 4th-order, incised-valley-fills); and (5) coalescence of coals westwards. High-frequency, fourth-order sequence boundaries postulated at the base of each conglomerate continue as the boundary between coalesced coals.

be interpreted as the 3rd-order transgressive systems tract of Sequence F (Figs. 2 and 8). The two parasequences capped by the Victoria Tunnel and Fern Valley Coals downlap onto the Nobbys Tuff as part of a progradational parasequence set and can be interpreted as the lower part of the 3rd- order highstand systems tract of Sequence F (Figs. 2 and 8). Conglomerate (Bamsley Member) again marks the western terminus of the paralic parasequences. The Victoria Tunnel and Fern Valley Coals cap parasequences and coalesce westwards with the West Borehole Coal as did previous coal seams.

3. Sequence G

Sequence G is composed of the basal 88-m- thick Charlestown Conglomerate (Fig. 2) overlain by upward-fining sandstone and conglomerate with subordinate upward-coarsening parasequences, coal, and tuff of the Mount Hutton and Warners Bay Formations. The characteristics of Sequence G are considered intermediate between those of Sequences F and H and are not discussed further.

4. Sequence H

quences are only a metre or two thick. Conglom- erates in Sequence H are generally thicker, coarser- grained, and more extensive than those in Sequences F and G (except for the Charlestown Conglomer- ate which is a 3rd-order incised valley fill, Herbert, 1995). Backing et al. (1988) mapped part of the Bolton Point Conglomerate as a 5-km-wide curvi- linear, channel-like feature for a distance of 50 km and found that it passed southwards from conglomerate to sandstone (Fig. 10d). Splits of the Fassifem Coal define 3 offlapping channel-fills within the Bolton Point Conglomerate (Fig. 9). In addition, conglomerates stratigraphically above the Bolton Point Conglomerate also form imbricated or offlapping channel-till bodies. The positions of successively younger channel-fills migrated eastward and are separated by coal and tuff. Palaeocurrent measurements for the conglomerates indicate a general southerly palaeoflow (Diessel, 1992a), and isopachs (Branagan and Johnson, 1970) define north-south sinuous channel belts. The lowermost conglomerate in Sequence H, the Belmont Conglomerate has a mean palaeocurrent direction of 210” (Diessel and Moelle, 1988). Note that the encasing coal seams coalesce both to the east and to the west forming condensed sections in the absence of siliciclastic sediments.

Sequence H is dominated by conglomerate with The conventional vertical profile (Fig. 2) imply- relatively minor coal, tuff, and shale (Figs. 2 and ing thick vertically stacked and areally extensive 9). Rare upward-coarsening, shale/sandstone parase- conglomerates is misleading. The vertical profile is


w Lochinvar Anticline

Martinswile h.

Lake Macquarie Syncline E Wang1

Fig. 9. Subsurface cross-section of the upper part of the nonmarine Sequence H from Martinsville, near the crest of the Lochinvar Anticline (west), to Wangi in the axis of the Lake Macquarie Syncline (east) (d in Fig. l), showing thick sigmoidal bodies of conglomerate and intercalated coal and tuff. Modified from Backing et al. (1988). Note the eastward migration of fluvial conglomerate- filled channels, was probably assisted by greater rates of subsidence in that direction. Fourth-order sequence boundaries postulated at the base of each incised-valley-fill conglomerate continue as the boundary between coalesced coals. Coal should be decompacted, perhaps IO-fold, to envisage the geometry during deposition.

in fact a composite section, which although cor- rectly representing age relationships, misrepresents by exaggeration the limited extent and lateral stacking geometry of each conglomerate as shown more realistically in Fig. 9.

5. Palaeoenvironment

It is proposed that deposition in the Newcastle Coal Measures took place on a coastal plain landward of a marine barrier shoreline (Waratah Sand- stone) (Fig. 10). Paralic and marine parasequences, fluvial conglomerates, coals, and tuffs deposited in these environments are discussed below.

5.1, Parasequences

Two types of repeated upward-coarsening, shale to sandstone intervals are interpreted as marine and paralic parasequences.

Stacked marine parasequences occur only in the Dempsey Formation and Waratah Sandstone and are interpreted as offshore silt and prograding sandy beach ridges bounded by marine flooding surfaces (Herbert, 1995) (Fig. 12a). The Dempsey Formation contains abundant bioturbation and synaeresis cracks. Agglutinated and calcareous foraminifera have been

identified in the stratigraphically equivalent Singleton Coal Measures (Scheibnerova, 1980a,b,c).

Paralic parasequences occur in the overlying Newcastle Coal Measures and are interpreted as crevasse splays, crevasse subdeltas, and small deltas which prograded into shallow, brackish lagoons, interdistributary bays and lakes landward of the Waratah Sandstone barrier shoreline (Fig. 10~). The term paralic is used to indicate a coastal plain environment rather than implying a particular salinity. All parasequences in the previous outcrop and subsurface discussions of Sequences F, G and H are considered examples of paralic parasequences.

Brackish conditions are suggested by the presence of burrows, in sandstones at the top of parasequences, which resemble the ichnofossil Maca- ronichnus found in Cretaceous, marine shoreface and estuarine sandstones in the U.S.A. (MacEachern and Pemberton, 1992; Male, 1992). These burrows are considered to have been produced by organisms anal- ogous to modem, deposit-feeding polychaete worms (Clifton and Thompson, 1978). Acritarchs indicative of marine and coastal plain environments have been found throughout the Newcastle Coal Measures to a stratigraphic level as high as the top of the Bel- mont Conglomerate in Sequence H (McMinn, 1982, 1984) (mfs in Fig. 2). Bymes et al. (1981) found

176 C. Herbrrt/Sedirnentq Geology 107 (1997) 167-187

(a) LST - Sequence F (b) TST - Sequence F

(c) HST Sequence F (4 LST - Sequence H

Fig. IO. Palaeogeographic reconstructions for a single representative ilth-order sequence within the larger 3rd-order Sequence F (a-c) during one cyclical 4th-order change in relative sea level (large arrows depict cyclicity) and Sequence H (d) during a 4th-order lowstand. See text for detailed discussion. The marine shoreline prograded southward during lowstands, and the paralic delta complex prograded westward during highstands. N = Newcastle, NE0 = New England Orogen, ,!,A = Lochinvar Anticline, OU = Offshore Uplift. (a) Relative sea level lowstand to initial rise (lowstand systems tract). I = Ferndale Conglomerate, 2 = Cockle Creek Conglomerate, 3 = Signal Hill Conglomerate. (b) Early relative sea level rise to maximum rate of rise (transgressive systems tract). (c) Slowing rates of relative sea level rise to early fall (highstand systems tract). Location of backstepping paralic delta fronts based on seam convergence lines from Branagan and Johnson (1970). Warbrooke (1981) Bowman and Whitehouse (1984). I = Borehole/Yard Coals, 2 = Yard/Dudley Coals, 3 = Dudley/Nobbys coals. (d) Relative sea level lowstand to initial rise (lowstand systems tract). Location of main fluvial channel based on Bolton Point Conglomerate. I = Bolton Point Conglomerate, 2 = Teralba Conglomerate, .? = Marks Point Conglomerate, 4 = Karignon Conglomerate (cf. Fig. 9 for stratigraphic context).

C. Herbert/Sedimenta~ Geology 107 (1997) 167-187 177

inarticulate brachiopods above and below the Won- gawilli Coal in the southern Sydney Basin (probably equivalent to the Hartley Hill Coal in the Newcastle Coal Measures; Herbert, 1995), and Scheibnerova (1980~) identified foraminifera from equivalent coal measures to the northwest.

Each parasequence is sheet-like, but appearing mound-like near the pinchout which is interpreted as the delta-front terminus where coal drapes and defines, to some extent, the original deposition slope on the surface of abandoned paralic deltas (e.g. Ps2 in Fig. 5 and Psl in Fig. 6). Inclined heterolithic strata, constituting each parasequence, can be seen downlapping onto coal (e.g. Ps 3, Fig. 5) and indi- cates progradational direction. It is considered that compaction of underlying thick peat by depositional loading steepened downlap inclination contributing to an increase in water depth at the toe of the prograding parasequence (Fig. 11). Rapid sedimentation is suggested by the presence of abundant ripple-drift cross-lamination, crossbedding, macerated plant de- bris, buried in situ trees, rare sporadic escape burrows, and lack of body fossils. Rapid lateral facies changes from shale to sandstone indicate proximal delta front and top delta fluvial facies.

Major avulsion-related fluvial crevasse-splay complexes, which are invading the Recent inland Cumberland Marshes in Canada (Smith et al., 1989), are similar in vertical profile to these paralic parasequences but, although extensive, are only a few metres thick. They are not considered an analogue of the thicker parasequences (lo-15 m) in the New- castle Coal Measures because (a) they are only a few metres thick, (b) they are prograding into an inland alluvial swampland rather than a subaque- ous environment on a marine coastal plain, and (c) they are clearly coeval with a peat mire; whereas the Newcastle Coal Measures parasequences were deposited after inundation of the entire peat surface. Base-level change, rather than autocyclic avulsion, is considered more important here. Relative sea level rise is supported by the common occurrence of in situ fossil trees up to 6 m high, with roots in the top of coal seams, that have been buried by the deposition of overlying parasequences. Conversely, relative sea level falls are indicated by erosional incision of the parasequences and subsequent deposition of fluvial conglomerate. The presence of

in situ fossil trees above these erosional surfaces also attests to exposure of the deltaic parasequences. Smaller-scale elastic splits within coals are probably the result of avulsion and are more likely to be similar to the Canadian analogue discussed above. Lacustrine crevasse-splay and crevasse sub-delta ori- gins for large-scale, heterolithic ‘giant’ crossbeds in the Late Permian coal measures of the Bowen Basin at Goonyella (Johnson, 1984) and Moura (Flood and Brady, 1985) are similar to the proposed paralic origin of the deltaic parasequences discussed here. An alternative point-bar origin for heterolithic sediments similar to these parasequences has recently been proposed by Fielding et al. (1993).

5.2. Conglomerates

Most conglomerates in the Newcastle Coal Mea- sures are characterised by large inclined bedding planes with dips up to 45” (principal surfaces of deposition, Sp, Diessel, 1992a) (Sp in Figs. llc, f). In the Redhead Conglomerate, inclined beds con- tain internal crossbeds with foreset dips up to 55” trending at right angles to the dip of the inclined beds (Sf in Fig. llf). Diessel (1992a, fig. 6.50) in- ferred that the principal surfaces of deposition were originally deposited as more gently inclined fluvial lateral accretion beds, and agreed with Britten et al. (1975) that compaction of the underlying peat during deposition steepened bedding into forms that mimic giant crossbeds (Fig. 11). The overall effect imparts a sigmoidal or convex upwards shape to each conglomerate body (Figs. 6-9). Other suggested ori- gins for the conglomerates, such as Gilbert deltas (Conaghan, 1982) and alluvial fans (Hunt and Hob- day, 1984) are at odds with the linear channelised geometry, erosional bases, and the lack of laterally equivalent finer-grained delta-front and prodelta deposits. Peat compaction induced and accentuated inclined or downlapping bedding, not only in fluvial conglomerates, but also in the previously discussed heterolithic strata in paralic deltaic parasequences. However, although displaying similar inclined bedding, they were deposited in different environments and, as suggested by Flood and Brady (1985), should not be forced into one palaeoenvironmental model.

In Sequence F, in the Newcastle area, most conglomerates were deposited in channel belts which, in

17s C. Herben/Sedimetmr) Geology 107 (19971 167-f%

RSL

Rising Falling

I

VH=lO 1

0 m L Approx

100m

Fig. II. (a-d) Schematic sections showing the deposition of 4th-order paralic delta parasequences and an alluvia! incised-valley-t% (circles) in Sequence F during progressive peat compaction and chanpin g relative sea level. Relative sea level curve at right of each panel shows the rising (R) and falling (F) inflection points and range of relative sea level (vertical line between horizontals). Horizontal to slightly inclined principal bedding surfaces (Sp) steepen as the underlying peat compacts during siliciclastic deposition. (e) Fully compacted section as seen in coastal outcrop at Redhead, based on the Merewether Conglomerate and parasequences Ps I and 2 in Fig. 6. (f) Plan view showing the orientation of the principal bedding planes (Sp) and internal fooresets (.Sf) in the conglomerate, after Diessrl (1992a, p. 334). Note that sediment supply for the paralic deltas is east to west (left to right) whereas the supply for the alluvial incised valley fill is at right angles, i.e. north to south. Vertical and horizontal scale is applicable to (a)-(c).


plan, were convex to the west and outline the distal terminus of paralic delta parasequences (Figs. 8, lOa, c and 11). Basal erosional surfaces are nearly parallel to the delta-front depositional slope and commonly cut through the entire parasequence down to the underlying coal (Figs. 6, 8 and 11). This sug- gests that the entire paralic delta front was subaeri- ally exposed to stream incision by a fall in relative sea level producing a 4th-order sequence boundary. Subsequently fluvial channels were constrained by the moats, formed by compaction of the underlying peat in front of the advancing delta-front slope, and filled with coarse alluvium (Fig. 11). Thus the conglomerates are considered here to be 4th-order incised valley fills.

According to Backing et al. (1988), conglomerates in Sequence H were deposited in braided alluvial channels as lateral facies equivalents of raised peats but with ‘a notable absence of overbank sediments’. The Bolton Point, Teralba, Marks Point, and Karignan Conglomerates were apparently deposited by the episodic eastward migration of successively younger channels (Fig. 9). After each episode of alluvial sedimentation, compaction of the underlying peat would have deformed the conglomerate into sigmoidal or convex-upwards bodies forming esker- like ridges over which blanket peat mires expanded. Subsequent alluvial channels were displaced to the east of these ridges in the direction of increasing subsidence (Fig. 12~). The Fassifern to Wallarah Coals and associated splits coalesce westwards to form condensed coal sections, equivalent to at least six episodes of fluvial deposition; each episode was probably initiated by a fall in relative sea level.

5.3. Coal

Coal is considered to have formed from peat mires which developed at the same time as, and adjacent to, areas of active siliciclastic sedimentation (e.g., Diessel, 1992a). However, stratigraphic relationships indicate that the major coals in the Newcastle Coal Measures were formed from extensive peat mires that blanketed abandoned sedimentary surfaces during a substantial decline in siliciclastic deposition. Similarly, Bamberry et al. (1989) concluded that in the coeval Illawarra Coal Measures of the southern Sydney Basin, [The Bulli and Balgownie] “Coals . .

exhibit very few signs of contemporaneous development with the fluvial sequence and hence, formed following abandonment of the fluvial setting”. War- brooke (1981), in a study of the lower Newcastle Coal Measures, also observed that although “Seam formation does not start and finish in all areas at the same time . . . . Time surfaces within the seam (pyro- elastic bands) show that the swamps were not significantly time transgressive, ie. seams did not slowly prograde through the area but tended to rapidly cover the entire area at one time” [my emphasis]. Also McCabe (1984), when discussing the formation of low-ash coals (as occur in the Newcastle Coal Measures) considered that the most important factor was that peat deposition was not contemporaneous with local elastic sedimentation.

Most abandoned sedimentary surfaces in the New- castle Coal Measures were probably flat except for gentle slopes on abandoned paralic delta-fronts and ridges caused by differential compaction over fluvial conglomerate and sandstone channel bodies. Peat mires which covered these surfaces were probably woody and raised in rain-fed (ombrotrophic) bogs at 70”s palaeolatitude (Embleton, 1984), similar to those forming today in the boreal wetland regions of Canada (Martini and Glooschenko, 1984, 1985). Only very minor amounts of peat may have been contemporaneous with deltaic or fluvial deposition. Most of the widespread coals in the Newcastle Coal Mea- sures are 2 m or more thick and, in some areas, ex- ceed 5 m. A 10: 1 compaction ratio for peat to coal (Ryer and Langer, 1980) implies decompacted 20-m to 50-m-thick peats. Autocompaction during growth would have decreased these thicknesses somewhat. Nevertheless, the raised peat mires probably built up the highest surfaces on the coastal plain, reducing or excluding alluvial siliciclastic sedimentation and pre- venting the marine shoreline from transgressing the coastal mires (as suggested by McCabe and Shanley, 1992 for the Cretaceous of Utah). No open marine sediments have been recorded to be intercalated with the 400 m thickness of the Newcastle Coal Measures. Peat aggradation, stimulated by a rising water table, probably kept pace with early rates of rising relative sea level. However, near the maximum rate of relative sea level rise, the peat mires were terminated by paralic inundation, not marine inundation. This indi- cates that an effective shoreline barrier to open ma-

180 C. Herbert/Srdirnentu~ Grolo~y 107 (1997) 167-187

rine conditions persisted during the highstands. Most thick peat mires in the lower Newcastle Coal Mea- sures (Sequences F and G) developed in the transgressive systems tracts of high-frequency, ilth-order sequences (cf. Model 2, Diessel, 1992b).

Coal seams generally become thicker from the lower to the upper part of the Newcastle Coal Mea- sures, indicating that longer periods of uninterrupted peat growth were available in the uppermost Se- quence H. This is considered to result from the increasing rate of 2nd-order falling relative sea level towards the end of the Permian during Newcastle Coal Measures time (Herbert, 1995). This successively slowed marine and paralic 4th-order transgressions which terminated peat-mires later and allowed longer periods for growth. Eventually during the deposition of Sequence H most transgressions failed to penetrate into the presently preserved basin. Thus, in Sequence H, peat mires continued to develop during both the transgressive systems tract and the highstand systems tract (i.e., for most of the relative sea level cycle), interrupted only during lowstands by relatively brief pebbly stream deposition or, at other times, by catastrophic ash falls.

From a study of coal lithotypes, microlithotypes, and macerals, Warbrooke (1987) suggested that peat mires evolved from low-lying, “brackish influenced, wet forest swamps” (Borehole to Victoria Tunnel Coals), through “fresh-water influenced, wet forest swamps” (Fern Valley to Upper Pilot Coals), and fi- nally to “fresh-water influenced, dry forest swamps” (Fassifern to Wallarah Coals). This is consistent with a coastal plain interpretation for Sequence F and for a landward, dominantly alluvial environment for Sequence H. High ratios of vitrinite to inertinite in coals indicate autochthonous, woody peat under consistently high water-table levels, whereas lower ratios indicate aerobic oxidation (Hunt, 1984). Hunt and Hobday (1984) used this criterion to confirm the upward stratigraphic trend to a lower water-table. Gould and Shibaoka (1980) found a similar trend and invoked the onset of aridity during deposition of the uppermost coals prior to the less humid or seasonal Triassic. All these changes are consistent with regional regression.

Intraseam elastic sediments thin westward from the present coast across the Lake Macquarie Syn- cline to the flank of the Lochinvar Anticline, and

coals converge and coalesce, reflecting faster subsidence in the east; possibly ten times faster (War- brooke, 198 1). Warbrooke ( 198 1) considered that individual coal seams split from west to east by ‘tectonic splitting’ and ‘sedimentary splitting’. However, the difference between the two types is probably one of scale. ‘Sedimentary splits’ were probably caused by the local short-term avulsion of fluvial channels into a continuously growing peat mire. In contrast Warbrookes’ ‘tectonic splits’ were developed between two temporally separclte peat mires by the deposition of areally extensive paralic deltas after rising relative sea level inundated and terminated the earlier, lower peat. The later, upper peat did not develop on the abandoned delta-top until relative sea level had fallen and begun to rise again. Peat also blanketed the delta-front slope and converged onto the surface of earlier defunct peats, as the delta thinned to the west (Fig. 8). Seam convergence lines (Fig. 10~) define the westward distal extent of deltaic parasequences (Fig. 8) and outline the lobate shape of the delta front. Repeated delta progradation and subsequent draping by peat mires led to the successive convergence of the Borehole, Yard, Dudley, and Nobbys Coals to form the composite West Borehole Coal in the western part of the Newcastle Coalfield (Fig. 8).

5.4. Tufl

Ubiquitous volcanic ash in the Newcastle Coal Measures and the Dempsey Formation was produced by explosive volcanicity (Diessel, 1992a) located about 30 km east of the present coast near the crest of the ‘Offshore Uplift’ (Bradley, 1993a,b). Intermittent ash falls and pyroclastic surges during the deposition of the Newcastle Coal Measures were preserved intact only on hiatal surfaces or in areas of starved siliciclastic sedimentation. For example, in the marine Dempsey Formation, most tuffs are found in the dark-grey to black shale of siliciclastic sediment- starved condensed sections. Thick tuffs, such as the Nobbys Tuff (15 m) and the Reids Mistake For- mation, terminated peat mire growth by smothering the vegetation. These tuffs probably correlate basin- wards with marine maximum-flooding-surfaces and represent non-marine condensed sections (Herbert, 1995). Tuffs also occur on hiatal surfaces on top


of paralic deltaic parasequences and conglomerates, and above, below, and within coals.

6. Regional palaeogeography

The Newcastle Coal Measures was deposited in a north-south half-graben, about 50 km wide, situ- ated between the synsedimentary flexural Lochinvar Anticline (Herbert, 1993) and the Offshore Uplift (Bradley, 1993a,b) (Fig. 1). As discussed above, the coal measures thicken eastwards into the subsiding Offshore Syncline and thin westwards onto the more slowly subsiding flank of the Lochinvar Anticline. The Newcastle Coal Measures also thin to the south in the direction of shoreline progradation and decreasing subsidence rates (Fig. 10a). Note that in this foredeep basin the most rapid subsidence took place to the northeast, towards the source of sediment supply, not in the seaward direction as in a shelf margin basin. Increasing subsidence rates in the landward direction provided increased accommodation space for the deposition of a thick paralic/nonmarine interval coupled with a laterally equivalent thin marine interval (the reverse of a shelf margin basin).

The Waratah Sandstone and Dempsey Forma- tion comprise stacked, marine shoreface parasequences that downlap to the southwest onto marine maximum-flooding-surfaces in the Dempsey Forma- tion (Herbert, 1995). Landwards, to the northeast, the shoreface parasequences pass into paralic deltaic parasequences, coal, and fluvial conglomerate of Se- quence F. The base of the Newcastle Coal Measures becomes younger to the southwest as it passes laterally into the marine Dempsey Formation at the top of the Tomago Coal Measures. The entire regressive interval, from a sequence boundary in the Dempsey Formation to the top of the Newcastle Coal Measures, was deposited as three 3rd-order depositional sequences (Fig. 2). Smaller-scale cycles of conglomerate to coal to paralic delta within these sequences are interpreted as higher-frequency, 4th-order sequences.

7. Relative sea level change and Sequence F

Interpreted depositional patterns are discussed and related to changes in relative sea level during the depositional of an idealised, single, 4th-order sequence within the 3rd-order Sequence F.

7.1. Relative sea level fall

As a fall in relative sea level exposed the coastal plain the lower base-level rejuvenated streams drain- ing the New England Orogen where a repository of gravelly alluvium had accumulated during the previous highstand. Hinterland tributaries incised the alluvium and flanking Piedmont fans to provide immediately available detritus for transport to the basin. The tributaries merged into a single trunk stream before they entered the Newcastle half-graben from the northeast (Fig. 10a). Finding the lowest topography, the trunk stream was deflected around the toes of abandoned and exposed paralic delta-front sediments in a moat created by compaction of the underlying peat (Fig. 11). Fluvial sediments crossed the coastal plain in confined channels that fed directly to the marine shoreline. Abundant sediment supply to the shoreline initiated seaward progradation of upward- coarsening marine shoreface parasequences of the Waratah Sandstone/Dempsey Formation in the early part of the 4th-order lowstand systems tract.

7.2. Rising relative sea level

Rising relative sea level caused as much as 40 m of alluvial gravel to aggrade in the channel incised during the previous fall (late lowstand systems tract). As the rise accelerated, the locus of alluvial deposition migrated upstream north of the basin, probably to hinterland valleys in the New England Orogen and fringing Piedmont fans. Correspondingly, sediment supply to the marine shoreline declined and the Waratah Sand- stone beach ridges were transformed into transgressive barrier islands above a transgressive surface. As siliciclastic deposition on the coastal plain declined the surface landward of the sandy barriers (Fig. lob) was covered by forested peat mires whose growth was stimulated by the rising water-table (transgressive systems tract). During the transgression sandy streams with only minor gravel intermittently crossed the peat mire to form the ‘sedimentary splits’ and elastic members as described by Warbrooke (1981).

7.3. Maximum rate of rising relative sea level

The vertical accumulation of peat initially kept pace with the early rates of rising relative sea level

182 C. Herbert/Sedimentary Geology 107 (1997) 167-187

and inhibited shoreline transgression. During the maximum rate of rise, near the rising inflection point on the relative sea level curve (R in Fig. 1 l), peat growth was insufficient to prevent inundation by expanding lagoons and lakes landward of the barrier complex. The paralic flooding surface across the top of the peat mire is marked by a lithologically sharp transition from coal to paralic/lacustrine prodelta siltstone. Trees on the surface of the peat mire were drowned and preserved in growth position or as fallen logs. At this time, or during the next phase, the barriers may have been submerged as shoals, but still protected the paralic environment from open marine processes (cf. the Mississippi delta plain ‘inner shelf shoals’ of Penland et al., 1988).

7.4. Relative sea level highstand

During relative sea level highstand lagoons and lakes, landward of the marine barrier shoreline or shoals, provided sufficient accommodation space for small paralic/lacustrine deltas to prograde westwards from the Offshore Uplift (highstand systems tract) (Fig. 10~; note that coastal cliffs near Newcastle mainly expose the northerly prograding sections of the deltas as shown in Figs. 5 and 6). Volcanoes from the Offshore Uplift not only showered the coal measures with air-fall and pyroclastic-surge ash but also provided volcano-lithic detritus by erosion of associated ignimbrite sheets (Jones et al., 1987). Any peat mires developed on the delta plain during this regressive phase were minor. High relative sea level caused the impounding of coarse gravelly alluvium in hinterland valleys of the New England Orogen while finer- grained sediment was trapped and deposited on the coastal plain. As a result the marine shoreface and

shelf was starved of siliciclastic sediments (Waratah Sandstone and Dempsey Formation respectively).

7.5. Relative sea level fall

Falling relative sea level ultimately terminated the deposition of paralic/lacustrine deltas. The entire coastal plain was then exposed leading to lo- calised fluvial incision and the creation of a 4th- order sequence boundary (Fig. 10a) above which another 4th-order sequence of conglomerate-coal- paralic delta could be deposited (Figs. 12a, b).

8. Relative sea level change and Sequence H

During the deposition of the 3rd-order Sequence H, 2nd-order relative sea level had fallen to such an extent that the marine shoreline was located out- side the present limits of the Sydney Basin and 4th-order, paralic transgressions rarely reached the Newcastle Coalfield. The interval is dominated by fluvial conglomerate deposited following base-level falls induced by 4th-order changes in relative sea level (Figs. 9 and 10d). Gravelly sediments were derived largely from the New England Orogen. Mi- nor sandy volcaniclastic detritus from the Offshore Uplift was either diluted by New England detritus or entered the basin south of the Newcastle Coalfield. During rising relative sea level and subsequent to fluvial deposition, new peat mires expanded basin- wide over the area occupied by previous alluvial-peat mire complexes. Without paralic transgressions peat growth could continue uninterrupted (except during ash falls and climatic change) throughout most of the relative sea level cycle (at least during the transgressive systems tract and the highstand systems tract)

Fig. 12. Sequence-stratigraphic elements of high-frequency, 4th-order sequences in the .?rd-order Sequence F (a-b) and Sequence H (c). Youngest sequence is depicted during HST deposition in (a) and (b), during LST in (c), and showin g uncompacted and compacting peat. For clarity, sequence-stratigraphic boundaries are shown only for the topmost sequence in each diagram, but should be extrapolated to apply for all repeated motifs in the diagrams. L.ST = lowstand systems tract, 7”T = transgressive systems tract, HST = highstand systems tract, SB = 4th-order sequence boundary, ivf = incised-valley-fill, ts = transgressive surface, ~fi = maximum flooding surface. (a) Section of the progradational upper part of Sequence F oriented N-S, approximately in the direction of LST marine shoreface progradation and decreasing subsidence (arrows), and perpendicular to HST progradation. The transgressive part of the barrier shoreface has low preservational potential and is prone to modification during subsequent relative sea level fall. (b) Section of the retrogradational lower part of Sequence F depicting only the coastal plain environments, oriented E-W, approximately in the direction of HST paralic parasequence progradation, coal seam coalescence, and decreasing subsidence (arrows). and perpendicular to LST progradation. (c) Section of the nonmarine Sequence H, oriented W-E, in the direction of successive migration of alluvial channels towards increasing subsidence (arrows).

C. tferber~/&ditnenrary Geology 107 (1997) 167-187

183

(4

Shale. tree Stump. coalfl

r3 Paralic delta pamsequences

pJ Marine shoreface parasequences

Lochinvar Anticline Lake Macquarie Syncline

E

Lochinvar Anticline

Lake Macquarie Syncline

E

184 C. Hrrhert/Sedirnentarv Geology 107 (19971 167-~187

until renewed fluvial deposition followed the next relative sea level fall. Thus the coals in Sequence H possibly represent an almost continuously develop- ing peat mire which was briefly, and only locally, interrupted and split by lowstand fluvial deposition restricted to discrete channels. This contrasts with Sequence F in which peat growth was restricted to the early rising part of the relative sea level cycle (transgressive systems tract only) and was terminated by paralic inundation near the rising inflection point (R in Fig. 11).

9. Sequence stratigraphy

The Newcastle Coal Measures was deposited during accelerating rates of relative sea level fall in the upper part of a highstand systems tract of a 2nd- order depositional sequence (Herbert, 1995). Accel- erating relative sea level fall accentuated stratigraphically higher 3rd- and 4th-order sequence boundaries, which successively became more obvious and were overlain by thicker and coarser-grained incised- valley-fill sediments until the end-Permian relative sea level fall (Herbert, 1993).

The highest-frequency sequences recognised here in the Newcastle Coal Measures are probably 4th-order and commence with an erosion surface (sequence boundary) overlain by fluvial incised- valley-fill conglomerate in a lowstand systems tract (Fig. 12). In most interfluvial areas, between incised valleys, coal and tuff directly overlie a coplanar sequence boundary and transgressive-surface. In Sequence F, incised-valley-fills probably pass seawards to marine shoreface/offshore parasequences of the Waratah Sandstone/Dempsey Formation, another component of the lowstand systems tract (Fig. 12a). The conformable submarine equivalent of the sequence boundary is located near the base of the marine parasequence, but just above the condensed section of the previous sequence. A transgressive surface separates the transgressive systems tract, barrier island part of the upper Waratah Sandstone from the lowstand shoreface sandstone below. The major coals with associated tuffs and minor clas- tic splits represent nonmarine equivalents of the transgressive systems tract and are separated from the underlying incised-valley-fills by transgressive surfaces. A marine maximum-flooding-surface over-

lies each Waratah/Dempsey parasequence, and an equivalent paralic maximum-flooding-surface overlies each coal in Sequence F (Figs. 12a, b), but not in Sequence H (Fig. 12c), which was mostly too far landward for paralic transgressions to reach. Paralic parasequences, which constitute the highstand systems tract in Sequence F, are rare in Sequence H where peat growth was probably continuous in both the transgressive and highstand systems tracts. Sili- ciclastic sediments in Sequence H were deposited during a brief period only in the lowstand systems tract whereas coal and tuff were deposited during a longer period in the transgressive and highstand systems tracts (Fig. 12~).

In shelf margin basins, where subsidence in- creases in a seaward direction, lateral movement of the bayline (paralic shoreline) away from the marine shoreline by regression and transgression produces a thick marine and a relatively thin paralic/nonmarine deposit. However, the Newcastle Coal Measures was deposited on the tectonically active side of a foreland basin where increasing subsidence rates were landward of the marine shoreline. Thus, whereas the marine shoreline was relatively static, the bayline could transgress much further landward, and for a longer time, creating greater amounts of subaerial accommodation space for the deposition of a thick non-marine/paralic section, but a correspondingly thin marine section. In this case marine shoreface parasequences were deposited only in the lowstand systems tract. During the highstand the marine shoreface was starved. because sediments were trapped on the extensive coastal plain and deposited as paralic delta parasequences. This contrasts with situations where marine parasequences were largely deposited in the highstand systems tract (Aigner and Bachmann, 1992) where subsidence rates increased seawards.

10. Conclusion

(i) Most models of coal-measure deposition re- gard alluvial, peat mire, deltaic, and shoreline environments as coeval, laterally equivalent facies. However, it is proposed here that the juxtaposition of three disparate lithologies in the Newcastle Coal Measures, such as coal, fluvial conglomerate, and upward-coarsening siltstone/sandstone deltaic


parasequences was caused by changes in relative sea level which initiated deposition of these facies at different times. In Sequence F, back-barrier par- alit deltas prograded during 4th-order relative sea level highstands and alternated with coarser-grained alluvial deposition during lowstands. Most thick peat mires developed as bogs, blanketing abandoned sedimentary surfaces between times of alluvial and par- alit delta deposition, coincident with a siliciclastic hiatus and stimulated by a rising water table.

(ii) Fourth-order depositional sequences com- menced with subaerial erosion surfaces (sequence boundaries) and were overlain by channelised fluvial conglomerate deposited in incised-valleys in lowstand systems tracts. Following lowstand fluvial deposition, peat mires extended over the entire non-marine area of the Newcastle Coalfield in transgressive systems tracts. In Sequence F, maximum flooding surfaces developed by submergence of peat mires and were overlain by prograding crevasse splays, crevasse subdeltas, and small deltas which extended into lagoons and lakes in highstand systems tracts. Marine shoreface parasequences were deposited during relative sea level lowstands, out of phase with paralic delta parasequences which were deposited during relative sea level highstands. In Sequence H, topographically higher areas, too far landward to be reached by paralic transgressions, supported peat mires which developed throughout both the transgressive and highstand systems tracts and were crossed briefly by streams which deposited gravelly alluvium in the lowstands systems tract.

(iii) Two source areas supplied sediment into the Newcastle half-graben. In highstand systems tracts an easterly source on the Offshore Uplift shed sandy volcanic detritus into the Newcastle Coalfield via streams feeding paralic and lacustrine deltas. In lowstand systems tracts a northern source in the New England Orogen shed gravelly volcano-lithic detritus via braided streams flowing in incised valleys. The supply from the New England Orogen was switched on or increased by a fall in relative sea level while supply from the Offshore Uplift was switched off or diverted elsewhere. A rise in relative sea level caused a reverse situation.

(iv) Multiple 4th-order sequences constitute three 3rd-order depositional sequences, which in turn constitute the Newcastle Coal Measures.

(v) The Newcastle Coal Measures was deposited during 2nd-order falling relative sea level, reflected in an overall upward regressive trend of increasing thickness and coarseness of fluvial conglomerate, and a corresponding decrease in coastal plain facies. Increasing up-sequence evidence for oxidation during peat mire development is consistent with lower water tables as a result of regional regression.

(vi) The deposition of thick, nonmarine/paralic sediments (Newcastle Coal Measures) and thin laterally equivalent marine sediments (Waratah Sand- stone and Dempsey Formation) resulted from higher rates of subsidence and subaerial accommodation in the landward direction, towards the proximal part of the foreland basin, and lower subsidence rates to seaward. In addition, high rates of peat aggradation probably inhibited the marine shoreface from transgressing the coastal mires. These combined effects led to the accumulation of as much as 400 m of siliciclastic sediments and peat in alluvial and paralic environments with no open-marine intercalations.

Acknowledgements

I thank R. Boyd, G.H. Bradley, P.J. Conaghan, C.F.K. Diessel, J. Hunt, and J.J. Veevers for review- ing the manuscript before submission and I thank T.A. Cross, C.R. Fielding, P.G. Flood and M.R. Gib- ling for final review. This study was supported by an Australian Research Council Special Investigator Award to J.J. Veevers.

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relative sea level control of deposition in the late permian newcastle coal measures of the sydney...

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