officer_structural_framework.pdf

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EASTERN OFFICER BASIN:STRUCTURAL FRAMEWORK FROM GEOPHYSICAL DATA

GEOINTERP CONFIDENTIAL REPORT 2003/2

ForOIL & GAS DIVISION

DEPARTMENT OF PRIMARY INDUSTRIES SAGrenfell St, Adelaide

L R RANKIN


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Geointerp 2003/1:- Structural Framework - Eastern Officer Basin PIRSA

This report and accompanying maps have been compiled by The Consultant from datasupplied by The Department of Primary Industries South Australia (PIRSA). Whilstevery effort has been made to carry out the work as diligently as possible, TheConsultant accepts no responsibility for technical or business decisions arising fromthis report and the accompanying maps.

Leigh R RankinDirector, Rankin Consultancy PLJune 2003


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Table of Contents

Table of Contents ............................................................................................. ii

1. EXECUTIVE SUMMARY................................................................................ 6

2.1. Preamble................................................................................................................. 9

2.2. Aims & Strategy ..................................................................................................... 9

3. DATA & INTERPRETATION METHODOLOGY .......................................... 14

3.1. Geophysical and Geological Data...................................................................... 14

3.2. Geological interpretation methodology............................................................. 17

3.3. Glossary of terms for magnetic data ................................................................. 21

4. RESULTS OF INTERPRETATION .............................................................. 23

4.1. Structural Framework.......................................................................................... 23

4.1.1. Basement.......................................................................................................... 23

4.1.1.1. Western Gawler Craton ................................................................................... 25

4.1.1.2. Ammaroodinna & Yoolperlunna Inliers ............................................................ 26

4.1.1.2. Coompana Block ............................................................................................. 32

4.1.1.3. Musgrave Block ............................................................................................... 32


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4.1.2.6. Sector 6 - Bitchera Ridge Boorthanna Trough.............................................. 59

4.2.1.7. Salt structures.................................................................................................. 59

4.2. Tectonic Development ........................................................................................ 61

4.2.1. Pre Off icer Basin............................................................................................ 61

4.2.2. Officer Basin Neoproterozoic ....................................................................... 62

4.3. Depth to Basement. ............................................................................................. 68

5. EXPLORATION TARGETing ...................................................................... 83

5.1. Officer Basin Hydrocarbons ............................................................................83

5.2. Basement Mineral Targets ............................................................................... 86

6. SUMMARY & RECOMMENDATIONS......................................................... 87

REFERENCES................................................................................................. 89

FIGURES

Figure 1. Outline of Officer Basin (green line) and adjacent regions of continentalAustralia (adapted from Gravestock, 1997).

Figure 2. Summary of current stratigraphic nomenclature for the eastern Officer Basin.

Figure 3. Summary of Archaean to earliest Neoproterozoic tectonic events basementto Officer Basin.

Figure 4. Schematic diagram of induced dipolar magnetic profile for a magnetic body atmoderate magnetic latitude.

Figure 5 Location of Middle Palaeozoic to Cainozoic basins of South Australia (from


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Figure 10. Total magnetic intensity image of Australia (after Geoscience Australia);Coompana-Isa Shear Zone highlighted.

Figure 11. RTP 1DV magnetic image, highlighting negatively magnetic maficintrusives of the Coompana Suite emplaced within the Coompana Block,Munyarai Subdomain and Gawler Craton.

Figure 12. Bouger gravity image (colour) superposed on RTP-1VD magnetic image.Significant NNW trending structures along the northern margin of the OfficerBasin are evident in the Bouger gravity data (black dashed lines).

Figure 13. Simplified tectonic sketch of the Musgrave Block (after Rankin & Newton,2002).

Figure 14. RTP-1VD magnetic image; basement subdomains highlighted.

Figure 15. General trend of Nurrai Ridge superimposed on Bouger gravity image(colour).

Figure 16. RTP-1VD magnetic image SE Officer Basin, highlighting interpretedintrabasement magnetic sources.

Figure 17a. RTP-1VD image Officer Basin; note high-frequency detail in magneticdata for shallow sectors of basin, particularly in the Ammaroodinna / MiddleBore Ridges & Tallaringa Trough areas.

Figure 17 b. Officer Basin structural framework superimposed on RTP-1VD image.

Figure 17c. Approximate location and boundaries of the tectonic sectors for the easternOfficer Basin.

Figure 18. RTP-1stVD magnetic image of Tallaringa Trough area.

Figure 19. Structural framework of Officer Basin superimposed on RTP-1VD magneticimage. Tallaringa Trough bounded by a) Karari FZ to SE, and b) complex NE-

trending fault zone to NW (part of Nawa Ridge complex).

Figure 20. Structural framework of Officer Basin. Red lines outline trend of regionalstructures evident in Bouger gravity data.

Figure 21. Officer Basin structural framework highlighting location of Nawa Ridge andBirksgate Coober Pedy Corridor


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Figure 27. Summary cartoons of Neoproterozoic Devonian tectonic development,

eastern Officer Basin.

Figure 28. SEEBASE model of depth to basement eastern Officer Basin (fromTeasdale et al, 2001).

Figure 29. Depth to magnetic basement and Eluder 2-D modelling (Calandro & Read,in press).

Figure 30. Location of modelled profile lines for SEEBASE depth to basement model(after Teasdale et al, 2001). Lines superimposed on TMI magnetic image.

Figure 31. Location of seismic lines eastern Officer Basin.

Figure 32. Seismic profile 93 AGS03.

Figure 33. Seismic profile 93 AGS-04.

Figure 34. Seismic profile 93 - AGS05.

Figure 35. Seismic profile 93 AGS06

Figure 36. Seismic profile 86)F-01.

Figure 37. Location of magnetic profile lines E-W 1-6 and N-S 1 & 2.

Figure 38. Location of magnetic profile line N-S 3 (Ammaroodinna Ridge area).

Figure 39. Structural framework of eastern Officer Basin. Several zones of intersectingregional NW & NE structures have been highlighted as potential loci forstructural trap development (including salt tectonic structures).

TABLES1. Datasets used .16

APPENDICES

1. Geophysical images.912. Selected magnetic profiles..99


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1. EXECUTIVE SUMMARY

A structural geological framework for the eastern Officer Basin (South Australia) wascompiled from combined regional and detailed magnetic data. The existing regionalBouger gravity, drillhole and selected seismic data were also integrated with theinterpretation. The structural framework of both the Officer Basin and the underlyingbasement were analysed within the interpretation.

Basement

The basement to the eastern Officer Basin comprises several major Precambriancrystalline terranes, with varying structural grains; these were developed duringseveral superposed orogenic events:

NW & W Gawler Craton (including Hughes Subdomain)Intense NE transpressive structural grain predominantly developed during theKimban (1850-1700Ma) & Kararan Orogenies (1600-1400Ma). The NE structural

grain is intersected (quasi-episodically) by corridors of variable-intensity E-Wtrending dextral shear, N-S sinistral transpressive shear and NW-trendingdilation. The Hughes Subdomain (interpreted here as the western margin of theGawler Craton) is dominated by a series of elliptical granitoids; these aretentatively correlated here with late Mesoproterozoic (1200 1050Ma?)intrusives within the Musgrave Block.

Munyarai SubdomainThis comprises a series of NE to NNE-trending tectonic belts (completelyconcealed by the Officer Basin). The Subdomain is transitional between theGawler Craton (South Australia) and Albany Fraser Orogen to the west(Western Australia). It is interpreted as equivalent to the Palaeoproterozoic Mesoproterozoic protolith to the Musgrave Block. A series of strongly magneticintrusives (interpreted here as Kulgeran Suite) extending south of the MusgraveBlock form the geophysically defined Nurrai Ridge.

Coompana Block

This is a rhombic to irregular zone of late Mesoproterozoic mafic intrusives andvolcanics (Coompana Suite) emplaced within and on the SW Gawler Craton. It iscorrelated here with the Tollu Volcanics (Bentley Supergroup) of the westernMusgrave Block.

Musgrave Block


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b) Gairdner Dyke Swarm (~800Ma). This is a major swarm of dykes of variablemagnetic character (dominated by positively magnetic dykes). The swarm

generally trends NW-SE, and intersects all of the basement domains to theEastern Officer Basin. The dyke swarm represents initial dilation / ?rifting ofthe crust prior to development of the Officer Basin and AdelaideGeosyncline.

Eastern Officer Basin

The eastern Officer Basin has been separated here into 6 structural subdomains,separated by both discrete structures and / or subtle structural corridors:

Sector 1:- comprises the Murnaroo Platform in the SW, and the NW-trendingWatson Ridge to the north. The Watson Ridge is a subtly expressed structuralcorridor which acted in part as a structural high during basin deposition. The SEend of the Ridge was involved with localised rifting and thicker sedimentationduring the Cambrian (associated with rift development of the Tallaringa Trough).

Sector 2:- Tallaringa Trough. This was initiated during Neoproterozoicsedimentation, but predominantly developed by NW-SE rifting during theCambrian (coupled with weak E-W sinistral shear along the trend of the CooberPedy Ridge). The Trough is separated from the main Officer Basin by the Nawaand Watson Ridges.

Sector 3:- Nawa Ridge. This is a NE-trending complex zone of rhombic faultblocks forming a structural high during both Neoproterozoic and Cambrian

sedimentation. Faulting is interpreted as predominantly transtensile. The Ridge isseparated from transpressive deformation developed to the north by theBirksgate-Coober Pedy Corridor (a subtly expressed SE- to ~E- trendingstructural zone).

Sector 4:- This comprises the central and northern sector of the basin, andincludes the deep Birksgate Subbasin and Munyarai Trough. The sector initiallydeveloped by NE-SW dilation during Neoproterozoic sedimentation, and was

subsequently overprinted by:a) N-S compression / NW dextral shear during the Petermann Orogeny

(550Ma)b) Cambrian and Ordovician sedimentation episodes (NW-SE dilation)c) Localised Devonian sedimentation and inversion (Alice Springs Orogeny

~400Ma)


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A brief comparison of existing seismic data and previous depth to basementmagnetic modelling has highlighted numerous discrepancies in the magnetic depth

model. A review of depth to magnetic basement along several selected magneticprofile, combined with the qualitative structural framework interpretation, highlightsseveral significant structural trends not evident in previous interpretations.

Exploration Potential

A series of structural trends with potential for hydrocarbon accumulation (structural

traps) have been highlighted in the current interpretation. These include potentialNW- and NNW-trending structural highs (including the Watson Ridge), andintersection zones of dilation / transfer fault zones (considered potential loci for saltdiapirism).


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2. INTRODUCTION

2.1. Preamble

The Neoproterozoic to mid-Palaeozoic eastern Officer Basin (part of the CentralianSuperbasin complex of Australia) covers an area of >100 000km2. The Basin issignificantly underexplored for both hydrocarbons and minerals, with only 7petroleum and 42 deep stratigraphic (mineral exploration) drillholes (PIRSA, 2001)within the region to date.

To promote petroleum exploration, the Oil & Gas Division of PIRSA has previouslycontracted studies on the available geological and geophysical data, including anatlas of geological interpretation maps based on seismic and drilling data (Lindsay,1995), a seismic interpretation study of the Marla & Munta areas (Mackie, 1994) andthe Officer Basin SEEBASE Project (Teasdale et al, 2001).

During 2001/2002, PIRSA acquired detailed magnetic data over the Musgrave Block

(at 200 400m line spacing). As part of the acquisition programme, detailedmagnetic data was also acquired over sectors of the northern Officer Basin.

Geointerp was contracted by PIRSA to review the structural framework of theeastern Officer Basin using the combined regional and detailed magnetic data, with aview to determining structural style and possible hydrocarbon leads within the SouthAustralian sector of the Basin. The location of the eastern Officer Basin and theinterpretation area are shown in Figure 1. A summary of the stratigraphy anddeformation history of the eastern Officer Basin is shown in Figure 2. A summary ofthe tectonic history of the basement to the eastern Officer Basin is shown in Figure 3.

2.2. Aims & Strategy

The principal aims of the project were:

Review the structural framework of both the eastern Officer Basin, and thebasement to the basin from the available magnetic and gravity data;

Where possible, indicate timing of specific structures;

Review the current depth to basement data produced by SRK (Teasdale et al,2001) and PIRSA (Calandro & Read, in press);

Highlight favourable zones or key structures for petroleum exploration.


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Figure 1. Outline of Officer Basin (green line) and adjacent regions of continentalAustralia (adapted from Gravestock, 1997).Area of current geophysical study shown in red.


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RANKIN CONSULTANCY PL 13

Figure 3. Summary of Archaean to earliest Neoproterozoic tectonic events basement to Officer Basin.


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3. DATA & INTERPRETATION METHODOLOGY

3.1. Geophysical and Geological Data

Datasets

The magnetic data used for the interpretation comprise a mosaic of several surveysof differing age, resolution and quality. The majority of the basin at present is covered

by poor-resolution, regional (1.6km) data; resolution of structure within these areaswas limited principally to basement structures in areas of variably magneticbasement. Intra-basin structures are poorly resolved, with some major structuresevident from major variations in thickness of sedimentary cover (and thereforefrequency or sharpness of magnetic anomalies). In the north and east of the basin,several recent surveys (400m-line spacing) provided high resolution of shallowstructure within the basin, as well as the basement structures.

Several significant levelling busts are evident within the regional (1.6km line-spaced) data. These linear discontinuities in amplitude and position of anomalies areartefacts caused by a) poor control on aircraft location, and b) temporal changes inthe amplitude of the Earths magnetic field between data collected on different daysnot adequately corrected for.

The Bouger gravity data for the basin comprises a coarse, regional dataset. Large-scale structures within both basin and basement are evident, but resolution ofdetailed structure is poor. It is suggested that the Bouger gravity data be reproduced

as a detailed colour contour image to assist further interpretation.

Seismic and drillhole data are limited throughout the majority of the basin. Themajority of data is concentrated in the NE of the region (Marla & Munta areas). Theseismic data was reformatted & reviewed by P Boult (PIRSA).

Depth to basement and stratigraphic information was taken from the PIRSA digital

drillhole database. Surficial geology was reviewed using the PIRSA digitalcompilation of 1:100 000 scale geological mapping.

Table 1 outlines the various datasets used. Magnetic and gravity images used arereproduced in Appendix 1.


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A short glossary of terms for magnetic data is provided in section 3.3.

Interpretation Scale

The scale of interpretation is dependent on 3 main factors:a) Purpose of project - is the interpretation designed to i) examine crustal-

scale features and broad tectonic domains, ii) highlight structure andlithological distribution at a prospect or district scale, or iii) target specificstructures, lithologies or geophysical anomalies for drilling;

b) Area to be covered (a detailed interpretation of an entire province may bedesirable, but will be dependent on time involved, and density of existinginformation);

c) Resolution of geophysical data 200m data is suitable for 1: 50 000 scaleinterpretation and smaller, but will be generally inadequate for larger scale interpretation, particularly where there is little detailed geology tointegrate with the geophysics.

Due to the regional nature of the proposed study, and the limited nature of anydetailed geological and geophysical data within the basin, a scale of 1:500 000 wasselected.


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Table 1. Datasets utilised for interpretation.

DATASET IMAGES (1:500 000 scale) COMMENTSMagneticsMagnetic data comprises numerous merged datasets of varying line spacing (1600mto 200m), line direction (both N-S & E-W) and resolution. Datasets were acquiredfrom 1956 to 2002.

RTP 1stVD GreyscaleRTP ColourRTP + RTP1st VD Composite image (greyscale 1stVD

with colour drape of RTP)TMI a) Colour (used for check on RTP

process and remanently- magneticsources). Sun-angle illuminated.

b) Greyscale (sun-angle illuminated)

GravityBouger Regional data of varying station

spacing (stations predominantly

acquired along access tracks).

GeologyPublished surface geology PIRSA digital dataset (from 1:100 000

scale digital geological maps).Drillhole data Displayed both as hardcopy and digital

images of location, with attachedstratigraphic log data.

SeismicVarious selected seismicprofiles from PEPSAdatabase

Data as Tiff scans of original seismicprofiles. Various scales.


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3.2. Geological interpretation methodology

Geological interpretation of the magnetic data followed the general interpretationmethodology outlined by Isles et al (2000), and routinely used by The Consultant.This methodology is strongly oriented to the use of a qualitative photogeological style approach to the magnetic data, rather than a quantitative, geophysicalmodelling approach. The following outlines a complete interpretation methodology formagnetic data.

Methodology

The interpretation methodology follows a series of steps in the compilation process.These steps are typically followed sequentially, although there is generally someiterative review of earlier phases during the compilation.

Observation Layer

The first step in the interpretation process is the compilation of an observationlayer (or worm map) from the magnetic data (Map 1). This involves therecording of the position of magnetic units. The 1stor 2ndvertical derivative of theRTP data is used for this process: the RTP data positions magnetic anomaliesover the nearest edge of the causative magnetic body (for normal, inducedmagnetism), while the vertical derivative sharpens the peak and maximumgradients of an anomaly (highlighting the centre or edge of a body). A greyscaleimage is typically used, as geometrical relationships are easier to resolve than in

colour images (physiological effect).

The observation layer provides a relatively objective series of observations ofmagnetic layering, contacts and zonation within the area. This is therefore kept asa separate overlay or digital file, and the interpretation is compiled using theobservations as a reference.

Note that the vertical derivative highlights shallow (high frequency) magnetic

features at the expense of deeper (low frequency) magnetic signatures. Lowfrequency (deeper source) features are typically recorded from a combination ofthe 1stVD and the regional RTP data.

Note:- Map 1 highlights magnetic trends from both basement and sedimentarycover sources. These have been separated on Maps 2 & 3. Many of the very


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Structural Framework

The compilation of the structural framework is typically much more subjectivethan recording the magnetic trends within the observation layer. It is thereforecompiled on a separate overlay: this can be altered as the interpretation conceptschange, or further data comes to hand.

The structural framework comprises an interpretation of all the structuralelements either directly observable in the magnetic data, or interpretable from thedata and other information. This includes:

a) Faults / shears: These are commonly separated into major & minorstructures, depending both on strike extent and displacement. Theposition of faults and shears within the magnetic data may bedirectly observed by the presence of magnetic anomalism alongthe structure (magnetite addition or destruction), or the presenceof magnetically anomalous intrusives within the structure (eg maficdykes). However, the majority of faults / shears are interpreted bythe presence of discordant terminations or inflexions within the

observed magnetic trends. Note the confidence with which theorientation, or even the existence of a particular fault may beinterpreted commonly decreases with increasing scale for any onedataset.

Many of the fault zones interpreted within or at the base of thenonmagnetic basin sequences have been interpreted by subtlechanges in frequency and resolution (fuzziness) within the

underlying basement magnetic anomalies (due to changes indepth to magnetic source).

b) Geological contacts: These may include conformable andunconformable contacts, intrusive contacts and unrecognisedfaulted contacts. It should be noted that there may be severaldifferent possible geological interpretations for any one series ofmagnetic trend patterns. For example, a discordant contact

evident in the magnetic data may represent an unconformity, faultor intrusive contact.

The structural framework is typically compiled using a combination of thegreyscale vertical derivative data, and the observation layer. The colourcomposite RTP-1stVD image is also typically used to highlight subtle structures atthi t


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texture may relate to layered vsmassive magnetic character, magnetically flatvsnoisy etc. Texture is commonly best observed in the greyscale imagery. The

overall magnetic intensity of different units is also used to define differentlithologies. Note that lithological discrimination based solely on magnetic intensityis generally invalid; there are no unique magnetic susceptibilities for anyparticular lithology, and numerous lithotypes can have the same susceptibilities(see Grant, 1985a,b; Clark, 1983).

Note also that the different lithologies interpreted in a solid geology map areapproximations only. Typically, each lithological zone comprises a suite ofdifferent rocks as recognisable at mapping scale. Any one lithology or domain

within the interpretation therefore represents a localised grouping of individuallithologies and secondary geological processes. The magnetic data typicallyhighlight not only primary lithologies, but also secondary processes, such asmetamorphism, metasomatism and diagenesis.

A detailed solid geology map for the basement lithologies has not been compiledhere. Rather, both basin and basement domain maps have been compiled,highlighting the various tectonic domains.

Potential inaccuracies in compilation of solid geology maps noted in previousregional interpretations are:

a) Inappropriate matching of similar magnetic responses over largearea (there are no unique magnetic signatures for particularlithologies See Grant, 1985a,b);

b) Matching of a magnetic signature to a volumetrically insignificant,but outcropping unit in areas of poor outcrop.

c) Lack of recognition of secondary processes (deformation,alteration within both outcrop and magnetic signature).

Tectonic Summary

A summary of the interpreted principal tectonic elements for each scale of

interpretation is generally compiled on a separate overlay. This process is highlyinterpretative (and therefore subjective). 1st order structures bounding litho-tectonic domains, and 2ndorder transfer fault systems are typically highlighted. Inaddition, large-scale intrusive complexes, including possible concealed plutoniccomplexes are commonly outlined. Subtle structural zones or corridors may bealso observed or inferred at this stage. Commonly such structural corridors

i i l li t f tl t d t d t t


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such as mapped lithology, alteration, geochemical data (etc) is integrated with thegeophysical interpretation.

The known geology of the Basin was integrated with the structural frameworkusing both the mapped geology and the drillhole database. Existing seismic datawas reviewed against the interpretation.


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3.3. Glossary of terms for magnetic data

The following terms are used commonly throughout this report. For a comprehensivedescription of the key elements of magnetic data, processing and imaging, the readeris referred to Isles et al(2000).

Total Magnetic Intensity (TMI):-

Amplitude of Earths magnetic field as measured at any given point. Thismeasurement is the sum of both the Earths regional field strength, and the local fieldproduced by magnetic sources within the local crust. The TMI data has typically beenprocessed to remove non-geological noise from the signal prior to interpretation.

Both the Earths regional field and magnetic fields from crustal sources are vectorquantities: the sum of the 2 fields is controlled by both magnitude and direction (seeFigure 4).

The orientation of the Earths magnetic field varies around the globe. This produces

different geometries of (induced) magnetic anomaly at different magnetic latitudes; At the magnetic poles (Inclination = 90o), the field is vertical. Induced

magnetic anomalies appear as simple peaks (or troughs), with the peak(or trough) directly over the centre (or nearest edge, depending on width)of the magnetic body.

At other magnetic latitudes, the anomalies form characteristic dipolarprofiles. Both the peak and the trough of the anomaly are migrated awayfrom above the centre of the causative body. The position of the maximum

gradient along the anomaly indicates the position of the centre or nearestedge of the body. The lower the magnetic latitude, the greater the dipolarnature of the anomaly.


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4. RESULTS OF INTERPRETATION

4.1. Structural Framework

The magnetic and gravity data for the eastern Officer Basin area highlights structureswithin the Basin, the underlying Archaean (?)-Mesoproterozoic basement, and thesurrounding basement domains.

The internal structural framework of the Musgrave Block and the majority of theGawler Craton has been highlighted in numerous previous studies (including Rankin& Newton, 2002; Daly et al, 1994; Teasdale et al, 2001). Although these have notbeen replicated within this study; the principal structural elements are summarisedbelow, along with the current interpretation. A description of the structural frameworkof the basement to the Officer Basin is considered essential to an understanding ofthe later development and deformation of the Basin.

The following description of the structural framework has been divided into structureswithin the basement, and those interpreted to influence the basin architecture (bothduring deposition and/or deformation).

4.1.1. Basement

The eastern Officer Basin is bounded by several different crystalline basementterranes (see Maps 2 & 3). These are:

Western Gawler Craton: - the Basin is both in faulted contact along its SEmargin (along the Karari Shear Zone), and onlaps the Craton in the NEand south. The western Gawler Craton comprises the Christie, Nawa,Coober Pedy Subdomains, the Fowler Suture Zone, the SW GawlerCraton, and the Ammaroodinna and Yoolperlunna Inliers.

Munyarai Subdomain: (name modified from Teasdale et al, 2001)- this is aNE-trending domain beneath the central Officer Basin.

Hughes Subdomain: (new name) - this is a NE-trending belt concealed bythe eastern margin of the Officer Basin. This domain is consideredtransitional to the western Gawler Craton.

Coompana Block: - the Basin onlaps the Coompana Block to the south.The Coompana Block is entirely concealed by the later Denman andE l B i (Fi 5)


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The basement terrane terminology of Teasdale et al (2001) has been modified in thisinterpretation, due to differences in interpretation of specific basement domainboundaries.

ConcealedCoom ana Block


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4.1.1.1. Western Gawler Craton

The structure of the western Gawler Craton is dominated by:

a) NNE - to NE-trending Archaean Mesoproterozoic gneissic belts of theChristie and Nawa Subdomains and Fowler Suture Zone. Major orogen parallel to subparallel shears trend NNE to NE, with both Archaean andProterozoic folding trending N- to NNE. Intense folding and shearingwithin the Fowler Suture Zone and the NW margin of the ChristieSubdomain are likely related to NW collision and transpressive wrenchingalong the western Gawler Craton during the 1600 1400 KararanOrogeny (see Figure 6). The Karari Shear Zone (Figure 6, Map 2) likely

was initiated as a terrane boundary between an Archaean Palaeoproterozoic cratonic nucleus in the east from a Proterozoic mobilebelt to the west. It is interpreted to have undergone numerousreactivations, including possible folding about a regional N-trending foldon the western margin of the exposed Craton (Figure 6). Orientation ofsecond-order folds adjacent to NNE trending shears in the Fowler SutureZone suggest a strong component of dextral shear. This wasaccompanied by development of a (? Palaeoproterozoic) wrench grabenalong the Coorabie SZ (BARTON Mapsheet; see Figure 7).

b) SW Gawler Craton. This subdomain is completely concealed by OfficerBasin sediments. It is described in detail in section 4.1.1.4.

c) E-W trending structural belts. There are several E-trending, regional-scalezones of dextral(?) transpressive shear that intersect the western andcentral Gawler Craton (see Figure 8). Two of the most obvious structuralcorridors are the Tarcoola and Coober Pedy Corridors. These likely reflect

earlier (Archaean Palaeoproterozoic) structures, reactivated as dextraltranspressive zones during the Kararan Orogen. These corridors wouldhave allowed bulk lateral expulsion of crust during the NW-vergingcollisional orogeny. The E-W structural corridors occur as quasi-episodicstructures throughout the Gawler Craton: the Polda Trough (Palaeozoic toMesozoic sedimentation) in the southern Gawler Craton represents areactivated Proterozoic dextral structural zone.

d) NW-trending dykes of the Gairdner Dyke Swarm (~800Ma?). These cutacross the entire Craton, and extend beneath the Officer Basin into theMusgrave Block (Figure 9). Some N- to NNE- trending dykes also occur inthe western Gawler Craton: these may represent a separate dyke swarm.

e) N-trending regional fold (Figure 7). The SW limit of the Gawler Craton


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4.1.1.2. Ammaroodinna & Yoolperlunna Inliers

The Ammaroodinna Inlier is a strongly magnetic NE-trending subdomain along themargin of the Gawler Craton. Its close proximity to the Musgrave Block suggests itlikely has an early Musgravian Orogeny structural overprint. However, the lack of astrong E-W structural grain within the Inlier implies it has not been strongly affectedby the late Musgravian (1080-1050Ma) and Petermann (550Ma) Orogenies.

The Yoolperlunna Inlier, (NW of the Ammaroodinna Inlier), lies along the inferrednorthwestern margin of the Gawler Craton. It has a highly variable magnetic

character, and includes both haematite and tourmaline breccia bodies. The exposedInlier is coincident with a broad NW-trending structural corridor, extending from theNE Gawler Craton: this structural corridor is interpreted to have acted episodically asa province-scale dilation zone both during and after the Kararan Orogeny. Thisstructural corridor was subsequently reactivated as part of the PalaeozoicBoorthanna Trough (see Maps 2 & 3).

The Ammaroodinna & Yoolperlunna Inliers are interpreted here as transitional

subdomains between the Western Gawler Craton (NE-trending structural graindominated by Kimban and Kararan Orogeny deformation), and the Musgrave Block /Munyarai Subdomain (deformational fabric strongly overprinted by early MusgravianOrogeny (1200Ma).


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Figure 6. RTP-1VD magnetic image of Officer Basin and surrounding basementdomains.

Intense NW-trending structural grain, plus rotated fold axial trends withinChristie Subdomain and Fowler Suture Zone (not obvious at this scale)suggest transpressive wrench deformation during Mesoproterozoic along NE- trending shear zones.Y ll li hi hli ht i l N t di f ld i l t d


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Dextralwrench basin


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Figure 8. TMI image of SA (from Geoscience Australia).Solid black lines highlight major Mesoproterozoic E-W dextral transpressivestructural zones within Gawler Craton. Note Polda Trough episodicallyreactivated in Neoproterozoic to Mesozoic.Black polygon (dashed) outlines area of current interpretation.


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Figure 9. RTP 1st VD magnetic image highlighting mafic dykes of the GairdnerDyke Swarm.

These are evident as narrow, positively magnetic NW-trending bodies. Greenarrows highlight two of the highest density zones of dyke emplacement.

Gairdner Dyke Swarm

Gairdner Dyke Swarm

G i t 2003/1 St t l F k E t Offi B i PIRSA


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Mt Isa Inlier

Geointerp 2003/1: Structural Framework Eastern Officer Basin PIRSA


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4.1.1.2. Coompana Block

The Coompana Block (see Figures 6 & Map 2) comprises a rhombic to irregularsubdomain of mafic volcanics and sediments overlying gneiss and granitoids of theGawler Craton and Munyarai Subdomain(?). Major mafic- ultramafic intrusives, likelycoeval with the volcanics, were emplaced within the felsic basement and the volcanicpile. A K-Ar age of 1159Ma (biotite-hornblende in gneiss; Webb et al., 1982)suggests that the Coompana Block represents a southern zone of MusgravianOrogeny (equivalent) overprint along the far-western Gawler Craton, with a majorepisode of mafic magmatism during the latter stages of the Orogeny. The mafics may

therefore be compared to the Giles Complex & Tollu Volcanics in the westernMusgrave Block (see Figure 3). The presence of volcanics indicates the CoompanaBlock region was emergent at least during the early Musgravian Orogeny (~1200Ma).

The Coompana Block is dominated by a series of NE and NW trending ?brittle faults(narrow / linear traces). The NE trending faults are parallel to possible layeringwithin the volcanics, and to the underlying gneissic fabric. A series of NNW trendingnegatively magnetic dykes are associated with a major series of sub-circular mafic-

ultramafic plutons (herein informally named the Coompana Suite; Map 2 & Figure11). The Coompana Suite are not restricted to the Coompana Block, but are alsoemplaced within the western Gawler Craton, and to the NW beneath the central andnorthern Officer Basin. The dykes also continue beneath the Officer Basin. Theseintrusives are considered older than the Gairdner Dyke and are likely related toanomalous, negatively magnetic mafic intrusives within the Musgrave Block (seeRankin & Newton, 2002). The northern margin of the Coompana Block is poorlydefined, being concealed by increasing sediment cover. It may be coincident with amajor NW positively magnetic dyke swarm extending from the Head of Bight coastalarea (South Australian) NW across into Western Australian (Figure 11).

The NNW dykes are parallel to a series of NNW-trending basement / basin structureswithin the Officer Basin area evident in the Bouger gravity data (Figure 12). Thisimplies that the NNW structures evident near the northern margin of the Officer Basinmay have developed during the late Musgravian Orogeny.

4.1.1.3. Musgrave Block

The following description is a brief summary of the tectonic framework of theMusgrave Block (see Figure 13). A comprehensive description of the structure of theMusgrave Block is given in Rankin & Newton (2002).The structural framework of the Musgrave Block is dominated by:


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Figure 12. Bouger gravity image (colour) superposed on RTP-1VD magnetic image.

Significant NNW trending structures along the northern margin of theOfficer Basin are evident in the Bouger gravity data (red dashed lines andhatching).

These are parallel to the Mesoproterozoic NNW trending negatively magnetic dykesassociated with the Coompana Suite to the south. The NNW structureswithin the basin are therefore interpreted as being initiated during the late



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RANKIN CONSULTANCY PL 35

Figure 13. Simplified tectonic sketch of the Musgrave Block (after Rankin & Newton, 2002).Early NE-trending tectonic fabric (Palaeoproterozoic - Mesoproterozoic) overprinted by intense E-W and NW-trending shears associated with

Musgravian Orogeny (1200 1050Ma) and Petermann Orogeny (550Ma). Levenger and Moorilyanna Grabens developed as wrenchgrabens along Mann Hinckley SZ. Southern margin of Musgrave Block thrust over Officer Basin to the south during both thePetermann (550Ma) and Alice Springs (350Ma) Orogenies.



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p

4.1.1.4. Concealed Basement

The concealed basement beneath the Officer Basin is magnetically variable, withboth linear (gneissic)? belts and elliptical, zoned granitoids evident.

The basement may be subdivided as below (see Map 2, Figures 14, 15).

Note: large areas of the Nawa Subdomain and Ammaroodinna and YoolperlunnaInliers are also concealed beneath variable thickness Officer Basin sediments onthe eastern margin of the Basin (see sections 4.1.1.1. & 4.1.1.2)

a) SW Gawler Craton.The SE margin of the Officer Basin conceals a subdomain ofthe Gawler Craton characterised by an overall low (quiet) magnetic signatureand a low Bouger gravity signature (similar to the granite dominated zones ofthe central Gawler Craton).

Several intensely magnetic bodies appear to trace a folded shear zone throughthe subdomain (?proto-Karari SZ?). These magnetic bodies are modelled at

~2500 3000m depth (from magnetic profiles E-W 1 5; Figure 36). Thesedepths have been previously interpreted as reflecting the presence of asignificant, narrow N-trending subbasin / canyon intersecting the basement(Teasdale et al 2001). The deep magnetic sources are interpreted here asintrabasement sources (Figure 16); note that similar deep seatedintrabasement magnetic bodies also occur throughout the eastern Gawler Craton.

The inferred granitoids within the subdomain may be either a) late KararanOrogeny intrusives (1600 1400Ma), or b) a series of unrecognised lateMesoproterozoic intrusives (Musgravian / Grenvillian age). A similar lateMesoproterozoic age is inferred for zoned granitoids within the HughesSubdomain (see below).

b) Hughes Subdomain. This is a major NE-trending belt, dominated bynonmagnetic to weakly magnetic elliptical intrusives, with occasional stronglymagnetic rims. The magnetic character and shape of the intrusives is bestexpressed in the south and centre of the domain (Watson Ridge - Murnaroo

Platform area (Map 3). To the NE, increasing sediment thickness obscures thebasement character.

The NW margin of this subdomain is regionally coincident with a continental scale structural corridor (evident in magnetic and gravity datasets), extendingfrom the northern margin of the Coompana Block in the SW to the southern



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Figure 14. RTP-1VD magnetic image; basement subdomains highlighted.

The far-western Gawler Craton, Hughes & Munyarai Subdomains are concealed byOfficer Basin sediments. The NW Gawler Craton (including the NawaSubdomain and Yoolperlunna & Ammaroodinna Inliers) is partly concealedby Officer Basin sediments.

Th N i Rid ( ll li ) i li ti b lt ith



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Figure 15. General trend of Nurrai Ridge superimposed on Bouger gravity image(colour).

The magnetic ridge is near coincident with a significant gravity high belt. This isinterpreted as a Mesoproterozoic subdomain, in part reactivated by NNEtrending block faults during development of the Officer Basin.



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Figure 16. RTP-1VD magnetic image SE Officer Basin, highlighting interpretedintrabasement magnetic sources. Yellow lines show approximate boundaryof deep (+3000m?) sedimentary trough interpreted by Teasdale et al(2001).

Profile modelling of the magnetic data indicates depths of 3000m for the positive



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c) Munyarai Subdomain. This comprises a major portion of the basement to the

eastern Officer Basin, and extends into the WA sector of the basin (see Figure 14& Map 2). The magnetic character of the basement is poorly defined, due tosignificant basin cover. The most significant structural elements evident in themagnetic data are (see Map 2):i) A series of NNE-trending elliptical intrusives (nonmagnetic, with

moderately magnetic rims, similar to those within the HughesSubdomain).

ii) NNE- to N- trending, arcuate fault / shear zones. These are oblique to the

major domain-bounding Coompana-Isa SZ. They likely representMesoproterozoic fold / thrust belts, verging to the WNW, and extendinginto the NE trending Palaeoproterozoic Mesoproterozoic fold / thrustbelts of the Musgrave Block. There is some evidence of NNE trendingregional scale folding; this may be related to major dextral shear along theCoompana Isa SZ.

iii) Nurrai Ridge. This is a series of significant, discontinuous linearmagnetic bodies extending from to Coompana-Isa SZ to immediately

south of the Musgrave Block (Figures 14, 15). The magnetic bodies arenear coincident in the north with a NNE-trending gravity gradient (gravityhigh block to east). They have been previously interpreted as an irregularbelt of mafic intrusives (responsible for the gravity high; see Teasdale etal, 2001). The current interpretation suggests that the gravity gradient isthe western edge of a wide basement block, rather than being causedspecifically by the magnetic bodies. The magnetic bodies appear to occurin 2 belts, separated by an arcuate N- to NNW shear zone (Map 2). Thereis some suggestion in the data that the northern magnetic belt may befolded around a N-trending fold axial trace to the east (Map 2, Figures 14,15). There are no significant mafic intrusives within the Musgrave Blockimmediately north of the Nurrai Ridge; however, the magnetic belt isnear coincident with a NNE to NE trending belt of magnetic KulgeranSuite granitoids within the southern Musgrave Block (see Rankin &Newton, 2002). It is suggested here that the Nurrai Ridge represents aseries of Kulgeran Suite intrusives (felsic to intermediate?) aligned along aseries of NNE-trending Palaeoproterozoic Mesoproterozoic shear

zones, (subsequently reactivated as horst / graben faults within the OfficerBasin).

The Munyarai Subdomain has been separated in Map 2 into:a) Munyarai Subdomain 1 (eastern sector); this subdomain is dominated by N- to

NNE str ct ral trends ith some e idence of NNE trending folds ithin the



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Intrusives & Basin Development

The eastern Officer Basin is flanked to the north by a major domain of anorogenic

granitoids (the Kulgeran Suite) emplaced within the Musgrave Block between 1200 1050Ma. The current interpretation highlights the presence of a major granitoid suitebeneath the eastern flank of the basin (Hughes Subdomain; see Map 2), possiblyextending well beneath the central basin (Munyarai Subdomain). The weak structuralgrain of the intrusives suggests emplacement post Kararan Orogeny (post -1400Ma).

Klein (1995) has proposed that Neoproterozoic intracratonic basins in severalcontinents developed in response to rifting above partial melting of lower crust and

intrusion of anorogenic granite during Neoproterozoic breakup of a supercontinent.The presence of the anorogenic granitoids is commonly concealed by lack of databeneath the sedimentary sequence. Cooper (1990) has documented the cyclicrecurrence of paired anorogenic granitoids and tholeiitic basal associated withintracratonic basin formation.

It is tentatively suggested here that the major suite of granitoids interpreted beneaththe Officer Basin are closely related to the Kulgeran Suite granitoids of the Musgrave

Block. Initial development of the basin was controlled by thermal weakening of thecrust during and post granitoid emplacement, rather than as a crustal warp / sagresponse to continental-scale N-S compression (as outlined by Teasdale et al, 2001).It is possible that some of the interpreted Adelaidean sedimentary sequence in thedeeper parts of the basin may include latest Mesoproterozoic earliestNeoproterozoic sediments / volcanics.



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4.1.2. Officer Basin

Structures evident from the magnetic data within the eastern Officer Basin aregenerally separated into several major tectonic subdomains: these are a combinationof both syn-depositional and syn-deformational structures. Detailed structure is moreevident in the shallower sectors of the basin; in particular the Ammaroodinna /Middle Bore Ridge area and Tallaringa Trough. Due to both the lack of magneticmarker units throughout a large proportion of the basin, and the weaker magneticcharacter of the basement within the western sectors of the basin, definition ofstructure in the thicker parts of the basin is at best poor in the magnetic data (see

Figure 17a,b). In addition, the higher density of structure evident in the NE of thebasin may also reflect a much stronger deformational regime within this area. Thestructural elements described below are highlighted in Map 3. The location of thesectors described below is shown in Figure 17c.

4.1.2.1. Sector 1 (SW region Murnaroo Platform A & Watson Ridge)

The SW sector of the basin is dominated by near orthogonal NE- & NW-trending

faults. The NW structural grain was developed as a series of normal faults duringinitial development of the basin in the Neoproterozoic, and cuts the earlier NE- toNNE- trending basement structural grain. A series of minor horst grabencomplexes with NW-SE trending axes were developed along a gently S shallowingbasin platform.

The NW-trending faults are sub-parallel to oblique to many of the (NW to NNW) faultsteps along the faulted / overthrust northern margin of the basin, and to the NNW-trending negative magnetic dykes of the Coompana Block.

The Murnaroo Platform (old name see Gravestock, 1997) has a general trend ofshallowing to the S & SE, with onlap of the basin sediments onto the CoompanaBlock & Gawler Craton. The overall shallowing of the platform is complicated bydissection of the region into several structural subdomains, principally superimposedduring Cambrian NW-SE dilation and sedimentation.

a) Sector 1a. This is the SW sector of the Murnaroo Platform (Murnaroo Platform A

on Map 3), and represents a platformal zone of both Neoproterozoic andCambrian sediments onlapping the crystalline basement to the south.

The trend of shallowing / thinning to the south is coincident with thickening to theNW due to NE block faulting during Cambrian sedimentation.



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The Watson Ridge includes a fault block of Adelaidean sediment which likelyacted as a structural high during Cambrian sedimentation (Map 3). The belt iscoincident with the appearance of horst block / rollover structures within at least 2

seismic sections (sections 93AGS 4& 5; see Figures 33, 34).

It is proposed here that the Watson Ridge acted a horst / ridge within theNeoproterozoic basin. The ridge has then been disrupted / overprinted by NE-trending block faulting, which controlled variable thickness deposition ofCambrian sediments.

The Watson Ridge continues to the SE, where it abuts the SW end of the NE-SW

trending Nawa Ridge (a NE-trending structural high during Neoproterozoicsedimentation; see Figure 21). The two ridge complexes form an orthogonalmargin to the northern depocentres of the Officer Basin.

At the SE end of the Watson Ridge, there is a localised thickening of the basinsediments (in part coincident with inferred subbasin / canyon of Teasdale et al,2001; see Figures 16 & 36). The sedimentary sequence appears to increase from~500-600m to ~1000m within several downthrown rhombic fault blocks. The

Watson Ridge appears to have been locally downthrown in this zone duringCambrian sedimentation. The zone of thicker sedimentation was offset from, butdeveloped in conjunction with, the Cambrian sedimentation within the TallaringaTrough. This change in sediment thickness along the ridge coincides with a bendalong the Watson Ridge from NW to NNW.

The Watson Ridge is interpreted to have acted as a basin transfer zone duringCambrian (and later) sedimentation.

4.1.2.2. Sector 2 (Tallaringa Trough, eastern region)

The Tallaringa Trough is a narrow NE-trending graben, which was active during bothNeoproterozoic, and (principally) Cambrian deposition. It is separated from the mainOfficer Basin by the Nawa Ridge a NE-trending structural high comprising acomplex series of rhombic fault blocks. These include blocks of:

a) Gawler Craton crystalline basementb) Neoproterozoic sediments with no Cambrian cover

c) Cambrian sediments unconformably overlying Gawler Craton basement.

At the SW end of the Trough there is also a complex zone of structural highscomprising crystalline basement and Neoproterozoic sediments (Map 3). These arecoincident with the SE end of the Watson Ridge.



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Structure within the Trough is dominated by a series of narrow rhombic fault blocks,typically defined by intersecting N to NNE & ENE trending faults. A series of NNWtrending faults intersect the western end of the trough: these are parallel to the SE

end of the Watson Ridge, and coincide with several regional structures evident in theBouger gravity data (see Figure 20). Tectonic style within the Trough appears to beprimarily dilational to transtensile, both during and post sedimentation. Similarly, theNawa Ridge appears to have acted principally a transtensile horst.

The Nawa Ridge & Tallaringa Trough are structurally partitioned from thetranspressive regime of the Ammaroodinna & Middle Bore Ridge areas by a subtle,broad E-trending structural zone, extending from the Birksgate Subbasin to the

Coober Pedy Subdomain (the Birksgate-Coober Pedy Corridor; see Map 3).



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Figure 17 b. Officer Basin structural framework superimposed on RTP-1VD image.Note contrast in resolution of structure in east / northeast of Basin (shallow basementwith high density of magnetic anomalies, and western sector of Basin (thickcover over relatively magnetically quiet basement).



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Figure 17c. Approximate location and boundaries of the tectonic sectors for theeastern Officer Basin.



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Figure 18. RTP-1stVD magnetic image of Tallaringa Trough area.The Palaeo- to Mesoproterozoic Karari SZ is outlined by a series of

intensely magnetic linear bodies. The southern fault margin of theTallaringa Trough is represents a late reactivation of this structure. Locally,the Palaeozoic sector of the fault zone (red line) is significantly displacedfrom the controlling Proterozoic structure.


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Figure 20. Structural framework of Officer Basin.

Dashed red lines and hatching outline trend of regional structures evident in Bougergravity data. These are parallel to NNW trending structures at SW end ofTallaringa Trough. See also Figure 12.

TallaringaTrough



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4.1.2.3. Sector 3 (Nawa Ridge)

This comprises a complex zone of rhombic fault blocks, with syn-depositionaldevelopment of various structural highs. The overall Ridge includes a narrow NE-trending ridge of crystalline basement, with thin Neoproterozoic cover (Map 3).Several blocks of relatively shallow basement have Cambrian sedimentary cover;these were likely emergent fault blocks during the Neoproterozoic, and subsequentlydownthrown during the Cambrian.

A large area in the north of the subdomain is covered by late Palaeozoic sediments,

with no drillhole data for the underlying sequences. At present, this area has beeninterpreted as a zone of thin Neoproterozoic sediments onlapping the Gawler Craton(magnetic units within the basement are relatively high frequency, indicating only thinto moderate sedimentary cover).

The SW end of the Ridge is intersected by a series of NNW to N-trending faultsextending from the Tallaringa Trough. These faults are ~ parallel to a series ofregional structures evident in the Bouger gravity data (see Figures 12 & 20); they areinterpreted as reactivated late-Mesoproterozoic (Musgravian?) structures.

The Nawa Ridge is separated from the transpressive deformation dominatedSector 5 by the subtly expressed Birksgate Coober Pedy Corridor (Map 3). Thisstructural zone appears to have restricted deformation associated with the lateMusgravian, Petermann & Alice Springs Orogenies into the north of the basin (seeFigure 21 & Map 3).

The Birksgate Coober Pedy Corridor may represent an original WNW to E-W

wrench structure intersecting the Munyarai Subdomain and Gawler Craton. Thestructure may represent incipient development of a Musgravian Orogeny dextralshear, parallel to major E- trending dextral shear zones throughout the MusgraveBlock.



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Figure 21. Officer Basin structural framework highlighting location of Nawa Ridge andBirksgate Coober Pedy Corridor.

Structure superimposed on RTP-1VD magnetic image.Transpressive deformation within Officer Basin appears largely restricted to area

north of the Birksgate Coober Pedy Corridor. The Tallaringa Troughappears bracketed by the Birksgate Coober Pedy Corridor and the



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4.1.2.4. Sector 4 (Birksgate & Munyarai Subbasins)

This domain comprises the main central and northern zone of the eastern OfficerBasin, and includes a) the Birksgate Subbasin, b) the Munyarai Trough, and c) thenorthern sector of the Murnaroo Platform (see Map 3, Figures 22, 23). It is boundedto the SW by the Watson Ridge, and to the SE by the Nawa Ridge andAmmaroodinna Ridge. To the north, it is overthrust by the Musgrave Block. Thisnorthern boundary is a complex of conjugate NW to NNW and NE trending and(locally) E-W trending faults.

Structural trends within this domain include: NW & NE faults. These vary from NW-trending transpressive (possible

thrust / oblique thrust) faults in the west (Birksgate Subbasin), to NE-trending transpressive faults in the east (Marla Overthrust area);

E-W folding. The major fold axes evident throughout the area are oriented~E-W, with reorientation towards NE in the Marla Overthrust area, andtowards ESE / SE in the Birksgate Subbasin (see Map 3). The folding is acomposite of Petermann & Alice Springs Orogeny (N-S compression,coupled with dextral NW SE trending shear). E-trending folds within thenorthern basin are also likely associated with E-trending thrust faults.

NNW-trending faults: these include significant step faults in the northernmargin of the basin, and parallel subtle structures evident in the Bougergravity data within the basin (Figure 20).

a) Birksgate Subbasin.This is a ~NW trending deep subbasin (up to 6km thick); itlies along the same NW structural axis as significant depocentres within theWaigen and Yowalga Subbasins within the western Officer Basin (WA; see Apak

& Moore, 2000). The principal subbasin is defined by an intense Bouger gravitylow anomaly. The principal axis of the subbasin is coincident with the (?thrust)contact between Neoproterozoic and Cambrian sediments at surface. This NWstructural axis (Birksgate Cobber Pedy Corridor) extends to the SE, where it iscoincident with the southern limit of Ordovician Devonian sedimentation withinthe Munyarai Trough. The structural axis then swings to the east, parallel, withthe E-trending Coober Pedy Subdomain.

The Birksgate Coober Pedy Corridor is intersected by several NE to NNEtransfer faults, which likely partitioned sedimentation during the Neoproterozoicdeposition, and were subsequently reactivated as block faults during Cambriandeposition.

Sh ll t t id t i th ti d t (i l di E t ESE t di


NE to NNE trending transfer faults These faults were reactivated in the Alice


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NE to NNE trending transfer faults. These faults were reactivated in the AliceSprings Orogeny, with sinistral en-echelon stepping of E-W folding throughout theOrdovician Devonian sequence evident.

The Trough is complex, with deposition focussed within the area during:

Early Neoproterozoic (NE-SW dilation, partitioned by NE-trending transferfaults);

Petermann Orogeny (550Ma) initial overthrust of Musgrave Block overTrough margin);

Cambrian NW-SE dilation, partitioned by reactivated NW-trending faults.

Ordovician sedimentation - ?structural controls ambiguous?

Devonian deposition during relaxation phase of Alice Springs Orogeny localisation of depocentre by weak sinistral shear couple along Birksgate Coober Pedy Corridor & axis of Boorthanna Trough.

Inversion along NE-trending Marla Overthrust Zone (SE margin of Trough).

c) Murnaroo Platform B. This is the northern continuation of the Murnaroo Platform;it is bounded to the south by the Watson Ridge, and to the north by the Birksgate

Subbasin (see Figure 22). The Platform is relatively featureless within themagnetic data, except for several NE-trending faults associated with the NurraiRidge.


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Figure 23. Outline of Sector 4 of northern Officer Basin (superimposed on colourBouger gravity image).

Birksgate Subbasin and Munyarai Trough are evident as significant gravity lowregions. The northern margins of the subbasins are overthrust by thesouthern margin of the Musgrave Block; fold/thrust structures are evident

in the detailed magnetic data (Map 3) and in field mapping. MurnarooPlatform B is evident as a broad gentle gravity gradient dipping to thenorth.

MurnarooPlatform B


4 1 2 5 Sector 5 NE transpressive Domain


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4.1.2.5. Sector 5 - NE transpressive Domain

This domain lies in the E of the Officer Basin (Map 3, Figure 24), with deformationdominated by a ~NE trending thrust / transpressive wrenching. The domain includes:

Marla Overthrust;

Ammaroodinna Ridge;

Manya Trough;

Middle Bore Ridge;

Wintinna Trough.

The domain is dominated by NE to NNE trending arcuate fault zones, commonlyintersected by E-W trending faults: it is bounded by

NE Bitchera Ridge / Boorthanna Trough;

E Gawler Craton;

S Birksgate Coober Pedy Corridor;

NW Munyarai Trough

The Ammaroodinna Ridge & Marla Overthrust comprise basement involved thrusts

/ transpressive wrench faults, generally verging to the SE.

Drilling in the Marla / Ammaroodinna Ridge area has highlighted the presence ofseveral minor fault blocks comprising Neoproterozoic sediment with no overlyingCambrian. These lie along a roughly E-trending structural zone (Figure 25). This mayreflect;a) Development of an E-trending lateral transfer zone within the thrust belt;b) Development of sinistral NE-trending transpressive wrench within the zone rather

than simple SE-verging thrusting;c) Later localised erosion (during deposition of Permian?).

The Middle Bore Ridge is a complex zone of arcuate fault blocks, commonly withCambrian sediments directly overlying basement. This suggests that the Ridge mayhave had an early history as a structural high during Neoproterozoic sedimentation(similar to the Nawa Ridge to the south). The arcuate fault pattern, and interferenceby common E-W faults suggest the Ridge was reactivated as transpressive pop-upblocks during the Alice Springs Orogeny (NE sinistral transpressive shear).

The Manya Trough lies between the Ammaroodinna & Middle Bore Ridges, and wasin part overthrust by the Ammaroodinna Ridge during the Alice Springs Orogeny.



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Figure 24. Structural framework of NE transpressive zone (Sector 5). The subdomainis bounded to the north by the Boorthanna Trough, to the west by theMunyarai Trough (Sector 4 yellow line), and to the east by the GawlerCraton. To the south, it is partitioned from the predominantly transtensileNawa Ridge Tallaringa Trough region by the Birksgate Coober PedyCorridor (dashed blue line).

Sector 5 comprises a series of SE-verging transpressive thrust and fold structures,with significant basement involvement. The structural zones from NW toSE are: Marla Overthrust Zone, Ammaroodinna Ridge, Manya Trough,Middle Bore Ridge and Wintinna Trough.



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Figure 25. Structural framework of Ammaroodinna Ridge Middle Bore Ridge area(detail from Map 3). ~E-W structural corridor (red lines) defined by

alignment of anomalous structural high blocks (Neoproterozoic sedimentswith no Cambrian cover).


4.1.2.6. Sector 6 - Bitchera Ridge Boorthanna Trough


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4.1.2.6. Sector 6 Bitchera Ridge Boorthanna Trough

This subdomain represents a partial-linking corridor between the Officer Basin andAdelaide Geosyncline during Neoproterozoic Early Palaeozoic sedimentation(Figure 26). The principal NW trending sector of the subdomain was reactivated as amajor Permian Carboniferous graben (Boorthanna Trough).

The subdomain can be separated into:a) Bitchera Ridge. This is a ~E-W to ESE- trending structural high, comprising

Neoproterozoic sediments and volcanics overlying (and in part tectonicallyintercalated with) crystalline basement (Musgrave Block and/or Gawler Craton).The Bitchera Ridge was developed in response to dextral transpressive shearalong the eastern continuation of major E-trending shear zones within theMusgrave Block during the Petermann Orogeny. The Ridge acted as a structuralhigh during Cambrian sedimentation. At least one bed of strongly magneticvolcanics , gently dipping to the north, and deformed by gentle NE-trending foldsare evident.

b) Boorthanna Trough. This is an obvious NW-trending graben in the NE of the

area; it comprises Neoproterozoic (and Cambrian?) sediments overlain by a thicksequence of Carboniferous-Permian glacigene sediment. The Boorthanna Troughlies on the western flank of the NW trending continuation of the Torrens HingeZone (Adelaide Geosyncline.

Deformation within the subdomain is likely complex, with overprinting of thePetermann, Delamerian & Alice Springs Orogenies.

4.2.1.7. Salt structures

Substantial salt and development of salt structures within Neoproterozoic sedimentshas been documented for the Munta and Marla areas (sector 5) (Lindsay, 1995,

Gravestock, 1997). One of the best expressions of salt structures is the developmentof a salt diapir / piercement in the Munta - Ungoolya area (Seismic section 86OF-01;see Figure 36). Salt structures are also known from the Yowalga Subbasin within theWestern Officer Basin. The lack of evidence elsewhere within the eastern OfficerBasin is considered here to be due not to a lack of salt, but a combination of factors:

a) Thickness of cover;


Salt diapirs may also be evident from weak radial or concentric fracturing above thedi i (thi b id t i ti d/ t llit i ) Thi ff t i


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diapir (this may be evident in magnetic and/or satellite imagery). This effect isdependent on the diapir being relatively shallow. Deeply buried diapirs within the

Officer Basin are not likely to be observed.

The development of salt piercement structures in the Munta - Marla is regionallycoincident with the intersection of the Sector5 Transpressive Domain, and a series ofNW-trending faults ~parallel to the Birksgate Coober Pedy Corridor. Otheranalogous structural loci are:

SW near the intersection of the Watson Ridge & Nawa Ridge;

Intersection of Birksgate Subbasin & NE transfer fault zones (including Nurrai

Ridge) Intersection of NNE/NE transfer Faults and NW faults within Munyarai Trough.

Figure 26. Officer Basin Sector 6. This comprisesa) The ~ E-W trending Bitchera Ridge (Neoproterozoic sediments & volcanic with

shallow to locally exposed crystalline basement) and;


4 2 Tectonic Development


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4.2. Tectonic Development

The eastern Officer Basin, and surrounding basement has undergone a complextectonic history; this extends from the Archaean (Mulgathing Orogeny, GawlerCraton) to the Mid-Palaeozoic Alice Springs Orogeny, and superposition of laterbasins (Pedirka, Denman, Eucla Basins; see Figure 5).

The following is a summary of the principal tectonic episodes, and their effect upondevelopment of the Officer Basin. The chronology of events is highlighted in Figures2 & 3, and the Neoproterozoic to Devonian tectonic development is highlighted in

Figures 27a-i.

4.2.1. Pre Officer Basin

The basement domains surrounding and underlying the Officer Basin haveundergone a complex series of tectonic events. A simplified overview of thesetectonic episodes includes:

Pre - 1600 Ma

Deformation of Archaean Mulgathing Complex (Gawler Craton) duringMulgathing Orogeny (2550-2450Ma)

Development of Palaeoproterozoic mobile belts, intrusives and episodicdeformation during Kimban Orogeny (Gawler Craton 1850 1700Ma).Combined deformations produced major zones of NNE- to NE- & E-trending structural grain.

Musgrave Block and Munyarai Subdomain formation of protoliths toMusgrave Block metamorphics with major NE to NNE structural grain.

Kararan Orogeny (1600-1400Ma Gawler Craton)

NW- trending compression / collision(?) of Gawler Craton with protoliths toMusgrave Block / Munyarai Subdomain. Development of strong NE-trending structural grain (transpressive / thrust), with quasi-regular spacedE-trending dextral N-trending sinistral & NW trending tensile structuralcorridors. NW-trending structural corridor in NE Gawler Craton acted as

focus for Iron Oxide Cu - Au mineralisation. Various structuralorientations also likely developed at this time in Munyarai Subdomain &Musgrave Block.

Late Kararan Orogeny (1400Ma - ?) inferred locking of NW collision /thrusting along western margin of Gawler Craton Strain accommodated


Gawler Craton / Coompana Block possible emplacement of majorgranitoid complexes along NE trending belts (Hughes Subdomain)


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granitoid complexes along NE trending belts (Hughes Subdomain).Thermal overprinting of Gawler Craton in area of Coompana Block.

1100-1050Ma Late Musgravian Orogeny.Musgrave Block major NW-SE to E-W transpressive shear. Furtheremplacement of granitoids, plus mafic-ultramafic intrusives. Extrusion ofTollu Volcanics.Coompana Block Gawler Craton: ?Age of Coompana Suite intrusivesand associated NNW trending dyke swarm. Extrusion of mafic volcanics(Coompana Block). Early development of NNW structural grain withinbasement to Officer Basin.Thermal weakening of crust in area of Officer Basin. Possible localiseddeposition of late Mesoproterozoic sediments prior to Officer Basinsequences.

4.2.2. Officer Basin Neoproterozoic

Gairdner Dyke Swarm emplacement (800Ma)

Initiation of rifting immediately prior to Officer Basin development. GairdnerDyke Swarm emplaced throughout Gawler Craton, Munyarai Subdomain,Coompana Block and Musgrave Block. Major concentration evident withinSW Murnaroo Platform.Interaction between NNW & NW tensile structures developed during thisevent and Late Musgravian Orogeny.

Neoproterozoic Deposition (800-550Ma)

Overall NE-SW dilation associated with thermally initiated crustal sag;development of NW-trending horst / graben architecture. Regionalarchitecture of the eastern Officer Basin / NW Adelaide Geosyncline forms amajor Z-vergent orthorhombic series of subbasins and structural highs.

Neoproterozoic sedimentation extends across majority of Musgrave Block,

linking Amadeus & Officer Basins.

Deep subbasin development immediately adjacent to overthrust margin ofMusgrave Block (previously interpreted as development of basin by N-Scompression & foreland sagging of crust). Current interpretation suggests


Southern margin of Musgrave Block possibly inverted & steeply thrustover Officer Basin Musgrave Block fully emergent at this time


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over Officer Basin. Musgrave Block fully emergent at this time.

Officer Basin Localised development of E-W to ESE trending folding (&

thrusting?) in Birksgate Subbasin & Munyarai Trough. Possible initiation oftranspressive thrusting in Marla Ammaroodinna area. Transpressiveeffects of orogeny do not appear to have extended south beyond theBirksgate Coober Pedy Corridor.

Note- Major transpressive strain developed during the PetermannOrogeny was concentrated in the northern half of the Musgrave Blockand Amadeus Basin. Effects within the Officer Basin appear minor.Development of structural highs (Middle Bore Ridge area) with noNeoproterozoic sedimentation, (or erosion prior to Cambriansedimentation) may have initiated at this time.

Cambrian Sedimentation

Relaxation post- Petermann Orogeny. NW-SE dilation, possibly associatedwith minor sinistral shear couple on existing NW structural corridors..

Reactivation of some NE-trending basement structures, an initiation of newstructures as normal faults. NW faults reactivated as tensile / basin transferfaults. Superposition of NE-SW trending horst-graben architecture onNeoproterozoic NW-trending horst-graben architecture.

Munyarai Trough localisation of thick Cambrian sedimentation by minorsinistral shear couple between Birksgate Coober Pedy Lineament &Boorthanna Trough Corridor.

Birksgate Subbasin possible localisation of thicker Cambriansedimentation (similar to Munyarai Trough), or more platformal drapecontinuous from Murnaroo Platform?

Tallaringa Trough NW-SE dilation, associate with minor sinistral shearcouple on Birksgate-Coober Pedy Corridor. Reactivation of Karari SZ asnormal fault. Nawa Ridge acts as NE-trending structural high. Dilationwithin Tallaringa Trough partitioned against SE end of NW trendingWatson Ridge. Minor increased opening & deposition occurs within minor

rhombic fault blocks at SE end of Ridge. Murnaroo Platform Cambrian sedimentation shallows / onlaps

Coompana Block & Gawler Craton in south. Sediment thickens to NW.


Ordovician Sedimentation


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O do c a Sed e tat o

Ordovician sedimentation appears restricted to the Munyarai Trough area.Teasdale et al(2001) suggest a narrow link to the Larapintine Sea Corridor(Canning Amadeus Basins), with continent wide NE-SW dilation / rifting.Distribution of sediments in the NE Officer Basin suggest its depocentre wasfocussed by weak sinistral shear couple along Birksgate Coober Pedy &Boorthanna Trough Corridors.

Devonian Sedimentation / Alice Springs Orogeny

Moderate thickness of Devonian sediments deposited within MunyaraiTrough, immediately prior to Alice Springs Orogeny (+3000m; Gravestocket al, 1995). This depocentre roughly coincides with Ordoviciandepocentre; sedimentation likely focussed by similar sinistral shear couplealong Birksgate Coober Pedy & Boorthanna Trough Corridors.

Devonian sediments are also known within parts of the western Officer

Basin; it is ambiguous at present whether these were linked to the NEOfficer Basin occurrences, or represent localised (wrench basin?)depocentres.

Alice Springs Orogeny (400-350Ma). Major transpressive thrustingwithin Arunta Inlier / northern Amadeus Basin. Musgrave Block onlyminor reactivation of existing transpressive structures?

Officer Basin transpressive dextral shear couple along Birksgate Coober Pedy & Boorthanna Trough Corridors. E-W en-echelon folding ofOrdovician Devonian sediments in Boorthanna Trough, associated withsinistral transpressive shear along Marla Overthrust / AmmaroodinnaRidge structures.

Development of transpressive thrusts along Marla Overthrust /Ammaroodinna & Middle Bore Ridge zones (oblique ramping of Officer

Basin and crystalline basement onto shallow Gawler Craton to SE.



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Figure 27. Summary cartoons of Neoproterozoic Devonian tectonic development,eastern Officer Basin Major extensional and thrust fault orientations shown

A B

C



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Figure 27 (continued). Summary cartoons of Neoproterozoic Devonian tectonic

development, eastern Officer Basin. Major extensional and thrust faultorientations shown as black lines. Transfer faults / structural zones shownas dashed blue lines.

e) Initial Cambrian sedimentation NW-SE dilation, with possible sinistral

E F

G H



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Figure 27 (continued). Summary cartoons of Neoproterozoic Devonian tectonicdevelopment, eastern Officer Basin. Major extensional and thrust faultorientations shown as black lines. Transfer faults / structural zones shownas dashed blue lines.

i) Alice Springs Orogeny (~350 400Ma). Weak S-verging overthrust ofMusgrave Block over N margin of Officer Basin. Associated with regionalNW-SE dextral shear couple. Reactivation of NE-SW and E-W(?) obliquethrusting within NE transpressive sector. Transpressive deformation

appears restricted to north of Birksgate Coober Pedy Corridor.

I


4.3. Depth to Basement.


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There have been several studies conducted to model depth to magnetic basementfor the Officer Basin, including:

a) Modelling of contour data by R Gerdes (1970s 80s; S Daly, persComm.);

b) SEEBASE modeling by SRK Consulting (Teasdale et al, 2001);c) Euler 2-D modeling (PIRSA; Calandro & Read, in press).

The SRK SEEBASE study involved computer modeling of magnetic profiles extractedfrom the gridded magnetic data (Figure 28). The computed depths were integratedwith structures interpreted from the imaged magnetic data.

Comparison of SEEBASE and PIRSA Models

A comparison of the SEEBASE model and the PIRSA Euler model show markeddifferences in the depth to magnetic source (Figures 28, 29). In general, depths to

magnetic source within the Officer Basin in the PIRSA model are commonly (overlarge areas) significantly shallower than those shown in the SEEBASE model. This isparticularly evident in the north of the basin. This suggests that the Euler 2-D modelhas highlighted minor shallow magnetic sources within the top 1km of sediment.These may be weakly magnetic volcanics and volcaniclastics, plus laterite profiles.

The magnetic bodies comprising the Nurrai Ridge are again highlighted differentlyin the 2 models:

a) The PIRSA model shows the ridge as a significant topographic low (>3000m)compared with surrounding shallow magnetic sources. This is due in part todisplay of both basement and sedimentary basin magnetic sources in the samedataset.

b) The SEEBASE model shows the ridge (and part of the Murnaroo Platform to theSE) as NNE to NE trending topographic highs (2000 3000m ridges against abackground basin floor of ~4000m); the ridges are coincident with magneticbodies in the basement.

A significant, narrow, N-trending topographic low is evident in both datasetsimmediately west of the exposed western limit of the Gawler Carton (SE OfficerBasin). This has been interpreted by Teasdale et al(2001) as a possible sedimentarytrough. The feature is coincident with a regional - scale N-trending structural corridor


Two possible interpretations can be proposed for this zone:

a) The isolated deep magnetic body within the zone is coincident with top of


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a) The isolated, deep magnetic body within the zone is coincident with top ofcrystalline basement, and that the zone represents a significant, narrowsedimentary trough or canyon of indeterminate age (SRK model), or;

b) The deep magnetic body lies significantly below the top of crystalline basement;this may be a magnetite alteration body associated with shearing within a zone ofmagnetically quiet granitoids emplaced within the anomalous Mesoproterozoic N-trending structural zone. Sedimentary cover is variable through the region, butdoes not appear directly associated with a significant N-trending trough. Note there are numerous magnetite alteration bodies throughout the Gawler andCurnamona Cratons, which lie beneath the top of basement (eg mt body

beneath Olympic Dam orebody).

A comparison of the SEEBASE Officer Basin image, and location of modelled profilesto the magnetic image (Figure30) suggests there are several possible discrepanciesor assumptions within the SEEBASE model:

a) The majority of major topographic structures evident in the SEEBASE model are

generally parallel to, and coincident to near coincident with magnetic bodieswithin the basement. While some reactivation of the NE tectonic grain within theGawler Craton occurred during deposition and deformation of the Officer Basin,the complex horst/graben block structure evident within the shallower parts of thebasin indicates that much of the basin structure intersects the earlier basementstructural grain.

b) The lack of numerous magnetic sources that can be modelled in the deepersectors of the basin will produce a biased image when the sparse data is gridded.

This effect may be responsible in part for the coincidence of basement magneticfeatures and basin topographic ridges within the main part of the basin.

Where well-constrained depth solutions are limited, it may be more valid to portraystructural domains (bounded by interpreted structures) with depth for each blockdetermined by the modelled depth for individual sources within the block, rather thanas a closely gridded (over interpolated?) image.



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Figure 28. SEEBA

officer_structural_framework.pdf

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