Eastern Officer Basin Research Articles

Organic-walled microfossils of the mid-Neoproterozoic Alinya Formation, Officer Basin, Australia

AbstractEstimates of Precambrian eukaryotic diversity and disparity indicate broad trends of increase in the Mesoproterozoic Era, leading to a peak and then rapid decline by ca. 750 Ma. The organic-walled microfossil assemblage presented here is representative of that mid-Neoproterozoic height of eukaryotic species richness. Organic-rich shales and siltstones of the mid-Neoproterozoic upper Alinya Formation, eastern Officer Basin, Australia, preserve an abundant and diverse assemblage of organic-walled microfossils deposited in a low-latitude, shallow marine setting. Use of scanning electron microscopy (SEM) revealed an unexpected level of morphological detail not visible in transmitted light microscopy. This led to the recognition of new species as well as establishment of degradational sequences, which aid in fossil recognition. In total, 26 taxa are described here; these include 21 previously named forms, four newly described species (Caelatimurus foveolatus, Culcitulisphaera revelata,Karenagare alinyaensis, andMorgensternia officerensis), and one new combination (Vidalopalla verrucata).

Locating a major Proterozoic crustal boundary beneath the Eastern Officer Basin, Australia

The Archaean to Mesoproterozoic basement of northern South Australia is almost completely overlain by thick Neoproterozoic and younger basins (≪1% outcrop), yet is likely to preserve an important record of the interactions between the Archean-Proterozoic Gawler Craton and the Proterozoic Musgrave Province during the amalgamation of Australia in the Proterozoic. However, constraints on the location and geometry of the boundary between these provinces are poor. We use potential field data to determine the 3D basement architecture and so constrain where this Palaeo-Mesoproterozoic boundary may be located beneath the Eastern Officer Basin. We establish the geometry and properties of the overlying basins and explicitly include them during forward and inverse modelling of potential field data to highlight the structure of the underlying basement. Our analysis identifies three crustal domains. (1) Southeast of the steep northeast–southwest Middle Bore Fault, positive gravity and magnetic anomalies are co-located and sourced from bodies in the upper crust, these bodies overlie middle to lower crust that is apparently uniform. (2) Between the Middle Bore Fault and the southern edge of the Munyarai Trough, the highest amplitude gravity and magnetic anomalies are not co-located and are sourced from large northwest dipping bodies. (3) To the northwest, the crust underlying the Munyarai Trough has similar properties to the Musgrave Province, suggesting that the Musgrave Province extends at least 50 km beneath the Eastern Officer Basin. Although details of the geology in the second (central) domain are poorly constrained, the domain preserves large crustal-scale Precambrian structures and is interpreted to mark the boundary between the Gawler Craton and the Musgrave Province. In particular, the multiply reactivated Middle Bore Fault forms a major crustal boundary and is interpreted to mark the northern limit of the Gawler Craton. The Middle Bore Fault may have formed as early as the Kimban Orogeny (∼1.7 Ga), although substantial reactivation and modification of the crustal architecture could have occurred between the Kimban Orogeny and intrusion of the cross-cutting Gairdner Dolerite Dykes (827 Ma).

Chapter 66 Mid-Ediacaran ice-rafting in the Adelaide Geosyncline and Officer Basin, South Australia

Abstract Sedimentary features characteristic of ice-rafting are present in the Bunyeroo Formation (Wilpena Group) of the Adelaide Geosyncline and in the coeval Dey Dey Mudstone (Ungoolya Group) of the eastern Officer Basin (Figs 66.1 & 66.2), providing evidence of a mid-Ediacaran glacial climate in South Australia. The Acraman asteroid impact, a negative shift in marine δ 13 C, and a major acritarch turnover coincided with this frigid epoch.

Structural and stratigraphic architecture of Australia's frontier onshore sedimentary basins: the Arckaringa, Officer, Amadeus, and Georgina basins

The Onshore Energy Security Program—funded by the Australian Government and conducted by Geoscience Australia—has acquired deep seismic reflection data in conjunction with state and territory geological surveys, across several frontier sedimentary basins to stimulate petroleum exploration in onshore Australia. Here, we present data from two seismic lines collected in SA and NT. Seismic line 08GA-OM1 crossed the Arckaringa and Officer basins in SA and the southern-most Amadeus Basin in NT. Seismic line 09GA-GA1 crossed the northeastern part of the Amadeus Basin and the complete width of the southern Georgina Basin in NT. Structural and sequence stratigraphic interpretations of the seismic lines will be presented here, followed by an assessment of the petroleum potential of the basins. Seismic line 08GA-OM1 also crosses the Neoproterozoic to Devonian eastern Officer Basin. The basin is structurally complex in this area, being dominated by south-directed thrust faults and fault-related folds—providing potential for underthrust petroleum plays. The northern margin of the basin is overthrust to the south by the Mesoproterozoic Musgrave Province. To the north, the Moorilyanna Trough of the Officer Basin is a major depocentre of up to 7,000 m deep. Both seismic lines cross parts of the eastern Amadeus Basin. Seismic line 08GA-OM1 shows that the southern margin of the basin is overthrust to the north by the Musgrave Province with the main movement during the Petermann Orogeny. In the northeast, seismic line 09GA-GA1 crosses two parts of the basin separated by the Paleoproteroozic to Mesoproterozoic Casey Inlier (part of the Arunta Region). The northern margin of the basin is imaged seismically as a southward-verging, thinned-skinned thrust belt, showing considerable structural thickening of the stratigraphic succession. Seismic line 09GA-GA1 was positioned to cross that part of the southern Georgina Basin that was considered previously to be in the oil window. Here, the basin has a complex southern margin, with Neoproterozoic stratigraphy being thrust interleaved with basement rocks of the Arunta Region. The main part of the basin, containing a Neoproterozoic to Devonian succession, is asymmetric, thinning to the north where it overlies the Paleoproterozoic Davenport Province. The well, Phillip–2, drilled adjacent to the seismic line, intersected basement at a depth of 1,489 m, and has been used to map the stratigraphic sequences across the basin.

Sedimentary basins reconnaissance using the magnetic Tilt-Depth method

We compute the depth to the top of magnetic basement using the Tilt-Depth method from the best available magnetic anomaly grids covering the continental USA and Australia. For the USA, the Tilt-Depth estimates were compared with sediment thicknesses based on drilling data and show a correlation of 0.86 between the datasets. If random data were used then the correlation value goes to virtually zero. There is little to no lateral offset of the depth of basinal features although there is a tendency for the Tilt-Depth results to be slightly shallower than the drill depths. We also applied the Tilt-Depth method to a local-scale, relatively high-resolution aeromagnetic survey over the Olympic Peninsula of Washington State. The Tilt-Depth method successfully identified a variety of important tectonic elements known from geological mapping. Of particular interest, the Tilt-Depth method illuminated deep (3 km) contacts within the non-magnetic sedimentary core of the Olympic Mountains, where magnetic anomalies are subdued and low in amplitude. For Australia, the Tilt-Depth estimates also give a good correlation with known areas of shallow basement and sedimentary basins. Our estimates of basement depth are not restricted to regional analysis but work equally well at the micro scale (basin scale) with depth estimates agreeing well with drill hole and seismic data. We focus on the eastern Officer Basin as an example of basin scale studies and find a good level of agreement between previously-derived basin models. However, our study potentially reveals depocentres not previously mapped due to the sparse distribution of well data. This example thus shows the potential additional advantage of the method in geological interpretation. The success of this study suggests that the Tilt-Depth method is useful in estimating the depth to crystalline basement when appropriate quality aeromagnetic anomaly data are used (i.e. line spacing on the order of or less than the expected depth to basement). The method is especially valuable as a reconnaissance tool in regions where drillhole or seismic information are either scarce, lacking, or ambiguous.

Vines 1 revisited: are older Neoproterozoic glacial deposits preserved in Western Australia?

The stratigraphy in Vines 1, a 2017.5 m-deep cored stratigraphic hole drilled by the Geological Survey of Western Australia in 2001 near the Western Australian – South Australian border, has been reinterpreted with implications for the Neoproterozoic to Cambrian geological history of the Officer Basin. A previous interpretation considered the intersected succession as a conformable stratigraphic package, the Vines Formation. An assemblage of palynomorphs, found throughout the hole and previously used to infer an age of no older than earliest Cambrian, is now thought to consist of contaminants. An older assemblage, which is considered to be reworked and inherited from underlying rocks, provides a new maximum age constraint of mid-Neoproterozoic. Based on sedimentological interpretations and comparisons with other drillholes in the western Officer Basin, and the succession in the eastern Officer Basin, the Vines 1 succession is reinterpreted as four discrete sedimentary packages, the Pirrilyungka (new name), Wahlgu, Lungkarta and Vines (redefined) Formations, in ascending order. The Pirrilyungka and Wahlgu Formations include glacigenic sediments and may correlate with similar glacial successions in Supersequences 2 and 3 (mid to late Cryogenian) of the Centralian Superbasin, and the Sturt Tillite and Elatina Formation and their equivalents in the Adelaide Rift Complex of South Australia, respectively. The eolian Lungkarta Formation and fluvial Vines Formation are considered, on regional evidence, to be most likely of Ediacaran to earliest Cambrian age.

Palaeoclimate implications from deep drilling of Neoproterozoic strata in the Officer Basin and Adelaide Rift Complex of Australia; a marine record of wet-based glaciers

Two prominent episodes of Neoproterozoic glaciation in Australia are recorded in the Cryogenian (Sturtian < 725 Ma) and during the Ediacaran (Elatina < c. 580 Ma). Deep stratigraphic drilling and coring of Sturtian and Elatina glacial strata within the Adelaide Rift Complex (Blinman 2), the eastern Officer Basin (Nicholson 2) and the western Officer Basin (Vines 1 and Empress 1/1A) permits detailed sedimentary logging of lithofacies in more than 3000 m of continuous drill core. Glacial intervals in widely spaced drill sites show a commonality of facies types consisting of diamictites intimately interbedded with graded conglomerates, sandstones and siltstones that were deposited by subaqueous sediment gravity flows below wave base. Direct evidence for a glacial source of sediment is provided by striated and outsized clasts in fine-grained laminated facies. Soft-sediment deformation structures produced by downslope slumping, together with intraclasts of reworked sediment, are common. Facies suggest a depositional setting such as that found on submarine fans within Pleistocene glaciated basins strongly influenced by floating ice and the delivery of poorly sorted debris by the rainout of ice rafted debris, fall out of fine-grained suspended sediment and episodic downslope resedimentation. The large volume and great thickness of these subaqueous glacial facies within the Adelaide Rift Complex and Officer Basin indicates that Neoproterozoic glaciers in Australia delivered large fluxes of meltwater and sediment to offshore basins in similar fashion to wet-based glaciers typical of younger Phanerozoic glaciations. Evidence of meltwaters as a key agent of transportation is difficult to reconcile with climate models of a fully permafrozen planet where the hydrological cycle was shut down by extreme cold and normal sedimentary processes were suspended.

Deposition and chemical composition of early Cambrian salt in the eastern Officer Basin, South Australia

Lower Cambrian salt deposits in the eastern Officer Basin occur in troughs along the western and northwestern margin of the Gawler Craton, and are interpreted to have been deposited in marginal to restricted marine environments. The salt horizons in two drillholes [Manya 6 (373554E, 6938478N: GDA94) and Wilkinson 1 (263545E, 6693435N: GDA94)] were studied utilising multiple techniques including mineralogical investigation, fluid inclusion and isotopic analysis. New data on Br values of 19 salt samples provide evidence of marine origin for the salt. Abundant two-phase (gas – liquid) inclusions and poor preservation of primary inclusions indicate intensive post-depositional recrystallisation. Homogenisation studies of fluid inclusions in halite from Manya 6 suggest that the upper part of the salt-bearing strata (Ouldburra Formation) has been heated to 80°C, whereas the lower part reached even higher temperatures. This heating event may be related to the Devonian – Carboniferous Alice Springs Orogeny. The chemical composition of fluid inclusions in Manya 6 and Wilkinson 1 samples indicates that parent brines were Ca-rich and SO4-poor, whereas modern seawater is SO4-rich and Ca-poor. Such a composition is similar to that recorded in the Lower Cambrian halite from the East Siberia Basin.

Neoproterozoic (Ediacaran) diversification of acritarchs—A new record from the Murnaroo 1 drillcore, eastern Officer Basin, Australia

The Neoproterozoic is characterised by global environmental change and the diversification of many groups in the fossil record including animals and photosynthesising microflora. The cause–effect relationships between environmental and biotic changes are poorly understood but drastic events such as low-latitude glaciations, ocean oxidation and perhaps a bolide impact (on a more regional scale) may have had a decisive influence on the biota at the time. A new study on organic-walled microfossils (acritarchs) from the Murnaroo 1 drillcore, eastern Officer Basin, Australia, reveals the presence of assemblages with great morphological disparity, taxonomic diversity and abundance. This microbiota represents a part of the Ediacaran diversification event recognised previously from the Officer Basin, and other sedimentary basins in south-central Australia. Earlier studies included only a cursory analysis of Murnaroo 1. Our more extensive study of this drillhole supports the acritarch-based zonation established by Grey for the middle Ediacaran System and both the uppermost and the lowermost acanthomorph zones in the Ediacaran biostratigraphic scheme have now been recorded in Murnaroo 1. Some species appear cosmopolitan, being known from Siberia and China, providing good potential for interregional correlation and palaeobiogeographic reconstructions. The significance of the present acritarch palynoflora, in addition to being more abundant, diverse and covering a wider stratigraphic interval, lies in the fact that (in Australia at least) it tightens the constraints on the first appearance of acanthomorphic acritarch species, and indicates that it is much closer to the Acraman impact ejecta layer than previously inferred. In the aftermath of the global glaciations and the Acraman impact, the Ediacaran acritarch diversification may have had a profound influence on the evolution of other life forms, such as the Ediacara fauna.

Nd isotopic and geochemical constraints on provenance of sedimentary rocks in the eastern Officer Basin, Australia: implications for the duration of the intracratonic Petermann Orogeny

Sm–Nd isotopic and geochemical data from Neoproterozoic to Cambrian sedimentary rocks in the intracratonic eastern Officer Basin in central Australia highlight the evolving provenance roles of the basement complexes that underlie and bound the basin. Initial ε Nd values of around −12 for the basal units indicate that both were largely derived from the late Archaean to Mesoproterozoic Gawler Craton, which bounds the basin to the south. At c . 720 Ma an influx of juvenile, glacially derived sediment indicates partial uplift of the Mesoproterozoic Musgrave Block along the basin's northern margin, in a regime interpreted to be broadly extensional. At around 600 Ma, synchronous with the development of a foreland architecture, there was a large influx of Musgrave Block-derived sediments. This is interpreted to mark the onset of the intracratonic Petermann Orogeny, which was a long-lived event or series of events, spanning more than 70 Ma. Subsequent to c . 600 Ma, the Nd isotopic composition of sequences within the Officer Basin indicates an increasing contribution from the Gawler Craton despite up to 45 km of denudation of part of the Musgrave Block. This suggests that the majority of sediment derived from the Petermann Orogen bypassed the eastern Officer Basin for much of the history of the Petermann Orogeny.

New records of Late Neoproterozoic Acraman ejecta in the Officer Basin

New occurrences of the Acraman impact ejecta layer were recently discovered in two drillholes, Giles 1 and Murnaroo 1, in the eastern Officer Basin, South Australia, using acritarch biostratigraphy and lithostratigraphy to predict the position. Fractured crystals were also observed in palynological preparations from drillhole SCYW 1a on the Stuart Shelf. These discoveries improve stratigraphic control of Late Neoproterozoic successions, especially in the eastern Officer Basin and will allow a test of the hypothesis that the Acraman impact event caused a global catastrophe.

New records of Late Neoproterozoic Acraman ejecta in the Officer Basin

New occurrences of the Acraman impact ejecta layer were recently discovered in two drillholes, Giles 1 and Murnaroo 1, in the eastern Officer Basin, South Australia, using acritarch biostratigraphy and lithostratigraphy to predict the position. Fractured crystals were also observed in palynological preparations from drillhole SCYW 1a on the Stuart Shelf. These discoveries improve stratigraphic control of Late Neoproterozoic successions, especially in the eastern Officer Basin and will allow a test of the hypothesis that the Acraman impact event caused a global catastrophe.

The thermal history of the eastern Officer Basin (South Australia): evidence from apatite fission track analysis and organic maturity data

The eastern Officer Basin in South Australia contains a Neoproterozoic to Devonian succession overlain by relatively thin (<500 m) Permian, Mesozoic and Tertiary deposits. Within the basin fill, there are several major unconformities representing uncertain amounts of erosion. Three of these surfaces are associated with regional deformational events. Regional unconformities formed between 560 and 540 Ma (Petermann Ranges Orogeny), approximately 510–490 Ma (Delamerian Orogeny), 370–300 Ma (Alice Springs Orogeny), 260–150 Ma; and 95–40 Ma. AFTA® results from 13 samples of Neoproterozoic, Cambrian and Permian sedimentary rocks in five wells (Giles-1, Manya-2, -5 and -6 and Lake Maurice West-1) show clear evidence for a number of distinct thermal episodes. Results from all samples are consistent with cooling from the most recent thermal episode beginning at some time between 70 and 20 Ma (Maastrichtian–Miocene). AFTA results from Giles-1 indicate at least two pre-Cretaceous thermal episodes with cooling beginning between 350 and 250 Ma (Carboniferous–Permian) and between 210 and 110 Ma (Late Triassic–Albian). Results from Manya-2, -5 and -6 and Lake Maurice West-1 show evidence for at least one earlier higher temperature event, with cooling from elevated paleotemperatures beginning between 270 and 200 Ma (Late Permian to Late Triassic). These episodes can be correlated with other cooling/erosional events outside the study area, and the AFTA-derived paleotemperatures are consistent with kilometre-scale erosion for each of the episodes identified. Integration of the AFTA data with organic thermal maturation indicators (MPI) in the Manya and Giles-1 wells suggests that the Cambrian and Neoproterozoic successions in the northern part of the study area reached peak maturation prior to the Permian, while limited data from Lake Maurice West-1 allows peak maturation to have occurred as young as the Late Permian to Late Triassic thermal episode revealed by AFTA. The approach outlined in this study is relevant to all ancient basins as it emphasises the importance of understanding events associated with neighbouring regions. The thermal history of the Officer Basin, as with most other ancient basins, has been strongly affected by significant tectonic events throughout its history, even though younger deposits are not preserved in the basin itself. The recognition of these younger events, and the implications of these events for the depositional history, is important as it allows identification of the best regions for preservation of early generated hydrocarbons, and in some cases, suggests areas where generation of hydrocarbons could have occurred more recently than previously thought.

Evolution of the intracratonic Officer Basin, central Australia: implications from subsidence analysis and gravity modelling

ABSTRACT The intracratonic basins of central Australia are distinguished by their large negative Bouguer gravity anomalies, despite the absence of any significant topography. Over the Neoproterozoic to Palaeozoic Officer Basin, the anomalies attain a peak negative amplitude in excess of 150 mGal, amongst the largest of continental anomalies observed on Earth. Using well data from the Officer and Amadeus basins and a data grid of sedimentary thicknesses from the eastern Officer Basin, we have assessed the evolution of these intracratonic basins. One‐dimensional backstripping analysis reveals that Officer and Amadeus basin tectonic subsidence was not entirely synchronous. This implies that the basins evolved as discrete geological features once the Centralian Superbasin was dismembered into its constituent basins. Two‐ and three‐dimensional backstripping and gravity modelling suggest that the eastern Officer Basin evolved from a broad continental sag into a region of intracratonic flexural subsidence from the latest Neoproterozoic, when flexure of the lithosphere deepened the northern basin. The results from gravity modelling improve when the crust is thickened beneath the northern margin of the basin and thinned at the southern margin, as has been suggested by recent deep seismic data. The crustal thickening beneath the basin's northern margin abuts the region of greatest topographic relief and is consistent with the observed structure at the edges of many orogenic belts. If the Officer Basin evolved as a foreland‐type basin from the late Proterozoic and has retained those features to the present, then one implication is that in the absence of any significant topography, cratonic lithosphere must be able to support stresses over very long periods of geological time.

Discussion on mantle plume uplift in the sedimentary record: origin of kilometre-deep canyons within late Neoproterozoic successions, South Australia

Scientific editing by Nick Rogers. Nicholas Christie-Blick writes: Kilometre-deep buried canyons in the upper Neoproterozoic successions of South Australia are unusual by any standard, and any viable explanation for their origin will probably involve an unusual set of circumstances. ⇓Williams & Gostin (2000) review published geological constraints and competing hypotheses, and suggest a role for uplift and crustal deformation associated with the passage of a mantle plume. On the basis of available evidence, I continue to favour an alternative explanation—that they are due to the short-lived evaporative drawdown of sea level in a marine embayment that became temporarily isolated from the global ocean (⇓von der Borch et al. 1989; ⇓Christie-Blick et al. 1990; ⇓Christie-Blick 1998). ### Spatial and temporal distribution of erosion Canyons more than 1 km deep in the Wonoka Formation of the Adelaide fold belt and up to 700 m deep in the subsurface of the eastern Officer basin are reasonably interpreted as coeval and of common origin, with a length scale of order 1000 km. Comparable erosional features are found nowhere else in Australia, even in the Kimberley region and western Officer basin (⇓Coats & Preiss 1980; ⇓Walter et al. 1994, ⇓1995), where flood basalts of possible plume origin are widespread (⇓Bultitude 1976; ⇓Jackson & van de Graaff 1981). ⇓Bender (2000) independently noted the distribution of late Neoproterozoic to early Palaeozoic magmatism in Western Australia, and analyzed the possible role of plume-related uplift and erosional thinning of the crust in early Palaeozoic subsidence of the Canning basin. However, that basin is some 2000 km to the northwest of the Adelaide fold belt, where the canyons are best known. A rather thornier problem is that in the Adelaide fold belt prominent erosion surfaces are present at three other horizons above the level of Marinoan glaciation and below Cambrian strata. …

Ediacarian sequence and isotope stratigraphy of the Officer Basin, South Australia

The Ediacarian was a time of active tectonism over large areas of the Australian craton and sedimentary rocks of this age are relatively widespread. In spite of their wide distribution and the perception that they offer some of the most likely Neoproterozoic petroleum source rocks, their stratigraphy is not well understood and interbasinal correlation has proved difficult. Using available seismic and well‐log data, 19 depositional sequences contained within two tectonically defined megasequences are recognised in the Ediacarian succession in the eastern Officer Basin. Systematic carbon and strontium isotope chemostratigraphy, combined with the sequence stratigraphy, has enabled correlation of the main sequence boundaries between the Officer Basin and the Adelaide Geosyncline and Amadeus Basin. A significant number of sequence boundaries recognised in the exceptionally complete Officer Basin succession are either missing or not expressed in the Amadeus Basin and Adelaide Geosyncline successions. This suggests that only the major tectonically enhanced sequence boundaries (megasequence boundaries) have been recognised in the adjacent basins. The megasequences bounded by these surfaces define major basin phases.

Evolution of a Neoproterozoic to Palaeozoic intracratonic setting, Officer Basin, South Australia

The Neoproterozoic basins of central Australia share many features of architecture and sedimentary fill, suggesting common large‐scale extrinsic causal mechanisms. In an attempt to improve understanding of these mechanisms we have gathered and analysed new deep seismic reflection data and re‐evaluated existing seismic and well‐log data from the eastern Officer Basin, the largest and most poorly known of Australia's intracratonic basins.The Officer Basin is asymmetric and has a steep thrust‐controlled northern margin paralleled by sub‐basins as much as 10 km in depth. Further south the basin shallows gradually onto a broad platform. The basin rests on a thick crust (≈42 km) that is pervaded by a complex of northward‐dipping surfaces most of which terminate erosionally against the sediments of the Officer Basin and are interpreted as prebasinal features, possibly faults. Some appear to have been zones of crustal weakness which were reactivated as thrust complexes and played a major role in basin evolution.The sedimentary succession can be subdivided into six megasequences separated by major tectonically and erosionally enhanced sequence boundaries. The megasequences have distinctive sequence stacking patterns suggesting that they were deposited in response to episodic subsidence induced by a major extrinsic tectonic event. The basin initially formed as part of a giant sag basin which incorporated all the present‐day intracratonic basins (Amadeus, Georgina, Ngalia, Officer and Savory Basins) in a single large ‘superbasin’ perhaps as a response to mantle processes. Subsidence then ceased for ≈100 Myr producing a regional erosion surface. Beginning in the Torrensian or Sturtian five more major events of varying regional significance influenced the basin's evolution. Four were compressional events, the first of which activated major thrust complexes along the present basin margins, forming deep foreland sub‐basins with elevated intervening basement blocks. Once activated, the thrust complexes and sub‐basins persisted throughout the life of the intracratonic basins. From this epoch the intracratonic basins of central Australia were decoupled from the giant sag basin and became interrelated but independent features.Available information suggests that the Officer, Amadeus, Georgina, Ngalia and Savory Basins are related and are perhaps products of major tectonic events associated with the assembly and ultimate dispersal of the Proterozoic supercontinent. The closing phases of these basins were strongly influenced by events occurring along the newly created active eastern margin of the Australian continent in the Palaeozoic.

Early Neoproterozoic sequence stratigraphy and acritarch biostratigraphy, eastern Officer Basin, South Australia

The early Neoproterozoic Torrensian (= latest Tonian, ∼ 800-750 Ma) succession in the central and eastern Officer Basin was deposited in an intra-cratonic rift basin. The outcropping Pindyin Sandstone consists of basal lowstand conglomerate, and upward-fining transgressive sandstone and is conformably overlain by marine highstand shale of the Alinya Formation. In the Giles 1 well, the Pindyin Sandstone contains aeolian to fluvial deposits and is overlain by peritidal and sabkha sediments of the Alinya Formation. Seismic interpretation suggests that the succession is thin on the Murnaroo Platform and thickens toward the Munyarai Trough. The two sedimentary provinces are separated by the Ungoolya Hinge which probably marks the southeastern margin of the Fraser-Musgrave fold belt. Highstand organic-rich shale in the Alinya Formation is probably an important petroleum source rock. Acritarchs occur in both chert and shale in the Alinya Formation and their distribution was probably controlled by sedimentary environments. Similar assemblages from outcrop and subsurface provide a reliable evidence for regional correlation. The assemblages in Giles 1 are dominated by benthic coccoids and contain many distinct spinose acritarchs which are particularly abundant in the subtidal grey shale, whereas mainly phytoplankton is present in the chert. The assemblage, represented by Trachyhystrichosphaera vidalii-T. aimica-Cymatiosphaeroides kullingii, can be used for global correlation in the early Neoproterozoic. The abundant microfossils from the Alinya Formation and equivalent beds around the world probably indicate a microorganism explosion at 750–800 Ma, prior to the Neoproterozoic glaciations. Thirty-seven acritarchs/spheroids and seven cyanobacterial filaments are briefly described in this study, including 5 new species: Comasphaeridium tonium, Eoentophysalis gilesis, Goniosphaeridium alinyum, Gorgonisphaeridium pindyium and Gorgonisphaeridium torrensium and a newly combined species Archaeoellipsoides karatavicus comb. nov.

Tectonic Development of the Eastern Officer Basin, Central Australia

Interactive interpretation of some 3000 km of seismic section, combined with the integration of well, potential field, subcrop and surface geological data with satellite imagery have shown that the structure of this prospective basin is controlled by compressive tectonics. Four stages of basin development between the late Proterozic and Carboniferous can be identified on calibrated seismic sections from the eastern Officer Basin. Recent work suggests that the Officer Basin was originally part of an extensional Centralian Superbasin which included the present Amadeus, Ngalia, Savory and Georgina Basins. Justification for this hypothesis comes from the broadly similar stratigraphy in each of these basins. A model is proposed which explains the deformation of the Centralian Superbasin into those basins recognised today, separated by basement uplifted to shallow depth and exposure by compression. The main deformation is believed to have commenced at or around the Proterozoic-Cambrian boundary, with a substantial pulse at the end of the Cambrian. As a result, the original broad platform depositional system was deformed into a series of foreland-style basins. Stacked thrust sheets of late Proterozic to Cambrian sediments, containing the best known source rocks in the basin, illustrate the intensity of the compressive deformation. lnterbedded reservoir units have been folded to form traps suitable for hydrocarbon retention. This model will enhance the probability of prospect generation and the search for commercial hydrocarbons.

POROSITY GENERATION AND RESERVOIR POTENTIAL OF OULDBURRA FORMATION CARBONATES, OFFICER BASIN, SOUTH AUSTRALIA

Porosity generation and reservoir potential of the early Cambrian Ouldburra Formation in the eastern Officer Basin is delineated by combining petrographical, petrophysical and sedimentological studies. The shallow marine Ouldburra Formation consists of carbonates, mixed carbonates and clastics, clastics and evaporites. Detailed analysis of more than 100 samples shows that dolomitisation resulted in substantial secondary porosity development within the carbonates. Secondary porosity has also been generated within the mixed siliciclastic-carbonate zone by carbonate matrix and grain dissolution as well as by dolomitisation. Prospective reservoir units correspond to highstand shallow marine facies where short periods of subaerial exposure resulted in diagenetic changes.Sedimentary facies and rock character indicate that sabkha and brine reflux models are applied to dolomitisation within the Ouldburra Formation. Dolomite mainly occurs in two stages: common anhedral dolomites formed early by replacement of pre-existing limestone, and saddle dolomite and coarse crystalline dolomite formed during the late stages of burial diagenesis, associated with hydrocarbon shows. The dolomite reservoirs identified are ranked on the basis of their porosity distribution and texture into groups I to IV. Dolomites with rank I and II exhibit excellent to good reservoir characteristics respectively.The Ouldburra Formation shows many depositional and diagenetic similarities to the Richfield Member of the Lucas Formation in the Michigan Basin of the USA. Substantial oil and gas production from middle Devonian shallow water to sabkha dolomites makes the Richfield Member an attractive reservoir analogue to the Ouldburra Formation.