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  • AGSO's 1995-96 Petrel Sub-basin Study was undertaken within AGSO's Marine, Petroleum and Sedimentary Resources Division (MPSR) as part of MPSR's North West Shelf Project. The study was aimed at understanding the stratigraphic and structural development of the basin as a framework for more effective and efficient resource exploration. Specifically, the study aimed to: - define the nature of the major basement elements underlying the Petrel Sub-basin and their influence on the development of the basin through time, - determine the nature and age of the events that have controlled the initiation, distribution and tectonic evolution of the basin; - define the nature and age of the basin fill, and the processes that have controlled its deposition and deformation; and, importantly, - determine the factors controlling the development and distribution of the basin's petroleum systems and occurrences.

  • The Argo-Apollo 1:50,000 regolith-landform map illustrates the distribution of regolith materials and the landforms on which they occur, described using the Residual-Erosional-Depositional (RED) mapping scheme developed by the CSIRO Division of Exploration and Mining

  • The Buckley River-Lady Loretta 1:85,000 paleosurface map illustrates the distribution of relict indurated materials described using the RTMAP scheme developed by Geoscience Australia

  • This paper was presented at the 13th Australian Geological Convention, Canberra, 1996

  • This paper was presented at the 13th Australian Geological Convention, Canberra, 1996

  • Gosses Bluff consists of a prominent circular ridge, 4.5 km in diameter, surrounded by a less well-exposed deformed outer ring, 24 km in diameter, which incorporates annular breccia troughs. The circular ridge, which forms part of an eroded central uplift, is composed of fractured and brecciated Ordovician to Devonian sandstone and shale, capped in places by overturned megabreccia. The structure was formed by the impact of an asteroid or comet. Evidence for an extraterrestrial impact origin includes: (1) the circular symmetry of the disturbed zone, which comprises outcrops of vertical to overturned strata whose original stratigraphic position would be at depths of <3-4 km; (2) the presence of shatter cones and rhomboidal fracture patterns diagnostic of intense shock; (3) shatter-cone axes that define a structurally central focus at shallow depth beneath the palaeosurface when reconstructed to their pre-impact orientation; (4) outward ejection of large blocks; (5) melting of sandstone and siltstone to form melt breccia; (6) a gradation with increasing depth from shock-melted breccia into recrystallised and unheated breccia, suggesting a high central heat source; (7) a depth limit of the structural disturbance defined by continuous seismic reflectors below about 3500 m; (8) the absence of gravity anomalies which would provide evidence for deep-seated mass excess or deficiency. Mineralogical and microstructural features diagnostic of instantaneously applied shock pressures abound. Quartz in both breccia and bed-rock shows shock-induced fractures and planar deformation features. The melt breccia at Mount Pyroclast records higher shock levels: quartz has been transformed to glass, partly recrystallised into tridymite, and subsequently converted to solid-state diaplectic quartz. The fusion of shale resulted in potassium-enriched hot solutions circulating below the crater floor, and recrystallisation into pumiceous aggregates of sanidine accompanied by zeolites and hematite. Ar-Ar plateau ages of this sanidine-rich material suggest recrystallisation at 142.5 ± 0.8 Ma, which - along with the orientation and reverse nature of the geomagnetic field at the time of the event - points to a latest Jurassic age. Calculations indicate that a crater the diameter of the Gosses Bluff structure reflects the release of energy in the order of 105_106 Mt, which could have been generated by an asteroid or comet estimated to have been about 2 km in diameter.

  • Geophysical studies of Gosses Bluff facilitate the resolution of its three-dimensional structure, which includes a broadly hemispherical low-velocity zone with a radius of about 4 km centred at the present ground level. This zone correlates with a deformed ring dominated by block-fauliing, megabreccia, and annular breccia troughs. Seismic data show a drop in shallow seismic velocities from 4.6 km s-1 over bedrock to 2.2 km s-1 over breccia outcrop. The base of the structure is defined by continuous reflectors at a maximum depth of 3450 m beneath the surface. The subsurface geology was deduced by fitting the extrapolated stratigraphy into the sequence of migrated seismic horizons. The uppermost little-disturbed reflectors are correlated with the tops of the Pertatataka Formation and Bitter Springs Formation of the Late Proterozoic--early Palaeozoic Amadeus Basin sequence. Seismic data for the Bitter Springs Formation define a domal structure and local thickening under Gosses Bluff. The gravity field associated with Gosses Bluff outlines a remarkably symmetrical circular gravity low of 440 µm s-2 with a radius of about 10.8 km. An annular gravity low bounded by steep gradients is associated with the central uplift. The outer limit of the near-surface gravity structure is nearly vertical. The symmetrical gravity features can be modelled by shallow density variations, and no net mass excess or deficiency is defined several thousand metres centrally under the bluff. Likewise, no gravity expression of a deep-seated Gardiner-Tyler anticline, suggested by the seismic reflection data, is apparent. Several anticlines around the bluff are shown by gravity to have low-density cores. Total magnetic intensity (TMI) contours suggest a depth to magnetic basement of about 10 km under the bluff. A negative anomaly of 4 nT occurs between the central ring and an outcrop of flow breccia at Mount Pyroclast. Similar short-wavelength airborne magnetic anomalies of shallow sources over the eastern, southern, and western flanks of the bluff within 6 km of the centre correspond to flow breccia. The anomaly sources represent both remanent primary components of magnetisation, and remagnetisation formed during cooling, as confirmed by ground magnetic surveys in the Mount Pyroclast area. The pole position is consistent with a lurassic-Cretaceous-boundary age.

  • Gosses Bluff is the remnant of a latest Jurassic asteroid or comet impact structure in the Amadeus Basin, central Australia. Conodonts have been successfully extracted from rocks exposed at the central uplift core of the impact structure, as well as from samples in petroleum exploration wells drilled into the structure. Maximum maturation of the source beds in the Early Ordovician Horn Valley Siltstone is reflected by the conodont colour-alteration index (CAl) value of 4 in Gosses Bluff No.2 well. A similar maturation level was attained by late Early Ordovician carbonates of the Stokes Siltstone, both in outcrop and drilled at shallow levels in Gosses Bluff No.1 well. A value of CAl 4 at the present surface indicates that these rocks were uplifted at least 4500m as a result of the impact. Combined with the CAl values from conodonts recovered from petroleum exploration wells to the northeast (Tyler No.1) and southeast (Palm Valley No.1 and West Waterhouse No. 1), the conodont CAl values derived from Gosses Bluff allow the construction of a palaeodepth trend, similar to the CAI-depth trend of the Appalachians and the Kidson Sub-basin of the Canning Basin. This indicates that about 500 m of rock has been eroded from above the Gosses Bluff and Tyler No. 1 well area, and that about 1700- 2800 m has been eroded from the Palm Valley and Waterhouse Range Anticlines. The CAl 4 value for the Horn Valley Siltstone, the primary rich hydrocarbon source rock in the basin, indicates that the organic matter in this formation in the Missionary Plain had reached a level of maturity equivalent to that for production of dry gas before about 140 Ma.

  • Two contrasting interpretations have been put forward recently for the structural evolution of the Wonga Belt, a north-south-trending anticlinal zone of strongly deformed metamorphic rocks, more than 80 km long and up to 5 km wide, in the central part of the Mount Isa Inlier. One interpretation involves major mid-crustal subhorizontal ductile extension with upper plate to the north, and the other involves north-south strike-slip faulting accommodating ditferential lateral transport of thrust sheets to either side, both the extension and strike-slip faulting taking place before the main folding event. A third interpretati on-relatively minor low-angle brittle extensional faulting and subsequent north-south strike-slip faulting predating folding-has been proposed for the southern continuation of the Wonga Belt. Stratigraphic continuity along the Wonga Belt and abrupt changes in stratigraphy across the belt are difficult to reconcile with the south-over-north mid-crustal extension hypothesis, but can be accounted for by the strike-slip faulting component of the other two interpretations. The postulated strike-slip displacement would have exceeded 80 km, and taken place between about 1780 Ma, the maximum age of the youngest metasediments affected, and 1115 Ma, the age of a dolerite dyke that post-dates cross-cutting faults.