Bancannia Trough Research Articles

Predicting sequence stratigraphic architecture and its implication for hydrocarbon reservoir potential of the uppermost Silurian through Lower Devonian Winduck Interval, central Darling Basin of western New South Wales, SE Australia

This paper provides the results of lithostratigraphy and depositional environment of the uppermost Silurian through Lower Devonian Winduck Interval section from three areas widely distributed in the central Bancannia Trough from Bancannia South 1 well, the southern Pondie Range Sub-basin from Pondie Range 1 well and the northwestern part of the Blantyre Sub-basin from two wells, the Booligal Creek 1 and 2. These wells were studied using wireline log, core and cutting data to predict sequence stratigraphic architecture and its implication for hydrocarbon sandstone reservoir potential. This study necessitated the use of detailed facies prediction relationships and geometry of lithostratigraphic concepts for the purpose of establishing a sequence stratigraphic architecture framework of the studied areas. Two units were established, lower and upper Winduck Interval, within the Winduck Interval identified by lithological data examination and wireline log characteristics of the sediments from the four wells in the study area. Results of the lithology type analysis showed that the Winduck Interval is interbedded richly in sandstone and siltstone complex deposits. This paper presents a clear picture of the sequence stratigraphic model, as well as that of the three third-order depositional sequences (DS1, DS2 and DS3, respectively) that are identified from changes in systems tracts patterns (composed of lowstand, transgressive and highstand), vertical and lateral facies sequence thickness changes and the lithostratigraphic units distribution in the Winduck Interval section. Potential reservoir prospectivity targets are identified in lowstand system tracts and highstand system tracts deposits consisting of sandstone facies and thin silts, while the transgressive system tracts deposits consist of shales and some interbedded shaly siltstones, many having excellent potential source and seal properties. The contribution of sequence stratigraphy as an approach to understanding lithostratigraphy of the Winduck Interval has important significance in regard to future hydrocarbon reservoirs exploration across the central Darling Basin.

Correlation of the lithostratigraphic facies relationships and depositional environments of the uppermost Silurian through Lower Devonian strata across the central Darling Basin, western New South Wales, SE Australia

This paper is focused on the detailed lithostratigraphic facies characterisation of the paleo-environments of the uppermost Silurian through Lower Devonian Winduck Interval strata from the Bancannia South 1, Pondie Range 1 and Booligal Creek 1 and 2 wells, across the central Darling Basin of western New South Wales. Based on data from both wireline logs and drill cores and cuttings, an interpretation of paleo-environments and evolution of the depositional facies is presented. Our approach involves two key interpretations. The first outcome was the delineation of the Winduck Interval section (maximum observed thickness of approximately 1475 m) and formal subdivision into lower and upper Winduck Intervals, based on well data from the central Bancannia Trough, southern Pondie Range Sub-basin and northwestern flank of the Blantyre Sub-basin. The second outcome is based on the internal lithostratigraphy of the Winduck Interval represented by 11 sedimentary lithofacies organised into three lithofacies associations (LFAs). LFA-X1 is interpreted as distributary-channel sandstone complex deposits, LFA-X2 is interpreted as distributary mouth bar with small-scale braided delta plain complex deposits and LFA-X3 is interpreted as tidal channel sands with proximal delta front complex deposits. Subsurface lithostratigraphic facies analysis relationships and depositional environments of the Winduck Interval section have implications for evaluating the hydrocarbon potential and may assist future exploration efforts in the central Darling Basin, especially the Bancannia Trough and Pondie Range Sub-basin. KEY POINTS A detailed lithostratigraphic facies characterisation of the paleo-environments of the uppermost Silurian–Lower Devonian Winduck Interval strata in the central Darling Basin is presented. Evaluation and interpretation of the Winduck Interval section in study area was subdivided into two informal stratigraphic units, a lower Winduck Interval and an upper Winduck Interval. Eleven distinct sedimentary lithofacies codes (LFs), which have been recognised in the internal lithostratigraphy of the Winduck Interval from the Bancannia South 1, Pondie Range 1 and Booligal Creek 1 and 2 wells, have been grouped into three lithofacies associations (LFAs). Lithostratigraphic facies analysis relationships and depositional environments of the Winduck Interval section has implications for evaluating the hydrocarbon potential and may assist future exploration in the Bancannia Trough and Pondie Range Sub-basin.

Facies analysis relationships depositional environments of the subsurface stratigraphy of the Snake Cave Interval in the Bancannia Trough, western Darling Basin, New South Wales, SE Australia

Abstract This work presents the first detailed research on the subsurface stratigraphy of the sedimentary Snake Cave Interval (late Early to late Middle Devonian) in the Bancannia Trough, the westernmost component of the Darling Basin, in western New South Wales, Australia. The study is based on data from wireline-log responses (especially gamma-ray and sonic logs), and core and cuttings descriptions in three exploration wells that are used to develop a model for depositional environments in the Bancannia Trough. This study provides re-evaluation and re-interpretation of the positions of the top and bottom boundaries of the Snake Cave Interval using distinct patterns changes in gamma-ray and sonic log signatures in relation to lithological characteristics in the Bancannia South-1, Bancannia North-1 and Jupiter-1 wells. The log facies interpretation of the Snake Cave Interval section enabled the detailed facies description of four lithostratigraphic units (A, B, C and D, in ascending order) in the study area. The Snake Cave Interval displays units with ten distinct sedimentary facies identified from core and then tied to the gamma ray log characteristics, forming facies associations that are visible in both vertical and lateral sections. Five facies associations have been identified: FA-I, braided fluvial multistory channel belt complex deposits; FA-II, meandering fluvial distributary channel sandstone complex deposits; FA-III, sandy braided fluvial distributary channels deposits; FA-IV, meandering fluvial deposits; and FA-V, fluvial channel-fill association with fluvio-tidal transition zone complex deposits. Detailed comparison of lithologic logs with wireline-log suite signatures (mainly gamma-ray logs) that are dominantly braided fluvial and meandering fluvial deposits, with minor fluvio-tidal transition zone complex deposits in this trough. This article presents a new depositional model that will promote exploration and discovery of potential petroleum resources and stratigraphic-trap configurations in this trough and adjacent regions of the Darling Basin.

Facies analysis, depositional environment interpretation and lithostratigraphic architecture to predict reservoir potential of the Devonian Ravendale Interval in the Bancannia Trough, western Darling Basin, SE Australia

This paper discusses the facies sequence character for sandstone rocks of the subsurface upper Middle Devonian and Upper Devonian Ravendale Interval in the Bancannia Trough, western Darling Basin. The integration of multiple correlation techniques (recognition of changes in core and cutting facies, electrofacies and lithostratigraphic-facies associations) has helped define an internal stratigraphic correlation framework for the Ravendale Interval. This provides a powerful tool for understanding the distribution of the lithostratigraphic architectural controls on reservoir potential. This study has re-evaluated the stratigraphic boundaries of the Ravendale Interval (bottom and top) using marked changes in gamma-ray and sonic log characteristics, and depth estimates of the boundaries were derived from the wireline log data in the Bancannia South-1, Bancannia North-1 and Jupiter-1 wells. The internal stratigraphy of the Ravendale Interval consists of 20 facies and 19 subfacies that define 4 facies associations (FA). These facies associations are characterised as facies association-A (FAA), a braided channel-fill system; facies association-B (FAB), a meandering channel-fill system; facies association-C (FAC), a multistory fluvial channel-fill complex system, and facies association-D (FAD), a meandering channel delta system. The Ravendale Interval has been subdivided into five wireline log lithostratigraphic units (RI1, RI2, RI3, RI4 and RI5, in ascending order) in the three available wells. Use of the suggested wireline log lithostratigraphic unit correlations versus facies association of the Ravendale Interval has the potential to refine existing stratigraphic sequence schemes and, given the higher resolution and more detailed correlation, may significantly improve subsurface sedimentological framework relationships in lithostratigraphic unit reconstructions. Wireline log correlations indicate that potentially productive sand-rich units of the major Ravendale Interval in the Bancannia Trough can be correlated over large distances, suggesting significant continuity into lightly drilled and undrilled areas, and aid in the prediction of potential hydrocarbon-bearing reservoirs within the Darling Basin region.

SEISMIC STRATIGRAPHIC ANALYSIS AND STRUCTURAL DEVELOPMENT OF AN INTERPRETED UPPER CAMBRIAN TO MIDDLE ORDOVICIAN SEQUENCE IN THE NW BLANTYRE SUB‐BASIN, DARLING BASIN (WESTERN NEW SOUTH WALES, AUSTRALIA)

In this paper, a new interpretation is presented of the seismic stratigraphy of units equivalent to the upper Cambrian to middle Ordovician Scropes Range Formation in the NW Blantyre Sub‐basin in the central part of the Palaeozoic Darling Basin, western New South Wales. This succession was previously unrecognised, attributed to the lower part of the overlying Winduck Interval, or regarded as basement. The structural development of the study area is described, and may contribute to a better understanding of the regional stratigraphic evolution of the Lower Palaeozoic section in this part of the Darling BasinThis study presents interpretations of a regional two‐dimensional seismic survey which investigated the upper Cambrian to middle Ordovician Scropes Range Formation in the NW Blantyre Sub‐basin. A shallow marine Cambro‐Ordovician succession is well established in the Bancannia Trough in the western Darling Basin but this interval appears to have been ignored in the central Darling Basin where a deeper‐marine environment and suitable kitchen conditions may have provided greater petroleum potential. Data from two shallow wells in the 831 km2 study area were available to support the seismic investigations. Seismic sequences were defined by a combination of horizon mapping, the configuration of internal reflections, termination patterns and thicknesses. Six seismic reflection horizons were defined and used to identify five seismic sequences named Units A, B, C, D and E. These features were identified in three seismic lines and demonstrate the continuity of the Scropes Range Formation throughout the NW Blantyre Sub‐basin. All seismic sequence boundaries are based on fair to good continuous markers, with strong amplitudes throughout the study area. The acoustic basement beneath the Scropes Range Formation was metamorphosed in the Delamerian Orogeny (around 500 Ma). Acoustic facies recognized in the three seismic lines can be correlated with sedimentary facies observed in outcrops in the Scropes Range, 130 km west of the study area, and are comparable to acoustic facies in modern fluvial sandstones associated with local braided delta deposits. Complex geological features suggest potential structural and stratigraphic hydrocarbon traps.This study emphasizes the reservoir potential of the Scropes Range Formation in the central Darling Basin where extensive exploration efforts are currently occurring. The detailed interpretation of the seismic stratigraphy may lead to the definition of new exploration plays in this under‐explored region.

Potential-field interpretation of the Kars Belt, western NSW

The Cambrian Ponto Group of the Koonenberry Belt is a suite of fore-arc rocks of the Mt Wright Arc, and includes basalts that are associated with copper mineralisation. Magnetic derivative maps allow the Ponto Group to be followed southwest under shallow cover, where they define the magnetic texture of the Kars Zone that flanks the southeastern margin of the Curnamona Craton. Shallow-sourced positive magnetic anomalies in the Kars one are superimposed on a longer-wavelength magnetic low; this contrasts with their equivalents along the Koonenberry Belt, which sit on broad magnetic highs. Gravity images of the Koonenberry Belt suggest that shallow dense sources associated with the Ponto Group overlie deeper mass excesses, while equivalent shallow and deep mass excesses appear to be uncoupled in the Kars Zone. Joint magnetic and gravity modelling of a profile across the Kars Zone suggests that high susceptibility, dense bodies representing the Ponto Group have been displaced to the northwest, over-riding in their path deep magnetic sources below the Menindee Trough. Similar deep magnetic sources below the Bancannia Trough have been interpreted as very large intrusive features forming the base of the Mt Wright Arc. Together, the potential field imagery and modelling suggests very large scale, low-angle thrusting of the Mt Wright fore-arc over a rigid basement of the Mt Wright Arc.

The Mount Wright Arc: A Cambrian subduction system developed on the continental margin of East Gondwana, Koonenberry Belt, eastern Australia

The Mount Wright Arc, in the Koonenberry Belt in eastern Australia, is associated with two early to middle Cambrian lithostratigraphic groups developed onto the Late Neoproterozoic volcanic passive margin of East Gondwana. The Gnalta Group includes a calc-alkaline basalt-andesite-dacite suite (Mount Wright Volcanics), interpreted to represent the volcanic component of the arc. Volcaniclastic Gnalta Group rocks now buried in the Bancannia Trough represent the continental back-arc, developed immediately behind the arc in a manner analogous to the modern Taupo Volcanic Zone of New Zealand. East of the Gnalta Group is the Ponto Group, a deep marine sedimentary package that includes tholeiitic lavas (Bittles Tank Volcanics) and felsic tuffs, interpreted as part of a fore-arc sequence. The configuration of these units suggests the Mount Wright Arc developed on continental crust in response to west-dipping subduction along the East Gondwana margin, in contrast with some models for Cambrian convergence on other sections of the Delamerian Orogen, which invoke east-dipping subduction and arc accretion by arc-continent collision. This convergent margin was deformed by the middle Cambrian Delamerian Orogeny, which involved initial co-axial shortening followed by sinistral transpression, and oroclinal folding around the edge of the Curnamona Province.

Fluvial Geomorphology Suggests Recent Tectonic Activity Near the Northern Barrier Range, western New South Wales

Large faults expressed at the ground surface often have clear effects on drainage networks. Smaller scale tectonic activity, or that which occurs beneath sedimentary cover, can have more subtle effects that arise from changes to down-valley slope. These include an increase meander sinuosity or the development of anabranching. Fowlers Creek arises in the northern Barrier Range (NSW) and its terminal floodout flows across the flat surface of the Bancannia Trough. In the floodout's proximal and medial reaches the channel is sinuous but generally without active meander development. Active meandering occurs in only in two reaches, and is associated with higher slopes and repeated avulsions, the most recent being during the 20thC. It is suggested that small-scale uplift may have occurred, increasing the down-valley slope and therefore promoting meander development and avulsion. More closely spaced topographic data is necessary to support this hypothesis, however Fowlers Creek is one of nine creeks flowing east from the Barrier Range which show planform irregularities several kilometres from the rangefront. This supports the possibility of a concealed tectonic feature, and suggests that it is large-scale and linear. Major structures (e.g. mines, waste dumps) may be built in sparsely monitored remote locations where earthquake hazard estimation can be difficult. In such places, maps of fluvial geomorphology can be useful indicators of neotectonic activity.

Stratigraphy, sedimentology, structure and tectonics of Lower Ordovician and Devonian strata of South Mootwingee, Darling Basin, western New South Wales

A small inlier of marine Bynguano Quartzite, of Early Ordovician age, was discovered in the eastern part of the mapped area, although most of the strata are Devonian in age. The mid‐Emsian to mid‐Eifelian Snake Cave Sandstone consists of seven informal units. In the west the Waverly Creek Formation (new) is unconformable on the Snake Cave Sandstone; it consists of The Rocky Tank Conglomerate Member (new) (?early Givetian in age), which is overlain by the Ramparts Creek Sandstone Member (new) (mid‐Givetian in age). In the west the late Givetian to Famennian Ravendale Formation is conformable on the Ramparts Creek Sandstone Member whereas further east where the Waverly Creek Formation is absent, the Ravendale Formation is largely paraconformable on Snake Cave Sandstone. The area is cut by the north‐northwest‐trending Mootwingee (in the west) and the Boorungie Faults (in the east). These faults were active during the Tabberabberan and the Kanimblan Orogenic Pulses of the Lachlan Orogen and they subdivide the area into West, Central and East Blocks. The West and the Central Blocks are an uplifted part of the Bancannia Trough (basin inversion) whereas the East Block is a southern part of the Wonominta Block.

Stratigraphy, sedimentology, structure and tectonics of Lower Ordovician and Devonian strata of South Mootwingee, Darling Basin, western New South Wales

A small inlier of marine Bynguano Quartzite, of Early Ordovician age, was discovered in the eastern part of the mapped area, although most of the strata are Devonian in age. The mid‐Emsian to mid‐Eifelian Snake Cave Sandstone consists of seven informal units. In the west the Waverly Creek Formation (new) is unconformable on the Snake Cave Sandstone; it consists of The Rocky Tank Conglomerate Member (new) (?early Givetian in age), which is overlain by the Ramparts Creek Sandstone Member (new) (mid‐Givetian in age). In the west the late Givetian to Famennian Ravendale Formation is conformable on the Ramparts Creek Sandstone Member whereas further east where the Waverly Creek Formation is absent, the Ravendale Formation is largely paraconformable on Snake Cave Sandstone. The area is cut by the north‐northwest‐trending Mootwingee (in the west) and the Boorungie Faults (in the east). These faults were active during the Tabberabberan and the Kanimblan Orogenic Pulses of the Lachlan Orogen and they subdivide the area into West, Central and East Blocks. The West and the Central Blocks are an uplifted part of the Bancannia Trough (basin inversion) whereas the East Block is a southern part of the Wonominta Block.

Post-early cretaceous landform evolution along the western margin of the Bancannia trough, western NSW.

Previously undated post-Devonian sediments outcropping north of Fowlers Gap station near the western margin of the Bancannia Trough are shown by plant macro- and microfossil determinations to be of Early Cretaceous (most likely Neocomian and/or Aptian) age, and thus part of the Eromanga Basin. They are assigned to the previously defined Teleplione Creek Formation. Study of the structural configuration of this unit and the unconformably underlying Devonian rocks suggests that the gross landscape architecture of the area results from post-Early Cretaceous monoclinal folding along blind faults at the western margin of the trough, combined with the effects of differential erosion. This study shows that. while landscape evolution in the area has been dynamic, the major changes that have occurred are on a geological rather than human timescale. Key words: geology, landscape evolution, Eromanga Basin, folding

Mid and Late Devonian arenites deposited by sheet-flood, braided streams and rivers in the northern Barrier Ranges, far western New South Wales, Australia

Extensive and well exposed, fine-grained fluvial sandstone and less common pebbly coarse-grained fluvial sandstone of Devonian age, crop out in the northern Barrier Ranges of far west New South Wales, Australia. These strata were deposited largely on low-gradient alluvial fans in a basin and range landscape and contain common sedimentary structures (especially streaming lineations and tabular cross-beds). Around 400 of these sedimentary structures were measured to determine the palaeoflow trends of the sheet floods, streams or rivers which deposited the sandstone.The strata are mapped as the Mid Devonian Coco Range Sandstone and the Late Devonian Nundooka Sandstone, which together are around 2.7 km thick. They were deposited at the western margin of the large Emsian to Early Carboniferous Darling Basin. The Coco Range Sandstone is Emsian to Eifelian in age (based on fragments) of fossil fish) and it is separated from the Frasnian-Famennian (Late Devonian) Nundooka Sandstone by the north-trending Nundooka Creek Fault. The eastern boundary of the Nundooka Sandstone is formed by the Western Boundary Fault. Eastward of this fault is the north-trending and 40 km wide Bancannia Trough, which contains gently folded Late Silurian to Early Carboniferous strata up to 7.5 km thick.Most of the Coco Range Sandstone and all of the Nundooka Sandstone are non-graded, fine and very fine-grained, light brown sub-litharenites which are considered to have been deposited mainly on low-gradient alluvial fans. Sedimentary successions of 1.75 to 5.25 m thickness in the fine-grained arenite usually commence with Sm (massive or slumped) → Sh (laminated arenite) or St (trough cross-beds) → Sp (tabular cross-bedded sandstone). An erosional surface commonly underlies the sedimentary successions and they are interpreted to be the result of deposition from decelerating sheet floods. Units composed of tabular cross-bedded strata several metres thick are rarely channelised and are interpreted to represent deposition within braided streams flowing upon the fans or deposited at the margin of sheet floods.In the Coco Range Sandstone there are two sheet-like coarse pebbly arenite units (The Valley Tank and Copi Dam Members) which together total 200 m in thickness. Unimodal palaeocurrent trends and heavy mineral suites from within the coarse-grained arenite indicate a derivation from the south near Broken Hill. Sedimentary structures within the coarse-grained arenite indicate a Platte River style of deposition upon distal braid plains, whereas local interdigitation of coarse-grained arenite with fine arenite strata shows that deposition was essentially continuous (i.e. the coarse arenite do not overlie unconformities) and the two lithotypes represent interdigitation of alluvial fans and braid plain deposition. The northward progradation of the coarse arenite units was probably due to a sudden retardation of basement downwarping.

Phanerozoic stratigraphy and structure of the northern Barrier Ranges, western New South Wales

Devonian strata near Fowlers Gap and Nundooka Stations, northern Barrier Ranges comprise ∼2.7 km of sparsely fossiliferous, fluvially deposited sandstones (Mulga Downs Group). These strata are subdivided into the Coco Range Sandstone (oldest, Emsian‐Eifelian) found west of the north‐trending Nundooka Creek Fault, and the Nundooka Sandstone (youngest, ?Frasnian‐Famennian found east of the fault). Eleven stratigraphic units are mapped and two of these in the Coco Range Sandstone are formally named as The Valley Tank Arenite and Copi Dam Arenite Members. The Coco Range Sandstone and Nundooka Sandstone are tentatively correlated with strata in the Bancannia Trough. Deposition of the Coco Range Sandstone and Nundooka Sandstone was, however, separate from that of the Bancannia Trough, probably due to topographic highs which occurred east of the Western Boundary Fault. The Coco Range Sandstone is cut by northeast‐trending faults splaying from the Nundooka Creek Fault. These faults have vertical planes and are th...

Geological evolution of the Wonominta Block

The Wonominta Block in western New South Wales is an area of over 20,000 km 2 of poorly to moderately exposed Palaeozoic and older rocks. The block is surrounded by Mesozoic or younger sediments and is separated from the Broken Hill and Euriowie Blocks by the Bancannia Trough. The Wonominta Block can be divided into a number of lithostratigraphic units. The oldest unit is a fault-bounded sequence of multiply deformed low- to medium-grade metasediments, basic volcanics and silicic tuffs, that may be correlated with the Middle Proterozoic Willyama Supergroup of the Broken Hill Block. The second unit is an extensive sequence of carbonaceous slates containing several clean quartzite beds, with some dolomitic limestones and rare polymictic conglomerates in its upper section. This unit is equated with the Upper Proterozoic Adelaidean sequence overlying the Broken Hill Block, but differs in having a sequence of alkalic basaltic pillow lavas and tuffs interbedded with quartz-rich rocks at one level within the unit. A thin slate bed at the top of this unit marks a transitional stratigraphie contact with a thick overlying sequence of graded lithic and quartzofeldspathic sandstones with thin slate interbeds. This thick turbidite sequence, forming the third unit, is widespread within the Wonominta Block, becomes less oxidised to the east, contains rare trace fossils and sponge spicule shale horizons and is isoclinally folded along with the two older units. This unit equates with the Kanmantoo Group in South Australia. The more fossiliferous Lower to Middle Cambrian Gnalta Group, exposed near Mount Arrowsmith and near Mount Wright on the western edge of the Wonominta Block, may represent a shallow water near shore sequence equivalent to this deeper water turbidite unit to the east. The fossiliferous Upper Cambrian to Lower Ordovician Mootwingee Group in the west and the Kayrunnera Group in the east overlie the older units with a pronounced unconformity. This unconformity represents the Delamerian Orogeny in western New South Wales, and the overlying units define the near shore facies at the western edge of an extensive sedimentary depository that was destined to become the Lachlan Fold Belt.

Northwestern New South Wales regional magnetics and gravity

Bouguer gravity maps and aeromagnetic maps of the Koonenberry 1:500 000 map area in northwestern New South Wales have been compiled from Bureau of Mineral Resources data bases. Low regional magnetic field strength within the area of the Precambrian Adelaide Fold Belt extends about 90 kilometres north of the northernmost Precambrian outcrop, suggesting some continuity of crustal structure through this region. On the western edge of the area, in the vicinity of 30°30¢ S, magnetic response from shallow subcrop indicates that a detailed aeromagnetic survey in this vicinity could clearly delineate the shallow subsurface structure. The Bancannia Trough is underlain by strongly magnetised rock units, possibly dacite/andesite. These magnetized rock units are terminated at their southern end by an ENE trending lineament which extends through the Wonominta Block into the basement of the Darling Depression. ERTS-1 images indicate that this lineament also extends WSW through Broken Hill. Bouguer gravity values over the Wonominta Block are markedly higher than over the Adelaide Fold Belt, reflecting a probable convergent plate margin associated with the Wonominta Block. The full extent of the Koonenberry Fault (which in this area is a major component of the boundary between the Kanmantoo and Lachlan Fold Belts) is clearly delineated by the magnetic data. The few near-surface magnetized rock units east of the Koonenberry Fault generally give rise only to isolated anomalies, except in the restricted area of Ordovician metasediments surrounding the Tibooburra Granite. The Eromanga Basin is distinguished from the Darling Depression by higher Bouguer values (indicating shallower sedimentary basement depths), and basement structures with generally northwesterly trends (compared with no discernable basement trend directions in the Darling Depression).

GEOLOGICAL APPRAISAL OF THE MOOTWINGEE AREA, NEW SOUTH WALES

The results of various photogeological, geological and geophysical programmes in the Mootwingee Area of northwestern New South Wales are discussed.In a structurally complex area, a central horst of outcropping Precambrian rocks trends north-northwestwards. This is flanked both to the east and west by basins containing considerable thicknesses of Palaeozoic sediments.The problem of whether the western structural basin, now represented by the Topar syncline, is coincident with the Cambro-Ordovician basin of sedimentation (?Bancannia Trough) is also discussed. The presence of infolded and down-faulted remnants of Cambro-Ordovician sediments within the Precanibrian basement, to the north of the area discussed, suggests that Cambro-Ordovician sedimentation may have extended further eastward than is now revealed in outcrop.The main source-potential of the area appears to be confined to the Cambro-Ordovician sequence with perhaps very limited potential in the thick Devonian sediments. The drilling of a stratigraphic well in each of the basinal areas respectively to the west and east of the centrally-outcropping Precambrian block, appears justified. Such drilling should resolve some of the above-mentioned problems of depositional limits, as well as assist in evaluating the petroleum potential of the region.

THE LAKE FROME EMBAYMENT AREA

The Lake Frome Embayment Area occupies a position in the southernmost extension of the Great Artesian Basin. It is bounded on the west by the North Flinders limb of the Adelaide Geosyncline and on the east by the Tibooburra Block. It is limited on the south by the Olary Region and the arbitrary northern limits are taken at the south-eastern extension of the Muloorina Gravity Swell.The structural configuration of the area is fault controlled, and is believed to be related to block movements in the basement. A complex of evolved troughs is present. Two major troughs are especially evident, the Bancannia Trough in the east and the Lake Frome Gravity Depression in the west.Remnants of Cambro-Ordovician sediments are preserved on both sides of the Embayment, although outcrop is dominated by rocks of Proterozoic and older age. Devonian sediments are present in the eastern part of the area. Jurassic-Cretaceous sediments related to the Great Artesian Basin sequence are present.