Eastern Officer Basin: Structural Framework from Geophysical Data

EASTERN OFFICER BASIN: STRUCTURAL FRAMEWORK FROM GEOPHYSICAL DATA

GEOINTERP CONFIDENTIAL REPORT 2003/2

For OIL & GAS DIVISION DEPARTMENT OF PRIMARY INDUSTRIES SA Grenfell St, Adelaide

L R RANKIN Consulting Geologist

PO Box 195, Aldgate, SA 5154, Australia

RANKIN CONSULTANCY PL ABN 26079486025 Geointerp 2003/1:- Structural Framework - Eastern Officer Basin PIRSA

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

Leigh R Rankin Director, Rankin Consultancy PL June 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

4.1.1.4. Concealed Basement ...... 36

4.1.2. Officer Basin ...... 42

4.1.2.1. Sector 1 (SW region – Murnaroo Platform A & Watson Ridge) ...... 42

4.1.2.2. Sector 2 (Tallaringa Trough, eastern region)...... 43

4.1.2.3. Sector 3 (Nawa Ridge) ...... 50

4.1.2.4. Sector 4 (Birksgate & Munyarai Subbasins) ...... 52

4.1.2.5. Sector 5 - NE transpressive Domain ...... 56

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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 – Officer 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 continental Australia (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 – basement to Officer Basin.

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

Figure 5. Location of Middle Palaeozoic to Cainozoic basins of South Australia (from Drexel et al, 1993).

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

Figure 7. RTP-1VD image of Officer Basin and surrounding basement, highlighting dextral wrench basin along Coorabie Shear Zone.

Figure 8. TMI image of SA (from Geoscience Australia).

Figure 9. RTP – 1st VD magnetic image highlighting mafic dykes of the Gairdner Dyke Swarm.

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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 mafic intrusives of the Coompana Suite emplaced within the Coompana Block, Munyarai Subdomain and Gawler Craton.

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 interpreted intrabasement magnetic sources.

Figure 17a. RTP-1VD image – Officer Basin; note high-frequency detail in magnetic data for shallow sectors of basin, particularly in the Ammaroodinna / Middle Bore 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 eastern Officer Basin.

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

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

Figure 21. Officer Basin structural framework highlighting location of Nawa Ridge and Birksgate – Coober Pedy Corridor.

Figure 22. Outline of Sector 4 of northern Officer Basin (superimposed on RTP-1VD image).

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

Figure 24. Structural framework of NE transpressive zone (Sector 5).

Figure 25. Structural framework of Ammaroodinna Ridge – Middle Bore Ridge area (detail from Map 3).

Figure 26. Officer Basin – Sector 6.

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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 (from Teasdale 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).

TABLES 1. Datasets used …………………………………………………………………………….16

APPENDICES

1. Geophysical images…………………………………………………………………….91 2. Selected magnetic profiles……………………………………………………………..99

MAPS

1. Observation layer from magnetic data. Compiled at 1:500 000 scale. 2. Structural framework (crystalline basement). Compiled at 1:500 000 scale. 3. Structural framework (Officer Basin). Compiled at 1:500 000 scale.

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1. EXECUTIVE SUMMARY

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

Basement

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

• NW & W Gawler Craton (including Hughes Subdomain) Intense NE transpressive structural grain predominantly developed during the Kimban (1850-1700Ma) & Kararan Orogenies (1600-1400Ma). The NE structural grain is intersected (quasi-episodically) by corridors of variable-intensity E-W trending dextral shear, N-S sinistral transpressive shear and NW-trending dilation. The Hughes Subdomain (interpreted here as the western margin of the Gawler Craton) is dominated by a series of elliptical granitoids; these are tentatively correlated here with late Mesoproterozoic (1200 – 1050Ma?) intrusives within the Musgrave Block.

• Munyarai Subdomain This comprises a series of NE to NNE-trending tectonic belts (completely concealed by the Officer Basin). The Subdomain is transitional between the Gawler 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 magnetic intrusives (interpreted here as Kulgeran Suite) extending south of the Musgrave Block form the geophysically defined Nurrai “Ridge”.

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

• Musgrave Block The Musgrave Block is dominated by an early structure of NE to NNE tectonic belts, overprinted by an intense E-trending dextral transpressive shear / thrust structural grain (developed during the Musgravian, Petermann & Alice Springs Orogenies).

• Mafic Dykes The basement domains are intersected by several different mafic dyke swarms. The most obvious and extensive dyke swarms are: a) NNW-trending negatively magnetic mafic dykes associated with the Coompana Suite. These are likely also related to negatively magnetic intrusives within the Munyarai Subdomain (WA sector) and western Musgrave Block.

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b) Gairdner Dyke Swarm (~800Ma). This is a major swarm of dykes of variable magnetic character (dominated by positively magnetic dykes). The swarm generally trends NW-SE, and intersects all of the basement domains to the Eastern Officer Basin. The dyke swarm represents initial dilation / ?rifting of the crust prior to development of the Officer Basin and Adelaide Geosyncline.

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-trending Watson Ridge to the north. The Watson Ridge is a subtly expressed structural corridor which acted in part as a structural high during basin deposition. The SE end of the Ridge was involved with localised rifting and thicker sedimentation during the Cambrian (associated with rift development of the Tallaringa Trough).

• Sector 2:- Tallaringa Trough. This was initiated during Neoproterozoic sedimentation, but predominantly developed by NW-SE rifting during the Cambrian (coupled with weak E-W sinistral shear along the trend of the Coober Pedy Ridge). The Trough is separated from the main Officer Basin by the Nawa and Watson Ridges.

• Sector 3:- Nawa Ridge. This is a NE-trending complex zone of rhombic fault blocks forming a structural high during both Neoproterozoic and Cambrian sedimentation. Faulting is interpreted as predominantly transtensile. The Ridge is separated from transpressive deformation developed to the north by the Birksgate-Coober Pedy Corridor (a subtly expressed SE- to ~E- trending structural zone).

• Sector 4:- This comprises the central and northern sector of the basin, and includes the deep Birksgate Subbasin and Munyarai Trough. The sector initially developed 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)

• Sector 5:- This comprises the NE area of the basin that underwent significant inversion / transpressive thrust deformation during the Alice Springs Orogeny. The sector includes the Ammaroodinna and Middle Bore Ridges, and the Manya Trough.

• Sector 6:- This comprises the ~E-trending Bitchera Ridge and NW-trending Boorthanna Trough; it marks the partial linking corridor to the Adelaide Geosyncline to the east.

Structure within the basin is dominated by near orthogonal NW and NE faulting; this represents a combination of reactivated basement, and newly activated structures. NNW and E- trending fault zones are also evident.

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A brief comparison of existing seismic data and previous depth to basement magnetic modelling has highlighted numerous discrepancies in the magnetic depth model. A review of depth to magnetic basement along several selected magnetic profile, combined with the qualitative structural framework interpretation, highlights several 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 potential NW- and NNW-trending structural highs (including the Watson Ridge), and intersection zones of dilation / transfer fault zones (considered potential loci for salt diapirism).

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2. INTRODUCTION

2.1. Preamble

The Neoproterozoic to mid-Palaeozoic eastern Officer Basin (part of the Centralian Superbasin complex of Australia) covers an area of >100 000km2. The Basin is significantly underexplored for both hydrocarbons and minerals, with only 7 petroleum 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 previously contracted studies on the available geological and geophysical data, including an atlas of geological interpretation maps based on seismic and drilling data (Lindsay, 1995), a seismic interpretation study of the Marla & Munta areas (Mackie, 1994) and the 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, detailed magnetic data was also acquired over sectors of the northern Officer Basin.

Geointerp was contracted by PIRSA to review the structural framework of the eastern Officer Basin using the combined regional and detailed magnetic data, with a view to determining structural style and possible hydrocarbon leads within the South Australian sector of the Basin. The location of the eastern Officer Basin and the interpretation area are shown in Figure 1. A summary of the stratigraphy and deformation history of the eastern Officer Basin is shown in Figure 2. A summary of the 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 the basement 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.

To address these aims, the following were undertaken: • Compilation of a regional structural framework at 1:500 000 scale for both the Officer Basin and underlying basement from regional magnetic and Bouger gravity data; • Integration of previous drilling and seismic data with the geophysically-derived framework; • Quantitative review of depth to basement by manual analysis of selected magnetic and seismic profiles, and comparison to existing depth to basement studies.

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

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Figure 2. Summary of current stratigraphic nomenclature for the eastern Officer Basin (after Gravestock, 1997; Palaeozoic dates after Tucker & McKerrow, 1995).

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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 surveys of 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 areas was limited principally to basement structures in areas of variably magnetic basement. Intra-basin structures are poorly resolved, with some major structures evident from major variations in thickness of sedimentary cover (and therefore frequency or “sharpness” of magnetic anomalies). In the north and east of the basin, several recent surveys (400m-line spacing) provided high resolution of shallow structure 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 are artefacts caused by a) poor control on aircraft location, and b) temporal changes in the amplitude of the Earth’s magnetic field between data collected on different days not 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 of detailed 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. The majority of data is concentrated in the NE of the region (Marla & Munta areas). The seismic 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 digital compilation of 1:100 000 scale geological mapping.

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

Data Presentation (magnetics)

The use of greyscale vertical derivative magnetic data is preferred by The Consultant over “sun-angle illuminated” data, as there is no directional bias imparted to the data. The use of Reduced to Pole (RTP) data rather than the original Total Magnetic Intensity (TMI) data is strongly recommended by the author for geological interpretation; the RTP data invariably provides a much more geologically- coherent dataset for moderate to low magnetic inclinations (see Isles et al., 2000 for a discussion on the merits of RTP imagery). Whilst the difference between RTP and TMI data may appear minimal for such moderate magnetic latitudes as South Australia, The Consultant has previously found the RTP data for the region provides a more coherent representation of the structural trends.

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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 and lithological distribution at a prospect or district scale, or iii) target specific structures, lithologies or geophysical anomalies for drilling; b) Area to be covered (a detailed interpretation of an entire province may be desirable, but will be dependent on time involved, and density of existing information); c) Resolution of geophysical data – 200m data is suitable for 1: 50 000 scale interpretation and smaller, but will be generally inadequate for larger – scale interpretation, particularly where there is little detailed geology to integrate with the geophysics.

Due to the regional nature of the proposed study, and the limited nature of any detailed geological and geophysical data within the basin, a scale of 1:500 000 was selected.

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Table 1. Datasets utilised for interpretation.

DATASET IMAGES (1:500 000 scale) COMMENTS Magnetics Magnetic data comprises numerous merged datasets of varying line spacing (1600m to 200m), line direction (both N-S & E-W) and resolution. Datasets were acquired from 1956 to 2002. RTP – 1st VD Greyscale RTP Colour RTP + RTP1st VD Composite image (greyscale 1stVD with colour drape of RTP) TMI a) Colour (used for check on RTP process and remanently- magnetic sources). Sun-angle illuminated. b) Greyscale (sun-angle illuminated)

Gravity Bouger Regional data of varying station spacing (stations predominantly acquired along access tracks).

Geology Published 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 attached stratigraphic log data.

Seismic Various selected seismic Data as Tiff scans of original seismic profiles from PEPSA profiles. Various scales. database

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3.2. Geological interpretation methodology

Geological interpretation of the magnetic data followed the general interpretation methodology 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, geophysical modelling approach. The following outlines a complete interpretation methodology for magnetic data.

Methodology

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

• Observation Layer

The first step in the interpretation process is the compilation of an observation layer (or “worm map”) from the magnetic data (Map 1). This involves the recording of the position of magnetic units. The 1st or 2nd vertical derivative of the RTP data is used for this process: the RTP data positions magnetic anomalies over the nearest edge of the causative magnetic body (for normal, induced magnetism), while the vertical derivative sharpens the peak and maximum gradients of an anomaly (highlighting the centre or edge of a body). A greyscale image 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 of magnetic layering, contacts and zonation within the area. This is therefore kept as a separate overlay or digital file, and the “interpretation” is compiled using the observations as a reference.

Note that the vertical derivative highlights shallow (high frequency) magnetic features at the expense of deeper (low – frequency) magnetic signatures. Low frequency (deeper – source) features are typically recorded from a combination of the 1stVD and the regional RTP data.

Note:- Map 1 highlights magnetic trends from both basement and sedimentary cover sources. These have been separated on Maps 2 & 3. Many of the very shallow magnetic trends evident in the sedimentary cover cross interpreted structures. This is caused by: a) Folded or unconformable units overlying concealed faults deeper in the sedimentary section, and; b) Flat – lying, weakly magnetic units within the overlying Mesozoic – Tertiary sediments (including weathering profiles) overlying the Officer Basin sequences. This is particularly evident with irregular magnetic trends in the far east of the Officer Basin area.

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• Structural Framework

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

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

a) Faults / shears: These are commonly separated into major & minor structures, depending both on strike extent and displacement. The position of faults and shears within the magnetic data may be directly observed by the presence of magnetic anomalism along the structure (magnetite addition or destruction), or the presence of magnetically anomalous intrusives within the structure (eg mafic dykes). However, the majority of faults / shears are interpreted by the presence of discordant terminations or inflexions within the observed magnetic trends. Note – the confidence with which the orientation, or even the existence of a particular fault may be interpreted commonly decreases with increasing scale for any one dataset.

Many of the fault zones interpreted within or at the base of the nonmagnetic basin sequences have been interpreted by subtle changes in frequency and resolution (“fuzziness”) within the underlying basement magnetic anomalies (due to changes in depth to magnetic source).

b) Geological contacts: These may include conformable and unconformable contacts, intrusive contacts and unrecognised faulted contacts. It should be noted that there may be several different possible geological interpretations for any one series of magnetic trend patterns. For example, a discordant contact evident in the magnetic data may represent an unconformity, fault or intrusive contact.

The structural framework is typically compiled using a combination of the greyscale vertical derivative data, and the observation layer. The colour composite RTP-1st VD image is also typically used to highlight subtle structures at this stage.

• Solid Geology

A “solid geology” interpretation may then be compiled from the structural framework by providing an interpreted lithology or other geologically - meaningful character to each area or unit defined within the structural framework. At this stage, any existing geological information can be integrated with the interpretation.

Magnetically – defined lithological units or sequences are generally based on recognition of areas with similar magnetic character. The magnetic “texture” of a unit or sequence is typically a significant marker in definition of the geology. The

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“texture” may relate to layered vs massive magnetic character, magnetically – flat vs noisy etc. Texture is commonly best observed in the greyscale imagery. The overall magnetic intensity of different units is also used to define different lithologies. Note that lithological discrimination based solely on magnetic intensity is generally invalid; there are no “unique” magnetic susceptibilities for any particular 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 are approximations only. Typically, each lithological zone comprises a suite of different rocks as recognisable at mapping scale. Any one “lithology” or domain within the interpretation therefore represents a localised grouping of individual lithologies and secondary geological processes. The magnetic data typically highlight not only primary lithologies, but also secondary processes, such as metamorphism, metasomatism and diagenesis.

A detailed solid geology map for the basement lithologies has not been compiled here. 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 previous regional interpretations are: a) Inappropriate matching of similar magnetic responses over large area (there are no “unique” magnetic signatures for particular lithologies – 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 highly interpretative (and therefore subjective). 1st order structures bounding litho- tectonic domains, and 2nd order transfer fault systems are typically highlighted. In addition, large-scale intrusive complexes, including possible concealed plutonic complexes are commonly outlined. Subtle structural zones or “corridors” may be also observed or inferred at this stage. Commonly such structural “corridors” comprise regional alignments of partly connected, or unconnected, structures evident in the data (the old-style “lineaments” commonly focussed on in early satellite image interpretation); these structural corridors commonly reflect the presence of steep, deep-crustal fault zones.

• Targeting

Exploration targets (either as specific drilling targets, or as broader target “zones”) may then be selected (at various scales) based on a combination of specific geological “concepts” or key indicators decided upon for a particular target type, plus association(s) of local and regional structural and geophysical constraints. At this stage of the interpretation, any and all other relevant data,

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

The known geology of the Basin was integrated with the structural framework using both the mapped geology and the drillhole database. Existing seismic data was 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 comprehensive description of the key elements of magnetic data, processing and imaging, the reader is referred to Isles et al (2000).

Total Magnetic Intensity (TMI):-

Amplitude of Earth’s magnetic field as measured at any given point. This measurement is the sum of both the Earth’s regional field strength, and the local field produced by magnetic sources within the local crust. The TMI data has typically been processed to remove non-geological “noise” from the signal prior to interpretation.

Both the Earth’s regional field and magnetic fields from crustal sources are vector quantities: the sum of the 2 fields is controlled by both magnitude and direction (see Figure 4).

The orientation of the Earth’s 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 dipolar profiles. Both the peak and the trough of the anomaly are migrated away from above the centre of the causative body. The position of the maximum gradient along the anomaly indicates the position of the centre or nearest edge of the body. The lower the magnetic latitude, the greater the dipolar nature of the anomaly.

Figure 4. Schematic diagram of induced dipolar magnetic profile for a magnetic body at moderate magnetic latitude. The position of the nearest edge of the magnetic body is coincident with the position of the maximum gradient (x) along the profile. For a body at the magnetic pole, the profile would be a simple peak, centred above the body.

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Reduced to Pole (RTP):-

This is a well-established mathematical procedure (Baranov, 1957) that transforms the dipolar magnetic anomaly for an induced magnetic field at any given magnetic latitude to an anomaly within a vertical Earth’s field (magnetic pole). The anomalies produced (for induced field anomalies) therefore appear as simple (“monopolar”) peaks, with the peak located over the centre (or nearest edge) of the magnetic body.

RTP magnetic data provides a better correlation than TMI data between the plan shape and position of a geological body, and its magnetic signature.

First Vertical Derivative (1st VD):-

The 1st VD is a high-pass mathematical filter designed to enhance the position of the centre / nearest edge of magnetic bodies. The filter specifically enhances shallow magnetic sources (high-frequency anomalies) at the expense of deeper magnetic sources (low-frequency anomalies).

The 1st VD is a calculation of the rate of change of the magnetic field in the vertical (z) direction (calculated from the measured rate of change in the horizontal directions x & y).

The 1st VD is the preferred filtering technique for discrimination of structural features, because: a) It has no directional bias (unlike Sun-illuminated filters); b) It provides a greater resolution of positions and magnetic “textures” of different lithologies.

Sun – angle Illumination:-

Sun-angle illumination is a common, simple filtering technique applied to gridded magnetic (and other) data to enhance both positions of magnetic highs and lows, and produce an artificial “shadowing” along structures.

Sun-angle illumination has the advantage that it is a simple mathematical procedure, and built into most available geophysical data processing packages. It has, however, several significant drawbacks if used for interpretation: a) It is strongly directionally biased. The filter is applied with a specific orientation and azimuth; structures orthogonal to the orientation are visually enhanced, whilst structures parallel to the orientation are subdued (or invisible). This means that numerous images must be produced and interpreted to obtain a relatively comprehensive view of all structural elements. b) The heavily biased shadowing can easily create false structures; c) Strong shadowing commonly hides subtle magnetic features on the sides of major magnetic anomalies.

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4. RESULTS OF INTERPRETATION

4.1. Structural Framework

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

The internal structural framework of the Musgrave Block and the majority of the Gawler Craton has been highlighted in numerous previous studies (including Rankin & Newton, 2002; Daly et al, 1994; Teasdale et al, 2001). Although these have not been replicated within this study; the principal structural elements are summarised below, along with the current interpretation. A description of the structural framework of the basement to the Officer Basin is considered essential to an understanding of the later development and deformation of the Basin.

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

4.1.1. Basement

The eastern Officer Basin is bounded by several different crystalline basement terranes (see Maps 2 & 3). These are: • Western Gawler Craton: - the Basin is both in faulted contact along its SE margin (along the Karari Shear Zone), and onlaps the Craton in the NE and south. The western Gawler Craton comprises the Christie, Nawa, Coober Pedy Subdomains, the Fowler Suture Zone, the SW Gawler Craton, and the Ammaroodinna and Yoolperlunna Inliers. • Munyarai Subdomain: (name modified from Teasdale et al, 2001)- this is a NE-trending domain beneath the central Officer Basin. • Hughes Subdomain: (new name) - this is a NE-trending belt concealed by the eastern margin of the Officer Basin. This domain is considered transitional 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 and Eucla Basins (Figure 5). • Musgrave Block:- the northern margin of the Basin is steeply overthrust by the Musgrave Block. Neoproterozoic sediments locally lie unconformable on Musgrave Block gneiss near the southern margin of the Block.

The crystalline basement beneath the eastern Officer Basin (Map 2) includes several NE- to NNE - trending belts (named the Ammaroodinna & Munyarai Subdomains by Teasdale et al, 2001). These belts represent transitional Palaeoproterozoic – Mesoproterozoic terranes between the Gawler Craton and the Albany-Fraser Orogen to the west (see Figure 1 for approximate location of Albany Fraser Orogen). These subdomains are likely to have a significant late Mesoproterozoic (Musgravian Orogeny) overprint.

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

Concealed Coompana Block

Figure 5. Location of Middle Palaeozoic to Cainozoic basins of South Australia (from Drexel et al, 1993). The Mesoproterozoic Coompana Block in the SW of the state is completely concealed by the Permo-Carboniferous Denman Basin and Tertiary Eucla Basin. The red dashed lines highlight the approximate boundary of the Officer Basin.

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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 the Christie and Nawa Subdomains and Fowler Suture Zone. Major orogen – parallel to subparallel shears trend NNE to NE, with both Archaean and Proterozoic folding trending N- to NNE. Intense folding and shearing within the Fowler Suture Zone and the NW margin of the Christie Subdomain are likely related to NW collision and transpressive wrenching along the western Gawler Craton during the 1600 – 1400 Kararan Orogeny (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 mobile belt to the west. It is interpreted to have undergone numerous reactivations, including possible folding about a regional N-trending fold on the western margin of the exposed Craton (Figure 6). Orientation of second-order folds adjacent to NNE trending shears in the Fowler Suture Zone suggest a strong component of dextral shear. This was accompanied by development of a (? Palaeoproterozoic) wrench graben along the Coorabie SZ (BARTON Mapsheet; see Figure 7).

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

c) E-W trending structural belts. There are several E-trending, regional-scale zones of dextral(?) transpressive shear that intersect the western and central Gawler Craton (see Figure 8). Two of the most obvious structural corridors are the Tarcoola and Coober Pedy Corridors. These likely reflect earlier (Archaean – Palaeoproterozoic) structures, reactivated as dextral transpressive zones during the Kararan Orogen. These corridors would have allowed bulk lateral expulsion of crust during the NW-verging collisional orogeny. The E-W structural corridors occur as quasi-episodic structures throughout the Gawler Craton: the Polda Trough (Palaeozoic to Mesozoic sedimentation) in the southern Gawler Craton represents a reactivated Proterozoic dextral structural zone.

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

e) N-trending regional fold (Figure 7). The SW limit of the Gawler Craton appears deformed by a regional-scale N-trending fold system. This is coincident with occurrence of a magnetically quiet zone along the Christie and Fowler subdomains (possible granitoids emplaced within the fold?). The proto-Karari Shear Zone may be folded within this zone. The concealed Hughes Subdomain is not folded. This fold may have developed during Mesoproterozoic dextral shear along the inferred Coompana – Isa SZ (Figures 6, 10 & Map 2).

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4.1.1.2. Ammaroodinna & Yoolperlunna Inliers

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

The Yoolperlunna Inlier, (NW of the Ammaroodinna Inlier), lies along the inferred northwestern margin of the Gawler Craton. It has a highly variable magnetic character, and includes both haematite and tourmaline breccia bodies. The exposed Inlier is coincident with a broad NW-trending structural corridor, extending from the NE Gawler Craton: this structural corridor is interpreted to have acted episodically as a province-scale dilation zone both during and after the Kararan Orogeny. This structural corridor was subsequently reactivated as part of the Palaeozoic Boorthanna Trough (see Maps 2 & 3).

The Ammaroodinna & Yoolperlunna Inliers are interpreted here as transitional subdomains between the Western Gawler Craton (NE-trending structural grain dominated by Kimban and Kararan Orogeny deformation), and the Musgrave Block / Munyarai Subdomain (deformational fabric strongly overprinted by early Musgravian Orogeny (1200Ma).

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Figure 6. RTP-1VD magnetic image of Officer Basin and surrounding basement domains. Intense NW-trending structural grain, plus rotated fold axial trends within Christie Subdomain and Fowler Suture Zone (not obvious at this scale) suggest transpressive wrench deformation during Mesoproterozoic along NE - trending shear zones. Yellow line highlights regional N-trending fold axial trend. Green line shows area of current interpretation.

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Dextral wrench basin

Figure 7. RTP-1VD image of Officer Basin and surrounding basement, highlighting dextral wrench basin along Coorabie Shear Zone. a) Yellow dashed area outlined in 7b. b) Palaeoproterozoic – early Mesoproterozoic dextral wrench basin (magnetically quiet zone) developed within dextral overstep in Coorabie Shear Zone (red), western Gawler Craton.

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

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Gairdner Dyke Swarm

Figure 9. RTP – 1st VD magnetic image highlighting mafic dykes of the Gairdner Dyke Swarm. These are evident as narrow, positively magnetic NW-trending bodies. Green arrows highlight two of the highest density zones of dyke emplacement.

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Mt Isa Inlier

Broken Hill Block B

Figure 10. Total magnetic intensity image of Australia (after Geoscience Australia); Coompana-Isa Shear Zone highlighted. a) Interpreted position of Coompana – Isa Shear Zone. b) Schematic reconstruction of Proterozoic terranes by removal of inferred ~300km dextral shear (magnitude of displacement inferred from potential alignment of Mt Isa Inlier & Broken Hill Block. Timing of inferred shearing ambiguous – likely at end of Kararan Orogeny (~1400Ma?). Dextral shear possibly responsible for development of regional N-trending fold evident in Figure 6.

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4.1.1.2. Coompana Block The Coompana Block (see Figures 6 & Map 2) comprises a rhombic to irregular subdomain of mafic volcanics and sediments overlying gneiss and granitoids of the Gawler Craton and Munyarai Subdomain(?). Major mafic- ultramafic intrusives, likely coeval with the volcanics, were emplaced within the felsic basement and the volcanic pile. A K-Ar age of 1159Ma (biotite-hornblende in gneiss; Webb et al., 1982) suggests that the Coompana Block represents a southern zone of Musgravian Orogeny (equivalent) overprint along the far-western Gawler Craton, with a major episode of mafic magmatism during the latter stages of the Orogeny. The mafics may therefore be compared to the Giles Complex & Tollu Volcanics in the western Musgrave Block (see Figure 3). The presence of volcanics indicates the Coompana Block 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 layering within the volcanics, and to the underlying gneissic fabric. A series of NNW –trending negatively magnetic dykes are associated with a major series of sub-circular mafic- ultramafic plutons (herein informally named the Coompana Suite; Map 2 & Figure 11). The Coompana Suite are not restricted to the Coompana Block, but are also emplaced within the western Gawler Craton, and to the NW beneath the central and northern Officer Basin. The dykes also continue beneath the Officer Basin. These intrusives are considered older than the Gairdner Dyke and are likely related to anomalous, negatively magnetic mafic intrusives within the Musgrave Block (see Rankin & Newton, 2002). The northern margin of the Coompana Block is poorly defined, being concealed by increasing sediment cover. It may be coincident with a major NW positively magnetic dyke swarm extending from the Head of Bight coastal area (South Australian) NW across into Western Australian (Figure 11).

The NNW dykes are parallel to a series of NNW-trending basement / basin structures within the Officer Basin area evident in the Bouger gravity data (Figure 12). This implies that the NNW structures evident near the northern margin of the Officer Basin may 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 the Musgrave Block (see Figure 13). A comprehensive description of the structure of the Musgrave Block is given in Rankin & Newton (2002). The structural framework of the Musgrave Block is dominated by:

a) NE to ENE trending orogen – parallel fold/thrust belts. These represent Palaeoproterozoic mobile belts (parallel to, and possibly part of the NW Gawler Craton). The NE trending belts were modified by continued NW transport during the early Musgravian Orogeny (~1200Ma). The NE- trend of these belts likely extends beneath the Officer Basin. Several major E-W and NW-trending structures that intersect these belts may have been initiated as transfer fault zones during development of the NE-trending mobile belts (c/f western Gawler Craton structures).

b) E-W to ESE- trending major shear zones; these include the Mann- Hinckley SZ. These were likely active as dextral transpressive structures during the late Musgravian Orogeny (1080-1050Ma), and later during the

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Petermann Orogeny (550Ma). Some reactivation as thrusts likely also occurred during the Alice Springs Orogeny. The southern margin of the Musgrave Block represents a complex zone of S-verging thrusting, likely active during both the Petermann & Alice Springs Orogenies.

c) NW-trending fault zones displace the earlier NE-trending structural belts. These represent both early mobile belt transfer faults, and later second- order dextral (domino-style) faults associated with dextral shear along major E-trending fault zones.

d) Numerous mafic dyke swarms intersect the Musgrave Block. The major swarms are oriented NW, E-W & N. The 800Ma Gairdner Dyke Swarm of the Gawler Craton is correlated with the Stuart Dyke Swam of the eastern Musgrave Block.

Figure 11. RTP – 1DV magnetic image, highlighting negatively magnetic mafic intrusives of the Coompana Suite emplaced within the Coompana Block, Munyarai Subdomain and Gawler Craton. These are tentatively correlated with negatively magnetic intrusives (Giles Complex equivalents?) within the western Musgrave Block (see Rankin & Newton, 2002). The yellow dashed line (parallel to a major NW dyke swarm) highlights the inferred original northern limit of the Coompana Block.

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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 the Officer Basin are evident in the Bouger gravity data (red dashed lines and hatching). These are parallel to the Mesoproterozoic NNW trending negatively magnetic dykes associated with the Coompana Suite to the south. The NNW structures within the basin are therefore interpreted as being initiated during the late Musgravian Orogeny (~1100 – 1050MaMa). The NNW structures are evident as fault steps along the northern margin of the Officer Basin within the magnetic data (see Map 3).

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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 wrench grabens along Mann – Hinckley SZ. Southern margin of Musgrave Block thrust over Officer Basin to the south during both the Petermann (550Ma) and Alice Springs (350Ma) Orogenies.

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4.1.1.4. Concealed Basement The concealed basement beneath the Officer Basin is magnetically variable, with both 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 Yoolperlunna Inliers are also concealed beneath variable – thickness Officer Basin sediments on the 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 of the Gawler Craton characterised by an overall low (quiet) magnetic signature and a low Bouger gravity signature (similar to the granite – dominated zones of the central Gawler Craton).

Several intensely magnetic bodies appear to trace a folded shear zone through the subdomain (?proto-Karari SZ?). These magnetic bodies are modelled at ~2500 – 3000m depth (from magnetic profiles E-W 1 – 5; Figure 36). These depths have been previously interpreted as reflecting the presence of a significant, narrow N-trending subbasin / canyon intersecting the basement (Teasdale et al 2001). The deep magnetic sources are interpreted here as intrabasement sources (Figure 16); note that similar deep – seated intrabasement magnetic bodies also occur throughout the eastern Gawler Craton.

The inferred granitoids within the subdomain may be either a) late Kararan Orogeny intrusives (1600 – 1400Ma), or b) a series of unrecognised late Mesoproterozoic intrusives (Musgravian / Grenvillian age). A similar late Mesoproterozoic age is inferred for zoned granitoids within the Hughes Subdomain (see below). b) Hughes Subdomain. This is a major NE-trending belt, dominated by nonmagnetic to weakly magnetic elliptical intrusives, with occasional strongly magnetic rims. The magnetic character and shape of the intrusives is best expressed in the south and centre of the domain (Watson Ridge - Murnaroo Platform area (Map 3). To the NE, increasing sediment thickness obscures the basement character.

The NW margin of this subdomain is regionally coincident with a continental – scale structural corridor (evident in magnetic and gravity datasets), extending from the northern margin of the Coompana Block in the SW to the southern (geophysically defined) margin of the Mt Isa Inlier in the NE. An inferred dextral displacement of over 300km during the Mesoproterozoic (1400-1200Ma?) may have displaced the Mt Isa Inlier from a roughly N-S alignment with the Curnamona Craton / Broken Hill Block (see Figure 10). A lack of significant shear fabric evident within the Hughes Subdomain suggests that either: i) The granitoids are correlated to the 1600Ma granitoids within the Gawler Craton, and that there was no significant post - 1600Ma movement along the Coompana-Isa SZ, or; ii) The Coompana – Isa SZ acted as a major (dextral) shear during the late Kararan Orogeny (1400Ma-?), and was subsequently the focus for major granitoid emplacement between 1400 and 800Ma (initial Officer Basin deposition). These granitoids therefor are likely equivalent to the Kulgeran Suite granitoids of the Musgrave Block (1200 – 1050Ma).

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Figure 14. RTP-1VD magnetic image; basement subdomains highlighted. The far-western Gawler Craton, Hughes & Munyarai Subdomains are concealed by Officer Basin sediments. The NW Gawler Craton (including the Nawa Subdomain and Yoolperlunna & Ammaroodinna Inliers) is partly concealed by Officer Basin sediments.

The Nurrai “Ridge” (yellow lines) comprises a linear magnetic belt, with a near coincident gravity high belt. Subtle folded magnetic trends suggest the magnetic “Ridge” comprises a folded belt of magnetic intrusives, possibly extending from a belt of magnetic granitoids to the north within the Musgrave Block. The NNE-trending faults defining the western margin of the intrusive belt were likely reactivated as block faults during the development of the Officer Basin.

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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 is interpreted as a Mesoproterozoic subdomain, in part reactivated by NNE trending block faults during development of the Officer Basin.

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

Profile modelling of the magnetic data indicates depths of 3000m for the positive magnetic body (1), and ~8000m for the negative magnetic body (2). Modelling of narrow mafic dyke anomalies within the same region indicates depths of <1000m. The magnetic bodies 1 & 2 are interpreted here are intrabasement magnetic bodies (similar to the negatively magnetic gabbro (3) within the Coompana Block to the west. The narrow N-trending trough is therefore considered here as an artefact of differing structural levels of magnetic bodies within the basement. An alternative interpretation to that of Teasdale et al (2001) is shown in Map 3.

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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 to significant basin cover. The most significant structural elements evident in the magnetic data are (see Map 2): i) A series of NNE-trending elliptical intrusives (nonmagnetic, with moderately magnetic rims, similar to those within the Hughes Subdomain). ii) NNE- to N- trending, arcuate fault / shear zones. These are oblique to the major domain-bounding Coompana-Isa SZ. They likely represent Mesoproterozoic fold / thrust belts, verging to the WNW, and extending into the NE trending Palaeoproterozoic – Mesoproterozoic fold / thrust belts of the Musgrave Block. There is some evidence of NNE – trending regional scale folding; this may be related to major dextral shear along the Coompana – Isa SZ. iii) “Nurrai Ridge”. This is a series of significant, discontinuous linear magnetic bodies extending from to Coompana-Isa SZ to immediately south of the Musgrave Block (Figures 14, 15). The magnetic bodies are near coincident in the north with a NNE-trending gravity gradient (gravity high block to east). They have been previously interpreted as an irregular belt of mafic intrusives (responsible for the gravity high; see Teasdale et al, 2001). The current interpretation suggests that the gravity gradient is the western edge of a wide basement block, rather than being caused specifically by the magnetic bodies. The magnetic bodies appear to occur in 2 belts, separated by an arcuate N- to NNW shear zone (Map 2). There is some suggestion in the data that the northern magnetic belt may be folded around a N-trending fold axial trace to the east (Map 2, Figures 14, 15). There are no significant mafic intrusives within the Musgrave Block immediately north of the “Nurrai Ridge”; however, the magnetic belt is near coincident with a NNE to NE – trending belt of magnetic Kulgeran Suite granitoids within the southern Musgrave Block (see Rankin & Newton, 2002). It is suggested here that the “Nurrai Ridge” represents a series of Kulgeran Suite intrusives (felsic to intermediate?) aligned along a series of NNE-trending Palaeoproterozoic – Mesoproterozoic shear zones, (subsequently reactivated as horst / graben faults within the Officer Basin).

The Munyarai Subdomain has been separated in Map 2 into: a) Munyarai Subdomain 1 (eastern sector); this subdomain is dominated by N- to NNE structural trends, with some evidence of NNE-trending folds within the basement; b) Munyarai Subdomain 2 (western sector); this subdomain is dominated by elliptical, variably magnetic granitoids (extending into Western Australia). The Nurrai “Ridge” represents the boundary between the 2 subdomains in the north.

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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 suite beneath the eastern flank of the basin (Hughes Subdomain; see Map 2), possibly extending well beneath the central basin (Munyarai Subdomain). The weak structural grain of the intrusives suggests emplacement post Kararan Orogeny (post -1400Ma).

Klein (1995) has proposed that Neoproterozoic intracratonic basins in several continents 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 data beneath the sedimentary sequence. Cooper (1990) has documented the cyclic recurrence of paired anorogenic granitoids and tholeiitic basal associated with intracratonic basin formation.

It is tentatively suggested here that the major suite of granitoids interpreted beneath the 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 the crust during and post granitoid emplacement, rather than as a crustal warp / sag response 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 the deeper parts of the basin may include latest Mesoproterozoic – earliest Neoproterozoic sediments / volcanics.

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4.1.2. Officer Basin

Structures evident from the magnetic data within the eastern Officer Basin are generally separated into several major tectonic subdomains: these are a combination of both syn-depositional and syn-deformational structures. Detailed structure is more evident in the shallower sectors of the basin; in particular the Ammaroodinna / Middle Bore Ridge area and Tallaringa Trough. Due to both the lack of magnetic marker units throughout a large proportion of the basin, and the weaker magnetic character of the basement within the western sectors of the basin, definition of structure 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 the basin may also reflect a much stronger deformational regime within this area. The structural elements described below are highlighted in Map 3. The location of the sectors 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 during initial development of the basin in the Neoproterozoic, and cuts the earlier NE- to NNE- trending basement structural grain. A series of minor horst – graben complexes with NW-SE trending axes were developed along a gently S– shallowing basin platform.

The NW-trending faults are sub-parallel to oblique to many of the (NW to NNW) fault steps 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 of shallowing to the S & SE, with onlap of the basin sediments onto the Coompana Block & Gawler Craton. The overall shallowing of the platform is complicated by dissection of the region into several structural subdomains, principally superimposed during 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 and Cambrian sediments onlapping the crystalline basement to the south.

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

The Murnaroo Platform was described by Gravestock, (1997) as extending to the NE of the Watson Ridge, overlying shallow Gawler Craton basement. The structural extent of the “platform” is poorly constrained within the available data, and this NE extension has been included here within Sector 4 (Murnaroo Platform B). b) Sector 1b (Watson Ridge – informal name). This is a NW-trending belt bounded by a series of NW-trending faults; it forms the northern margin of the Murnaroo Platform, and southern margin of Sector 4. The northern continuation of the Murnaroo Platform (Murnaroo Platform B on Map 3) lies between the Ridge and the Birksgate Subbasin.

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The Watson Ridge includes a fault block of Adelaidean sediment which likely acted as a structural high during Cambrian sedimentation (Map 3). The belt is coincident 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 the Neoproterozoic basin. The ridge has then been disrupted / overprinted by NE- trending block faulting, which controlled variable – thickness deposition of Cambrian 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 Neoproterozoic sedimentation; see Figure 21). The two ridge complexes form an orthogonal margin to the northern depocentres of the Officer Basin.

At the SE end of the Watson Ridge, there is a localised thickening of the basin sediments (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 during Cambrian sedimentation. The zone of thicker sedimentation was offset from, but developed in conjunction with, the Cambrian sedimentation within the Tallaringa Trough. This change in sediment thickness along the ridge coincides with a bend along the Watson Ridge from NW to NNW.

The Watson Ridge is interpreted to have acted as a basin transfer zone during Cambrian (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 both Neoproterozoic, and (principally) Cambrian deposition. It is separated from the main Officer Basin by the Nawa Ridge – a NE-trending structural high comprising a complex series of rhombic fault blocks. These include blocks of: a) Gawler Craton crystalline basement b) 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 highs comprising crystalline basement and Neoproterozoic sediments (Map 3). These are coincident with the SE end of the Watson Ridge.

To the SE, the Trough is bounded by the Karari Fault Zone. The fault zone represents a reactivation of the Proterozoic Karari SZ. The block-fault margin of the trough is locally displaced to the south of the magnetically defined ductile shear zone (Figure 18). The NE end of the Trough comprises a thinning sequence of Cambrian sediments.

The northern limit of the Trough is broadly aligned with a regional – scale E-trending structural corridor within the Gawler Craton (coincident with the Coober Pedy Subdomain; see Figure 19). It is likely that initial development of the Tallaringa Trough may have been associated with weak sinistral shear couple along this province-scale structural corridor during dominantly NW-SE dilation.

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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 NNW trending 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 the Bouger gravity data (see Figure 20). Tectonic style within the Trough appears to be primarily dilational to transtensile, both during and post sedimentation. Similarly, the Nawa Ridge appears to have acted principally a transtensile horst.

The Nawa Ridge & Tallaringa Trough are structurally partitioned from the transpressive 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).

Figure 17a. RTP-1VD image – Officer Basin; note high-frequency detail in magnetic data for shallow sectors of basin, particularly in the Ammaroodinna / Middle Bore Ridges & Tallaringa Trough areas. Resolution of structure in the western sector of the basin is hampered by a) increasing thickness of nonmagnetic cover, and b) an apparent lack of magnetic bodies within the basement.

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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 basement with high density of magnetic anomalies, and western sector of Basin (thick cover over relatively magnetically – quiet basement).

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

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Figure 18. RTP-1st VD 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 the Tallaringa Trough is represents a late reactivation of this structure. Locally, the Palaeozoic sector of the fault zone (red line) is significantly displaced from the controlling Proterozoic structure.

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Figure 19. Structural framework of Officer Basin superimposed on RTP-1VD magnetic image. Tallaringa Trough bounded by a) Karari FZ to SE, and b) complex NE-trending fault zone to NW (part of Nawa Ridge complex). Northern end of Trough merges with E-W basement corridor (parallel to Coober Pedy Subdomain. Internally, the Tallaringa Trough comprises a series of N- to NNE trending fault blocks. NW-SE extension / opening of the Trough may have been associated with minor sinistral shear along the E-W trending basement structural corridor (yellow arrows).

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Tallaringa Trough

Figure 20. Structural framework of Officer Basin. Dashed red lines and hatching outline trend of regional structures evident in Bouger gravity data. These are parallel to NNW trending structures at SW end of Tallaringa Trough. See also Figure 12.

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4.1.2.3. Sector 3 (Nawa Ridge)

This comprises a complex zone of rhombic fault blocks, with syn-depositional development 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 subsequently downthrown 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 been interpreted as a zone of thin Neoproterozoic sediments onlapping the Gawler Craton (magnetic units within the basement are relatively high frequency, indicating only thin to moderate sedimentary cover).

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

The Nawa Ridge is separated from the transpressive deformation – dominated Sector 5 by the subtly expressed Birksgate – Coober Pedy Corridor (Map 3). This structural zone appears to have restricted deformation associated with the late Musgravian, Petermann & Alice Springs Orogenies into the north of the basin (see Figure 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. The structure may represent incipient development of a Musgravian Orogeny dextral shear, parallel to major E- trending dextral shear zones throughout the Musgrave Block.

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Figure 21. Officer Basin structural framework highlighting location of Nawa Ridge and Birksgate – 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 Trough appears bracketed by the Birksgate – Coober Pedy Corridor, and the Watson Ridge (area of green stipple).

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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 Officer Basin, and includes a) the Birksgate Subbasin, b) the Munyarai Trough, and c) the northern sector of the Murnaroo Platform (see Map 3, Figures 22, 23). It is bounded to the SW by the Watson Ridge, and to the SE by the Nawa Ridge and Ammaroodinna Ridge. To the north, it is overthrust by the Musgrave Block. This northern 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, and towards ESE / SE in the Birksgate Subbasin (see Map 3). The folding is a composite of Petermann & Alice Springs Orogeny (N-S compression, coupled with dextral NW – SE trending shear). E-trending folds within the northern basin are also likely associated with E-trending thrust faults. • NNW-trending faults: these include significant step faults in the northern margin of the basin, and parallel subtle structures evident in the Bouger gravity data within the basin (Figure 20). a) Birksgate Subbasin. This is a ~NW trending deep subbasin (up to 6km thick); it lies along the same NW structural axis as significant depocentres within the Waigen and Yowalga Subbasins within the western Officer Basin (WA; see Apak & Moore, 2000). The principal subbasin is defined by an intense Bouger gravity low anomaly. The principal axis of the subbasin is coincident with the (?thrust) contact between Neoproterozoic and Cambrian sediments at surface. This NW structural axis (Birksgate – Cobber Pedy Corridor) extends to the SE, where it is coincident with the southern limit of Ordovician – Devonian sedimentation within the Munyarai Trough. The structural axis then swings to the east, parallel, with the E-trending Coober Pedy Subdomain.

The Birksgate – Coober Pedy Corridor is intersected by several NE to NNE transfer faults, which likely partitioned sedimentation during the Neoproterozoic deposition, and were subsequently reactivated as block faults during Cambrian deposition.

Shallow structures evident in the magnetic data (including E- to ESE-trending folds) suggest dextral transpressive movement along the major NW-trending structures (see Maps 1 & 3).

b) Munyarai Trough. This is a major ~NE-trending subbasin complex SE of the Musgrave Block. It is separated from the Birksgate Subbasin in part by i) The NNE – trending structural high associated with the “Nurrai Ridge”, and; ii) An inferred structural high trending NNW-SSE from the Musgrave Block towards the SW end of the Nawa Ridge.

The Trough is characterised by an intense Bouger gravity low anomaly. The magnetic and gravity data suggest the Trough is partitioned into several lobes by

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

The Trough is complex, with deposition focussed within the area during: • Early Neoproterozoic (NE-SW dilation, partitioned by NE-trending transfer faults); • Petermann Orogeny (550Ma) – initial overthrust of Musgrave Block over Trough 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 the magnetic data, except for several NE-trending faults associated with the Nurrai “Ridge”.

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Murnaroo Platform B

Figure 22. Outline of Sector 4 of northern Officer Basin (superimposed on RTP-1VD image). Sector 4 includes the NW trending Birksgate Subbasin and ~ NE trending Munyarai Trough, plus the northern extension of the Murnaroo Platform.

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Murnaroo Platform B

Figure 23. Outline of Sector 4 of northern Officer Basin (superimposed on colour Bouger gravity image). Birksgate Subbasin and Munyarai Trough are evident as significant gravity low regions. The northern margins of the subbasins are overthrust by the southern margin of the Musgrave Block; fold/thrust structures are evident in the detailed magnetic data (Map 3) and in field mapping. Murnaroo Platform B is evident as a broad gentle gravity gradient dipping to the north.

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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 deformation dominated 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, commonly intersected 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 of several minor fault blocks comprising Neoproterozoic sediment with no overlying Cambrian. These lie along a roughly E-trending structural zone (Figure 25). This may reflect; 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 with Cambrian sediments directly overlying basement. This suggests that the Ridge may have had an early history as a structural high during Neoproterozoic sedimentation (similar to the Nawa Ridge to the south). The arcuate fault pattern, and interference by common E-W faults suggest the Ridge was reactivated as transpressive “pop-up” blocks during the Alice Springs Orogeny (NE sinistral transpressive shear).

The Manya Trough lies between the Ammaroodinna & Middle Bore Ridges, and was in 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 subdomain is bounded to the north by the Boorthanna Trough, to the west by the Munyarai Trough (Sector 4 – yellow line), and to the east by the Gawler Craton. To the south, it is partitioned from the predominantly transtensile Nawa Ridge – Tallaringa Trough region by the Birksgate – Coober Pedy Corridor (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 to SE 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 sediments with no Cambrian cover).

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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 and Adelaide Geosyncline during Neoproterozoic – Early Palaeozoic sedimentation (Figure 26). The principal NW trending sector of the subdomain was reactivated as a major 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 tectonically intercalated with) crystalline basement (Musgrave Block and/or Gawler Craton). The Bitchera Ridge was developed in response to dextral transpressive shear along the eastern continuation of major E-trending shear zones within the Musgrave Block during the Petermann Orogeny. The Ridge acted as a structural high during Cambrian sedimentation. At least one bed of strongly magnetic volcanics , gently dipping to the north, and deformed by gentle NE-trending folds are 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 thick sequence of Carboniferous-Permian glacigene sediment. The Boorthanna Trough lies on the western flank of the NW – trending continuation of the Torrens Hinge Zone (Adelaide Geosyncline.

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

4.2.1.7. Salt structures

Substantial salt and development of salt structures within Neoproterozoic sediments has been documented for the Munta and Marla areas (sector 5) (Lindsay, 1995, Gravestock, 1997). One of the best expressions of salt structures is the development of 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 the Western Officer Basin. The lack of evidence elsewhere within the eastern Officer Basin is considered here to be due not to a lack of salt, but a combination of factors: a) Thickness of cover; b) Poor resolution magnetic & gravity data over the majority of the basin; c) Lack of suitable magnetic susceptibility contrast between salt & host sediments; d) Very limited seismic data throughout majority of Basin.

No structures or magnetic signatures characteristic of salt diapirs or piercement structures were evident within the magnetic data. Note that salt structures may commonly be evident in detailed magnetic data as magnetic lows with a weak to moderate magnetic rim. The magnetic rim can be produced as an edge effect of steep upturn of weakly to moderately magnetic sediments / volcanics, and / or a limited redox reaction between the halite and Fe within the host.

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Salt diapirs may also be evident from weak radial or concentric fracturing above the diapir (this may be evident in magnetic and/or satellite imagery). This effect is dependent 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 regionally coincident with the intersection of the Sector5 Transpressive Domain, and a series of NW-trending faults ~parallel to the Birksgate – Coober Pedy Corridor. Other analogous 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 comprises a) The ~ E-W trending Bitchera Ridge (Neoproterozoic sediments & volcanic with shallow to locally exposed crystalline basement), and; b) The NW trending Boorthanna Trough (thick Carboniferous – Permian sediments overlying Neoproterozoic sediments & volcanics.

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4.2. Tectonic Development

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

The following is a summary of the principal tectonic episodes, and their effect upon development of the Officer Basin. The chronology of events is highlighted in Figures 2 & 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 have undergone a complex series of tectonic events. A simplified overview of these tectonic episodes includes:

Pre - 1600 Ma • Deformation of Archaean Mulgathing Complex (Gawler Craton) during Mulgathing Orogeny (2550-2450Ma) • Development of Palaeoproterozoic mobile belts, intrusives and episodic deformation 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 to Musgrave Block metamorphics with major NE to NNE structural grain.

Kararan Orogeny (1600-1400Ma – Gawler Craton) • NW- trending compression / collision(?) of Gawler Craton with protoliths to Musgrave Block / Munyarai Subdomain. Development of strong NE- trending structural grain (transpressive / thrust), with quasi-regular spaced E-trending dextral N-trending sinistral & NW trending tensile structural corridors. NW-trending structural corridor in NE Gawler Craton acted as focus for Iron Oxide – Cu - Au mineralisation. Various structural orientations 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 by inferred dextral displacement along deformation zone between Gawler Craton & Munyarai Subdomain (Coompana – Isa SZ ; several 100’s kms displacement).

Musgravian Orogeny (1200 – 1050Ma) • 1200Ma - ? Early Musgravian Orogeny. Continued W- to NW trending compression. Thrusting of Albany Fraser Belt onto Yilgarn Craton (WA). Reactivation / overprinting of NE structural grain within Musgrave Block (and Munyarai Subdomain?). Emplacement of Kulgeran Suite intrusives along NE to ENE trending belts (Musgrave Block & Munyarai Subdomain). Possible early emplacement of Giles Complex (Musgrave Block).

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Gawler Craton / Coompana Block – possible emplacement of major 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. Further emplacement of granitoids, plus mafic-ultramafic intrusives. Extrusion of Tollu Volcanics. Coompana Block – Gawler Craton: ?Age of Coompana Suite intrusives and associated NNW trending dyke swarm. Extrusion of mafic volcanics (Coompana Block). Early development of NNW structural grain within basement to Officer Basin. Thermal weakening of crust in area of Officer Basin. Possible localised deposition of late Mesoproterozoic sediments prior to Officer Basin sequences.

4.2.2. Officer Basin – Neoproterozoic

Gairdner Dyke Swarm emplacement (800Ma)

Initiation of rifting immediately prior to Officer Basin development. Gairdner Dyke Swarm emplaced throughout Gawler Craton, Munyarai Subdomain, Coompana Block and Musgrave Block. Major concentration evident within SW Murnaroo Platform. Interaction between NNW & NW tensile structures developed during this event 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. Regional architecture of the eastern Officer Basin / NW Adelaide Geosyncline forms a major 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 of Musgrave Block (previously interpreted as development of basin by N-S compression & foreland sagging of crust). Current interpretation suggests initial deep subbasin development developed by steep normal faulting along Neoproterozoic (now concealed) southern margin of Musgrave Block, with later inversion / overthrust of basin margin. Mafic volcanics extruded along northern margin of basin.

Petermann Orogeny (550 – 530Ma) • Musgrave Block – Reactivation of major NW- to E-W transpressive dextral shear, principally along Mann – Hinckley Shear Zone. Development of Woodroffe Thrust & Petermann Nappe Complex in north. Levenger & Moorylianna Grabens developed as dextral wrench basins along major E- trending shear zones. Grabens in part compensate crustal thickening to north in area of Woodroffe Thrust.

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Southern margin of Musgrave Block possibly inverted & steeply thrust 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 of transpressive thrusting in Marla – Ammaroodinna area. Transpressive effects of orogeny do not appear to have extended south beyond the Birksgate – Coober Pedy Corridor.

Note- Major transpressive strain developed during the Petermann Orogeny was concentrated in the northern half of the Musgrave Block and Amadeus Basin. Effects within the Officer Basin appear minor. Development of structural highs (Middle Bore Ridge area) with no Neoproterozoic sedimentation, (or erosion prior to Cambrian sedimentation) may have initiated at this time.

Cambrian Sedimentation

Relaxation post- Petermann Orogeny. NW-SE dilation, possibly associated with minor sinistral shear couple on existing NW structural corridors.. Reactivation of some NE-trending basement structures, an initiation of new structures as normal faults. NW faults reactivated as tensile / basin transfer faults. Superposition of NE-SW trending horst-graben architecture on Neoproterozoic NW-trending horst-graben architecture.

• Munyarai Trough – localisation of thick Cambrian sedimentation by minor sinistral shear couple between Birksgate – Coober Pedy Lineament & Boorthanna Trough Corridor. • Birksgate Subbasin – possible localisation of thicker Cambrian sedimentation (similar to Munyarai Trough), or more platformal drape continuous from Murnaroo Platform? • Tallaringa Trough – NW-SE dilation, associate with minor sinistral shear couple on Birksgate-Coober Pedy Corridor. Reactivation of Karari SZ as normal fault. Nawa Ridge acts as NE-trending structural high. Dilation within Tallaringa Trough partitioned against SE end of NW trending Watson 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.

Delamerian Orogeny (~500Ma)

No significant effects of the Delamerian Orogeny have been noted within the Officer Basin. A possible disconformity occurs between Cambrian and Ordovician sediments in the north of the Basin.

The most likely zone affected by the Delamerian Orogeny is the Bitchera Ridge v- Boorthanna Trough area. This zone is regionally coincident with the link between the Officer Basin and the NW extension of the Adelaide Geosyncline. Gentle NE trending folds within the Neoproterozoic sediments may be related to this event.

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Ordovician Sedimentation

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 was focussed by weak sinistral shear couple along Birksgate – Coober Pedy & Boorthanna Trough Corridors.

Devonian Sedimentation / Alice Springs Orogeny

• Moderate thickness of Devonian sediments deposited within Munyarai Trough, immediately prior to Alice Springs Orogeny (+3000m; Gravestock et al, 1995). This depocentre roughly coincides with Ordovician depocentre; sedimentation likely focussed by similar sinistral shear couple along 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 NE Officer Basin occurrences, or represent localised (wrench basin?) depocentres.

• Alice Springs Orogeny – (400-350Ma). Major transpressive thrusting within Arunta Inlier / northern Amadeus Basin. Musgrave Block – only minor reactivation of existing transpressive structures?

Officer Basin – transpressive dextral shear couple along Birksgate – Coober Pedy & Boorthanna Trough Corridors. E-W en-echelon folding of Ordovician – Devonian sediments in Boorthanna Trough, associated with sinistral transpressive shear along Marla Overthrust / Ammaroodinna Ridge 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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A B

C D

Figure 27. Summary cartoons of Neoproterozoic – Devonian tectonic development, eastern Officer Basin. Major extensional and thrust fault orientations shown as black lines. Transfer faults / structural zones shown as dashed blue lines. a) 800Ma – emplacement of Gairdner Dyke Swarm (NE-SW dilation) b) Neoproterozoic sedimentation – NE-SW dilation c) Principal depocentre axes (yellow line) for Neoproterozoic sedimentation – NW Adelaide Geosyncline to western Officer Basin d) Petermann Orogeny (~550Ma). ~S-verging overthrust of Musgrave Block onto Officer Basin. Associated with NW-SE dextral shear couple. Initial development of oblique thrusting within northern sector of Officer Basin. Significant transpressive deformation restricted to north of Birksgate – Coober Pedy Corridor. Possible reactivation of N- to NNW structures as extensional faults (Nawa Ridge / Tallaringa Trough area).

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E F

G H

Figure 27 (continued). Summary cartoons of Neoproterozoic – Devonian tectonic development, eastern Officer Basin. Major extensional and thrust fault orientations shown as black lines. Transfer faults / structural zones shown as dashed blue lines.

e) Initial Cambrian sedimentation – NW-SE dilation, with possible sinistral shear couple along NW-SE transfer structures. f) Delamerian Orogeny (~500Ma). Restricted to minor NE-trending folding within Bitchera Ridge area. g) Ordovician sedimentation. Restricted to Munyarai Trough area. NE-SW extension possibly associated with weak sinistral shear couple along NW- SE transfer structures (including Birksgate – Coober Pedy Corridor). h) Devonian sedimentation. A second episode of NE-SW extension possibly associated with sinistral shear couple along NW transfer structures. Sedimentation again restricted to Munyarai Trough area.

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i) Alice Springs Orogeny (~350 – 400Ma). Weak S-verging overthrust of Musgrave Block over N margin of Officer Basin. Associated with regional NW-SE dextral shear couple. Reactivation of NE-SW and E-W(?) oblique thrusting within NE transpressive sector. Transpressive deformation appears restricted to north of Birksgate – Coober Pedy Corridor.

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4.3. Depth to Basement.

There have been several studies conducted to model depth to “magnetic basement” for the Officer Basin, including: a) Modelling of contour data by R Gerdes (1970’s – 80’s; S Daly, pers Comm.); 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 extracted from the gridded magnetic data (Figure 28). The computed depths were integrated with 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 marked differences 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 (over large areas) significantly shallower than those shown in the SEEBASE model. This is particularly evident in the north of the basin. This suggests that the Euler 2-D model has 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 differently in 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 to display of both basement and sedimentary basin magnetic sources in the same dataset. b) The SEEBASE model shows the “ridge” (and part of the Murnaroo Platform to the SE) as NNE to NE trending topographic highs (2000 – 3000m ridges against a background basin “floor” of ~4000m); the ridges are coincident with magnetic bodies in the basement.

A significant, narrow, N-trending topographic low is evident in both datasets immediately west of the exposed western limit of the Gawler Carton (SE Officer Basin). This has been interpreted by Teasdale et al (2001) as a possible sedimentary trough. The feature is coincident with a regional - scale N-trending structural corridor active in the Mesoproterozoic (involving folding & shearing of the Gawler Craton).

The magnetic character of the Gawler Craton sequences changes in this region from strongly (and highly variably) magnetic in the east, to overall magnetically quiet, with only occasional magnetic bodies evident (particularly several inferred magnetite alteration bodies associated with the proto – Karari SZ). The change to a quiet magnetic character is evident in shallow basement on the edge of the Craton, and cannot be wholly ascribed to a depth effect of increased sedimentary cover.

Simplified straight-slope modeling of magnetic profiles extracted from the gridded data across this zone suggests an average sedimentary cover of 600 – 800m, with an isolated magnetite body modelled at ~2500-3000m (see Figure 37, 38).

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Two possible interpretations can be proposed for this zone: a) The isolated, deep magnetic body within the zone is coincident with top of crystalline basement, and that the zone represents a significant, narrow sedimentary 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 of magnetically quiet granitoids emplaced within the anomalous Mesoproterozoic N- trending structural zone. Sedimentary cover is variable through the region, but does not appear directly associated with a significant N-trending trough. Note – there are numerous magnetite alteration bodies throughout the Gawler and Curnamona 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 profiles to the magnetic image (Figure30) suggests there are several possible discrepancies or 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 bodies within the basement. While some reactivation of the NE tectonic grain within the Gawler Craton occurred during deposition and deformation of the Officer Basin, the complex horst/graben block structure evident within the shallower parts of the basin indicates that much of the basin structure intersects the earlier basement structural grain. b) The lack of numerous magnetic sources that can be modelled in the deeper sectors 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 magnetic features and basin topographic ridges within the main part of the basin.

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

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Figure 28. SEEBASE model of depth to basement – eastern Officer Basin (from Teasdale et al, 2001). Note anomalous N-trending “trough” on western margin of Gawler Craton. This is interpreted by the author as an artefact caused by modelling of intrabasement magnetic sources.

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Figure 29. Depth to magnetic basement and Euler 2-D modelling (Calandro & Read, in press). This model appears to have highlighted shallow magnetic sources within the Officer Basin.

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

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Profile Modeling - Issues

There are several factors that must be addressed when either modeling or reviewing / interpreting modelled magnetic data (for a review of profile modelling constraints, see Isles et al, 2000): a) The ease of extraction of profile data along any profile orientation within modern software packages has meant that the majority of modelling compiled on large datasets is calculated on a gridded dataset. The profiles extracted are therefore a degraded dataset; gradients are typically smoother than the observed data. In addition, gridded data over large areas generally will comprise data collected at various line spacings and resolution. This also may provide discrepancies between observed and gridded profiles across extended profiles.

Although these discrepancies become less noticeable with significantly deep magnetic sources, the effects are noticeable as sources become shallower. This is important, as accurate depth modeling is dependent on the detailed geometry of the profile gradients.

Where possible, modeling of profile data should be conducted on the original flight – line data. b) For accurate depth modeling, profiles should be carefully selected to intersect near the centre of bodies. If the profiles merely cover the end of a magnetic anomaly, this will provide an inaccurate (deeper?) result. c) Both the geometry and orientation of the body with respect to the profile line needs to be taken into account.

If the profile line is not orthogonal to the magnetic anomaly, then a cosine correction must be calculated to convert the gradient width. This may be easily overlooked in automated (gridded) depth modelling procedures.

The depth calculated from a magnetic anomaly is dependent on the geometry of the body. For example: i) For a narrow, strike – length extensive body (dyke model), Depth = SS* ii) For a prism / pipe body (length:width < 1:3), Depth ~ 2xSS iii) For a spherical / irregular body, Depth ~3xSS (SS= horizontal width over which the anomaly gradient is straight)

Therefore, the overall interpreted shape of the modelled body must be taken into account when calculating depth. Most automated modelling packages allow for iterative input of anticipated shape; however this must be checked for each anomaly modelled.

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Comparison of Depth to Magnetic Basement and Seismic Data

A brief review of the SEEBASE depth to magnetic basement and existing seismic data was undertaken.

Two-way time algorithms were calculated from drillhole data for the northern Officer Basin by P Boult (PIRSA).

The SEEBASE depth to basement data was converted to 2-way time based on these algorithms, and plotted on selected seismic profiles. Profiles in the western sector of the basin were chosen to check NNE trending ridges outlined by the SEEBASE data. A brief comparison of relevant features is compiled below (see Figures 31 - 35).

• 93-AGS-03 (Figure 32) A broad topographic high within the SEEBASE data corresponds to the edge of a magnetic - high block in the magnetic data. The current interpretation suggests the seismic profile lies ~ parallel to a significant transfer fault zone within the eastern margin of the basin; the magnetic block is part of a NE-trending structural high, downthrown to the NW & NE. The seismic profile is ~ parallel to the structural high in this region. The regional stratigraphy appears relatively flat – lying. There are a series of minor horst-graben blocks within the sediment coincident with the SEEBASE topographic high., although there is no specific evidence of the topographic high within the seismic data.

It is likely the SEEBASE topographic high principally reflects the presence of a magnetic body rather than the detailed topography of the basement surface; however, some topographic relief likely occurs across the interpreted transfer faults.

• 93-AGS-04 (Figure 33) The seismic data show a series of relatively undeformed strata, with a shallow N- dip. There are several horst – graben dilational faults evident throughout the section. A very broad topographic high from the SEEBASE data corresponds to the positively magnetic bodies associated with the Nurrai “Ridge”. There are no other magnetic bodies close to these to provide a check on the background basement depth. The basement horizon in the seismic data does not appear to vary greatly, in contrast to the SEEBASE profile.

• 93-AGS-05 (Figure 34) The seismic data highlights a series of relatively flat-lying sediments, shallowly dipping to the south. At the southern end of the profile, the sediments appear downthrown by a series of normal faults. This zone of normal faulting is roughly coincident with a broad topographic low in the SEEBASE data; the low is associated with the NW edge of the Hughes Subdomain. The SEEBASE low likely reflects some downthrow along the bounding fault of the Hughes Subdomain.

• 93-AGS-06 (Figure 35) The seismic data shows a series of relatively undeformed sediments, dipping shallowly to the north. A broad topographic high shown in the SEEBASE depth profile is coincident with a swarm of positively magnetic dykes within a magnetically quiet host. The topographic high is roughly coincident with the base

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of sediments observed in the seismic data. The basement topography along this profile is interpreted here as relatively planar; the undulating topography within the SEEBASE data is an artefact of limited modelled depth solutions (magnetically quiet area given greater depth than modelled anomalies).

The SEEBASE basement topography appears to reflect the presence of magnetic units within a magnetically quiet basement, rather than basement topographic relief.

Figure 31. Location of seismic lines – eastern Officer Basin. Seismic lines reviewed are highlighted.

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Figure 32. Seismic profile 93 – AGS03.

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Figure 33. Seismic profile 93 – AGS-04.

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Figure 34. Seismic profile 93-AGS-05.

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Figure 35. Seismic profile 93 – AGS06

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Figure 36. Seismic section 86OF-01. Salt diapir outlined in yellow (P Boult pers Comm).

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N-S 1&2 N-S 1&2

E-W 6

E-W-5

E-W 1

E-W 2

E-W 3

E-W 4

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

Manually – determined depths (from straight-slope method) are highlighted. Profiles are shown in Appendix 2.

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

Manually – determined depths (from straight-slope method) are highlighted. Profiles are shown in Appendix 2.

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5. EXPLORATION TARGETING

5.1. Officer Basin – Hydrocarbons

Previous exploration within the basin has concentrated on the Marla – Ammaroodinna region (NE sector). This has been driven by: a) Known hydrocarbon shows within previous drilling; b) Presence of known salt structures within existing seismic data c) Presence of fold/thrust structures (from seismic data) d) Easy access compared to the rest of the basin (adjacent to Stuart Highway).

This study has highlighted the presence of significant structural trends throughout the rest of the basin that may have assisted in the development of structural traps (and possibly stratigraphic leads during sedimentation).

The key structural features noted with relevance for hydrocarbon exploration are:

• Wrench / transpressive tectonics (NE sector)

The northern half of the basin has undergone variable transpressive wrench / thrust deformation associated with both the Petermann & Alice Springs Orogenies. The most significant structures (Alice Springs Orogeny) may have developed post hydrocarbon development and accumulation.

Where deformation involved significant faulting as well as folding during the Alice Springs Orogeny (eg. Marla Overthrust / Ammaroodinna Ridge areas), it is possible that any major hydrocarbon traps have been breached. Any surviving hydrocarbon traps are therefore considered here unlikely to be large-scale.

• Wrench tectonics (regional)

Within other areas of the northern basin, wrench / transpressive tectonics appears less intense, and may be more associated with broad E-trending folding. En-echelon E- trending folds (and localised transpressive popup structures) may have developed above NW-trending basement faults (NW-dextral wrench couple interpreted during Petermann & Alice Springs Orogenies) in the central and western sectors of basin.

• Watson Ridge

The NW trending Watson Ridge is interpreted to have acted as a structural high during both Neoproterozoic and Cambrian sedimentation (more locally). It also acted as a basin transfer zone, particularly in the SE (partitioning of dilation and sedimentation in and adjacent to the Tallaringa Trough.

Limited seismic data shows some normal faulting and horst block development along the northern margin of the Watson Ridge. The Ridge is considered here to have regional potential for both stratigraphic onlap and later structural high / rollover style hydrocarbon traps.

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• Salt structures

Seismic and drilling data in the Munta – Marla area has confirmed the presence of remobilised salt structures within the Neoproterozoic section. The salt piercement structure at Ungoolya (Munta area) is regionally coincident with the intersection of NW- trending faults (parallel to the Birksgate – Coober Pedy Corridor), and the western margin of the NE transpressive structural zone (Sector 5). This structural locus would have provided a series of steep structural intersections for diapiric salt movement. Similar structural loci occur : i) Immediately north of the intersection of the Nawa & Watson Ridges; ii) Along the Watson Ridge and Birksgate – Coober Pedy Corridor (intersection by NE trending transfer faults). A series of regional structural intersections with potential for focussing of salt tectonics have been highlighted on Figure 38.

• NNW structural high (inferred).

A narrow NNW-SSE trending structural high / basement topographic gradient likely extends from the southern margin of the Musgrave Block (central sector) SSE towards the SW end of the Nawa Ridge (in part defined by gravity data). This structural corridor in part acts as the dividing zone between the Birksgate Subbasin and Munyarai Trough. This structural zone (likely initiate in the late Mesoproterozoic and reactivated in the Petermann & Alice Springs Orogenies) may provide a structural high focus for rollover – style structural traps.

In summary, areas of the Officer Basin associated with regional- scale (deep-seated) structural trends, but with only limited shallow complex structure evident, are considered here to have potential for subtle structural (and stratigraphic?) traps that have not been breached by late stage (Alice Springs Orogeny) structures. There is insufficient data at present in the central and western sectors of the Officer Basin (South Australia) to review potential source and reservoir rock distribution with respect to the structural trends.

• Potential Mesoproterozoic sedimentary section.

The current interpretation has suggested that initial basin development may have been triggered by thermal sag associated with (inferred) late Mesoproterozoic granitoid emplacement. Potential may therefore exist for localised zones of late Mesoproterozoic sedimentary section below the known Neoproterozoic evaporite horizon.

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Figure 39. Structural framework of eastern Officer Basin. Several zones of intersecting regional NW & NE structures have been highlighted as potential loci for structural trap development (including salt tectonic structures). Munta 1 DDH (Munta – Ungoolya area), which includes salt tectonic structures lies near the intersection of regional NE & NW structures. Note – regional distribution of evaporites throughout basin poorly understood.

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5.2. Basement – Mineral Targets

The basement to the Officer Basin has potential for Volcanogenic Massive Sulphides, Broken Hill - style base metals and Iron Oxide Cu-Au style mineralisation. However, depth to basement throughout the majority of the basin is considered here prohibitive for effective exploration.

The principal mineral targets within the region are considered: a) NiS / PGE mineralisation associated with the Coompana Suite intrusives. The structural setting of the intrusive suite has similarities to the Giles Complex / Tollu Volcanics of the western Musgrave Block (with known NiS mineralisation). In addition, the Suite has some similarities to the mafic extrusive / intrusive complex hosting the world-class Noril’sk NiS deposit: i) Emplacement of mafic / ultramafic intrusives and flood basalts along a major terrane – bounding fault zones (Coompana – Isa SZ and Karari SZ). ii) Locus of emplacement associated with major basement / basin margin. iii) Occurrence of both large and small-scale intrusives (Noril’sk associated with <6km wide pluton).

b) Kimberlites (both within basement and basin). Significant terrane – bounding and crosscutting structures beneath the Officer Basin (and adjacent basement areas) would have provided favourable loci for intrusion of kimberlitic – lamprophyric pipes within both basin and basement. The NNW trending structures evident in the Bouger gravity data, and the parallel structures associated with the Coompana Suite dykes are considered here to have potential for focussing of kimberlitic intrusives (known association with mafic – ultramafic intrusives, and episodic reactivation).

No anomalous magnetic signatures suggestive of kimberlites were noted in the detailed magnetic data for the northern Officer Basin. The magnetic data throughout the rest of the basin is of too coarse a resolution to be useful for kimberlite detection. More detailed magnetic data throughout the rest of the Officer Basin would be required to more comprehensively assess the potential of the region.

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6. SUMMARY & RECOMMENDATIONS

SUMMARY

The eastern Officer Basin has been subdivided into a series of tectonic subdomains. The subdomains are in part defined by a series of major NW-SE and NE-SW trending fault zones: these fault zones are in part reactivated basement structures (in particular NE-trending structures), and in part new structures cross-cutting the earlier basement trends.

The subdomains are: • Sector 1 (SW region) : Murnaroo Platform and Watson Ridge; • Sector 2 (SE region) : Tallaringa Trough; • Sector 3 (SE Region) : Nawa Ridge; • Sector 4 (Northern / Central Region) : Birksgate Subbasin and Munyarai Trough; • Sector 5 (NE Region) : NE transpressive zone, including Marla Overthrust, Ammaroodinna & Middle Bore Ridges & Manya Trough; • Sector 6 (NE Region) : Bitchera Ridge & Boorthanna Trough.

There are several subtly – expressed structural corridors throughout the eastern Officer Basin that appear to have partitioned strain, (acting as transfer structures), during both deposition and deformation. These include: • Ammaroodinna – Nawa FZ (NE- trending) – this is in part a reactivated basement structure. • Birksgate-Coober Pedy Corridor (NW-SE to E- trending) – this is aligned with the principal axis of the Birksgate Subbasin, and swings to the east to become aligned with a significant regional E-trending structural zone within the Gawler Craton. The structure in part defines the southern limit of Ordovician – Devonian sediments within the basin. The structural corridor also partitions zones of differing strain (transpressive wrench tectonics of the Petermann & Alice Springs Orogeny to the north, and transtensile deformation to the south). • NNW structural Corridor – this is a structural zone defined by NNW-trending faulting along the southern margin of the Musgrave Block, NNW-trending Bouger gravity gradients extending to the SSE, and NNW-trending faulting in the SW end of the Nawa Ridge. It separates the Birksgate Subbasin and Munyarai Trough, and may have acted as a structural high during both Neoproterozoic and Palaeozoic sedimentation. • Watson Ridge (NW-trending) – this extends from the NW of the region (southern limit of Birksgate Subbasin, to the SE where it in part bounds the western limit of the Tallaringa Trough. This structure likely acted as a structural high during Neoproterozoic sedimentation and (locally) during Cambrian sedimentation.

Previously calculated depth to basement models for the basin highlight different structures and apparent structural levels: a) The Euler 2D model of Calandro & Read, (in press) appears to highlight shallow magnetic sources at the expense of a detailed review of the deeper basin topography; b) The SEEBASE model of Teasdale et al, 2001 provides a relatively detailed image of the top of magnetic basement topography, which is not necessarily coincident with top of crystalline basement.

The current interpretation and review of geological constraints on magnetic source modelling (including drillhole, seismic, magnetic profile modelling and geological model concepts) highlight areas where the SEEBASE model may be refined. In particular:

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• The SEEBASE model highlights several topographic highs that are coincident or near coincident with magnetic bodies within the basement, but within areas where seismic data shows no such basement structures. The SEEBASE model appears to have modelled a deeper solution for magnetically-quiet zones adjacent to magnetic sources in some areas, rather than utilising the depth solution of the magnetic bodies as the overall depth for the zone; • Several deep magnetic bodies (>2.5km) in the SE of the basin have been used to model a significant N-trending graben / trough (Teasdale et al, 2001). The current interpretation suggests that localised thickness of the basin in this area is of the order of 1000m, and that the deeper magnetic sources are intrabasement.

A series of regional structural trends with for potential hydrocarbon accumulation (structural traps) have been highlighted, including potential NW- and NNW-trending structural highs and intersection zones of dilation / transfer fault zones (considered potential loci for salt diapirism).

RECOMMENDATIONS

• The current map of depth to basement should be revised to incorporate the interpreted structure. This will require further modelling of key magnetic profiles, and contouring of top of basement depth solutions within defined structural blocks. • More detailed gravity data should be acquired over the western sector of the basin, to cover the Watson Ridge and adjacent subbasin areas. • Acquisition of further seismic data should be promoted in the western sector of the basin. • The structural framework should be integrated with sedimentological and thermal maturation studies to refine hydrocarbon lead identification.

Leigh R Rankin

Consultant Geologist Director, Rankin Consultancy PL June 2003

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REFERENCES

Apak, S.N., & Moors, H.T., 2000. Basin development and petroleum exploration potential of the Yowalga area, Officer Basin, Western Australia. Geological Survey of Western Australia. Report 76.

Barenov, V., 1957. A new method for interpretation of aeromagnetic maps – pseudo gravimetric anomalies. Geophysics 22 (2).

Calandro, D. & Read, G., in prep. South Australia. Depth to magnetic basement map. Minerals & Energy Resources, PIRSA.

Clark, D., 1983. Comments on magnetic petrophysics. Bulletin of the Australian Society of Exploration Geophysics 14: 49 - 62.

Cooper, M.R., 1990. Tectonic cycles in southern Africa. Earth Science Reviews 28: 321-364.

Daly, S.J., Fairclough, M.C., Fanning, C.M. & Rankin, L.R., 1995. Tectonic evolution of the western Gawler Craton: a Palaeoproterozoic collision zone and likely plate margin. Geological Society of Australia. Abstracts 40: 35-36.

Drexel, J.F., Preiss, W.V. & Parker, A.J., 1993. The Geology of South Australia. Volume 1: The Precambrian. South Australia. Geological Survey. Bulletin 54.

Grant, F.S., 1985a. Aeromagnetics, geology and ore environments, I. Magnetite in igneous, sedimentary and metamorphic rocks: an overview. Geoexploration 23: 303 - 333.

Grant, F.S., 1985b. Aeromagnetics, geology and ore environments, I. Magnetite in igneous, sedimentary and metamorphic rocks: an overview. Geoexploration 23: 335 - 362.

Gravestock, D.I., 1997. Chapter 5. Geological setting and structural history. In Morton, J.G.G. & Drexel, J.F. The Petroleum Geology of South Australia. Volume 3: Officer Basin. Mines & Energy Resources South Australia. Report Book 97/19.

Gravestock, D.I., Alley, N.F., Benbow, M.C., Cowley, W.M., Farrand, M.G., Gatehouse, C.G., Kreig, G.W. & Preiss, W.V., 1995. Early and Middle Palaeozoic. In Drexel, J.F. & Preiss, W.V. (editors). The geology of South Australia. South Australia. Geological Survey. Bulletin 54 Vol. 2.

Isles, D.J., Rankin, L.R., Valenta, R., Cooke, A. & Anderson, H., 2000. Interpretation and structural analysis of aeromagnetic data. The Goongarie Trust. Workshop Manual. Current edition. Klein, G.D., 1995. Chapter 13. Intracratonic Basins. In Busby, C.J. & Ingersoll, R.V., (editors). Tectonics of Sedimentary Basins. Blackwell Science, Massachusetts; 579pp.

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Lindsay, J.F., (editor) , 1995. Geological Atlas of the Officer Basin, South Australia. Australian Geological Survey Organisation, Canberra, and Department of Mines & Energy, Adelaide.

Mackie, S., 1994. Seismic interpretation of the eastern Officer Basin (Marla, Munta areas). South Australia. Department of Mines & Energy. Report Book R97/00594.

Parker, A.J., 1993. Chapter 2: Geological framework. In Drexel, J.F., Preiss, W.V. & Parker, A.J., 1993. The Geology of South Australia. Volume 1: The Precambrian. South Australia. Geological Survey. Bulletin 54.

Preiss, W.V., Belperio, A.P., Cowley, W.M., & Rankin, L.R., 1993. Chapter 6: Neoproterozoic. In Drexel, J.F., Preiss, W.V. & Parker, A.J. (editors). The Geology of South Australia. Vol. 1. The Precambrian. South Australia. Geological Survey. Bulletin 54.

Rankin, L.R., & Newton, C.A., 2002. Musgrave block, central Australia: Regional geology from interpretation of airborne magnetic data Geointerp Report 2002/5.

Teasdale, J., Pryer, L., Etheridge, M., Romine, K, Stuart-Smith, P., Cowan, ., Loutit, T., Vizy, . & Henley, P., 2001. Officer Basin SEEBASE Project. SRK / PIRSA Report. Primary Industries, South Australia.

Tucker, L.R. & McKerrow, W.S., 1995. Early Palaeozoic chronology; a review in light of new U-Pb zircon ages from Newfoundland and Britain. Canadian Journal of Earth Sciences 32: 368-379.

Webb, A.W., Thomson, B.P., Blissett, A.H., Daly, S.J., Flint, R.B. & Parker, A.J., 1982. Geochronology of the Gawler Craton, South Australia. South Australia. Department of Mines & Energy. Report Book 82/86.

Webb, A.W., Thomson, B.P., Blissett, A.H., Daly, S.J., Flint, R.B. & Parker, A.J., 1986. Geochronology of the Gawler Craton, South Australia. Australian Journal of Earth Sciences 33: 119-143.

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APPENDIX 1

Geophysical Images

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Magnetic data:- RTP-1st Vertical Derivative

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Magnetic data:- RTP-1st Vertical Derivative Red dots represent drillholes (note paucity of drilling in central, southern & western sectors of Officer Basin).

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Magnetic data:- RTP 1st Vertical Derivative (greyscale) with RTP (colour drape).

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Magnetic data:- TMI (with NE sun-angle illumination filter).

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Magnetic data:- TMI greyscale with NE sun-angle illumination filter.

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Regional Bouger Gravity (colour) with E- sun angle illumination filter (against backdrop of RTP 1st VD magnetic data).

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Structural trends within Bouger gravity (black lines) superposed on RTP-1st Vertical Derivative magnetic data. NNW trending structural corridor extends from southern Musgrave Block to SW end Nawa Ridge.

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APPENDIX 2

SELECTED MAGNETIC PROFILES

(from RTP magnetic grid)

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Profile E-W-1

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Profile E-W – 2.

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Profile E-W-3

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Profile E-W-4

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Profile E-W-5

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Profile E-W-6

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Profile N-S-1

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Profile N-S-2

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Profile N-S-3 (northern sector)

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Profile N-S 3 (southern sector)

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