From 1 - 10 / 468
  • Australia's North West Margin (NWAM) is segmented into four discrete basins which have distinct rift and reactivation histories: Carnarvon, offshore Canning (Roebuck), Browse and Bonaparte. Bonaparte Basin incorporates Vulcan and Petrel sub-basins. The Bonaparte Basin stands out as an extensive sedimentary basin which has a geological history spanning almost the entire Phanerozoic, with up to 20 km of sediment accumulation in the centre. Browse Basin has considerably less thick sediment accumulation ? 12 km at maximum, which is still high for general hydrocarbon potential estimation. The structural architecture of the region is the product of a number of major tectonic events, including: ? Late Devonian northeast-southwest extension in the Petrel Sub-basin; ? Late Carboniferous northwest-southeast extension in the proto-Malita Graben, Browse Basin and proto-Vulcan Sub-basin; ? Late Triassic north-south compression; ? Early-Mid Jurassic development of major depocentres in the Exmouth, Barrow and Dampier sub-basins, and extension in the Browse Basin; ? Mid-Late Jurassic breakup in the Argo Abyssal Plain, onset of thermal sag in the Browse basin and extension in the Bonaparte Basin; ? Valanginian breakup in the Gascoyne and Cuvier abyssal plains, and onset of thermal sag in the Bonaparte Basin; and ? Late Miocene reactivation and flexural downwarp of the Timor Trough and Cartier Sub-basin Many of these events have involved processes of lower crustal extension and are strongly controlled by the pre-existing regional structural fabrics and basement character. Most reliable information on basement and deep crustal structure in the region comes from combined ocean-bottom seismograph (OBS) and deep reflection profiling along several regional transects (including Vulcan and Petrel transects in the Bonaparte Basin, and one transect in the Browse Basin). Average spacing between the OBSs of 30 km and shot spacing of 100 m with data recording to maximum offsets of 300 km enabled development of accurate crustal-scale seismic velocity models. Deep reflection data along the coincident profiles were recorded as part of Geoscience Australia?s regional grid of seismic lines. Consistent interpretation of several key horizons tied to petroleum exploration wells through the entire grid created the basis for co-interpretation of the OBS and deep reflection data supplemented by gravity field modelling.

  • The discovery of commercial oil in the Cliff Head-1 well in 2001 set an important milestone in the exploration history of the offshore northern Perth Basin. The region had been largely underexplored before then, partly due to the perception that the Hovea Member, a 10 to 40 m-thick unit straddling the Permian-Triassic boundary (PTB) and recognized as the main source of onshore petroleum accumulations, was not developed offshore (Crostella, 2001). The typing of the Cliff Head oil to the Hovea Member provided evidence that the key onshore petroleum system extends offshore and revitalized exploration in the area with 13 new field wildcat wells drilled since 2002. Three discoveries have been made subsequently further offshore in the Abrolhos Sub-basin with gas retrieved in Frankland-1 and Perseverance-1 and oil and gas in Dunsborough-1. A review of source rock and oil geochemical data was undertaken by Geoscience Australia in the offshore northern Perth Basin as part of a major integrated study aimed at reassessing the basin's prospectivity. This work supports the release of offshore exploration areas W13-19 and W13-20, two major blocks straddling the Houtman and Abrolhos Sub-basins with small portions extending into the Zeewyck and Gascoyne Sub-basins (Fig. 1). Well control is provided by 5 wells from the Wittecarra Terrace in the northern Abrolhos Sub-basin and Houtman-1 in the Houtman Sub-basin.

  • Predictive mineral discovery is concerned with the application of a whole of system process understanding to mineral exploration as opposed to an empirical deposit type approach. A mineral system process understanding can be derived from a consideration of five key questions, namely what is/are the: 1) geodynamic setting; 2) architecture; 3) sources and reservoirs; 4) drivers and pathways, and; 5) depositional mechanisms. The answers to these questions result in the identification of critical processes necessary for the function of a mineral system within a particular terrane, and permit the development of a targeting model. In this contribution we identify district scale critical orogenic gold mineral system processes for the late Archaean eastern Yilgarn Craton of Western Australia. During the geodynamic history of a terrane the critical processes which result in mineralisation change with time resulting in variations in mineralisation style. Proxies for critical processes have been mapped in an integrated GIS and are termed mappable mineral system process proxies (or MMSPP). In recognition of this, three separate time slices and a geochemical theme were analysed. Each MMSPP is given a weighting factor (WF) which reflects the spatial accuracy/coverage of the data and process criticality. For each theme/time-slice, a separate prospectivity map was created by summing the overlay or union of the spatial extent of each MMSPP, and adding the WF. A final target or prospectivity map was generated by a union of the four theme/time-slice prospectivity maps, and is tested against the known major deposits. The map 'discovered' the main gold camps and accounts for over 75% of the known gold in 5% of the area. This test verifies the process-based understanding and the appropriate mapping of the critical proxies. A further outcome from the map was the identification of a number of new target areas not known for significant gold mineralisation in what otherwise is thought to represent a mature terrane for gold exploration. The approach taken here has been to consider the Late Archaean gold deposits as a holistic system. Despite the recurring areas of uncertainty, this systems view has resulted in new findings that have generic applications to other mineral systems.

  • We measured the light absorption properties of two naturally occurring Australian hydrocarbon oils, a Gippsland light crude oil and a North West Shelf light condensate. Using these results in conjunction with estimated sensor environmental noise thresholds, the theoretical minimum limit of detectability of each oil type (as a function of oil thickness) was calculated for both the hyperspectral HYMAP and multispectral Quickbird sensors. The Gippsland crude oil is discernable at layer thickness of 20 micro metres or more in the Quickbird green channel. The HYMAP sensor was found to be theoretically capable of detecting a layer of Gippsland crude oil with a thickness of 10 micro metres in approximately six sensor channels. By contrast, the North West Shelf light condensate was not able to be detected by either sensor for any thickness up to 200 icro metres. Optical remote sensing is therefore not applicable for detecting diagnostic absorption features associated with this light condensate oil type, which is considered representative for the prospective Australian Northwest Shelf area. We conclude that oil type is critical to the applicability of optical remote sensing for natural oil slick detection and identification. We recommend that a sensor- and oil-specific sensitivity study should be conducted prior to applying optical remote sensors for oil exploration. The oil optical properties were obtained using two different laboratory methods, a reflectance-based approach and transmittance-based approach. The reflectance-based approach was relatively complex to implement, but was chosen in order to replicate as closely as possible real world remote sensing measurement conditions of an oil film on water. The transmittance-based approach, based upon standard laboratory spectrophotometric measurements was found to generate results in good agreement with the reflectance-based approach. Therefore, for future oil- and sensor-specific sensitivity studies, we recommend the relatively accessible transmittance-based approach, which is detailed in this paper.

  • The Munni Munni Complex (T Nd CHUR model age 2.85 Ga), located in the west Pilbara block of Western Australia, is one of the best preserved layered intrusions in Australia. Exposed over an area of 4 X 9 km, it is composed of a lower 1,850-m-thick ultramafic zone and an overlying gabbroic zone which has a minimum thickness of 3,630 m. The ultramafic zone contains rhythmically layered dunite, lherzolite, olivine websterite, clinopyroxenite, and websterite, with orthopyroxenite, norite, chromitite, and platiniferous websterite prominent near the top of the zone. The gabbroic zone consists of gabbronorite, anorthositic gabbro, and minor anorthosite which display a pronounced tholeiitic fractionation trend. The order of appearance of cumulus mineral assemblages in the complex is olivine, olivine + clinopyroxene, clinopyroxene + olivine, clinopyroxene, clinopyroxene + orthopyroxene, orthopyroxene + chromite, and plagioclase + clinopyroxene + ?orthopyroxene. This sequence is at variance with major platinum-group element-bearing intrusions in which crystallization of orthopyroxene generally precedes that of clinopyroxene.Trace-element data, obtained on samples collected across the entire intrusion to investigate the effects of crystal fractionation and S evolution on the distribution of the platinum-group elements, show that in the sulfide-undersaturated ultramafic zone, Pt, Pd, Au, Cu, S, Se, Cs, Rb, St, and Zr behaved incompatibly and were concentrated in the melt during fractionation. The S content of the melt began to increase above the 700-m stratigraphic level of the ultramafic zone, but Pt, Pd, and Au contents increased above background levels of approximately 3 ppb to 3 ppm Pt + Pd only with the attainment of sulfide saturation at approximately the 1,830-m stratigraphic level. The concentration trends of Zr, St, Cs, Rb, and Cu paralleled that of S, but Ir and Ni largely partitioned with early crystallizing olivine and decreased in concentration with increasing fractionation. In contrast to the ultramafic zone, Pt, Pd, It, and Au have depletion trends in the sulfide-saturated gabbroic zone. Hence, the evolution of S largely governed the behavior of the platinum-group elements during the fractionation of the Munni Munni magma(s).The platinum-group element mineralization occurs immediately below the ultramafic-gabbroic contact. It resulted from the combined magmatic processes of crystal fractionation (as evidenced by increasing Cu/(Cu + Ni) ratios and incompatible element trends with stratigraphic height), and magma mixing. Two models are presented. In model 1, a hot, buoyant sulfide-saturated tholeiitic magma (containing 1,700-2,600 ppm S) rose through the density stratified platinum-group element-enriched, sulfide-undersaturated resident ultramafic magma (containing 530 ppm S) until reaching its own density level near the top of the chamber, where it spread out laterally for a distance of at least 12 km. Due to crystallization of plagioclase and subsequent Fe-enrichment [of the melt], the density of the gabbroic melt increased until it overturned and mixed with the platinum-group element-enriched fractionated parts of the ultramafic magma. Model 2 is similar to model 1, except that it involves the fractionation and internal mixing of one magma. In both models, magma mixing triggered sulfide saturation in the hybrid magma and established a high R factor (the silicate/sulfide mass ratio).

  • Tropical cyclones affect storm-dominated sediment transport processes that characterise Holocene shelf deposits in many shelf environments. In this paper, we describe the geomorphology of reef talus deposits found in the Gulf of Carpentaria and Arafura Sea, Australia,that we attribute to tropical cyclones. The orientation of these deposits is also indicative of a consistent, along-coast transport pathway.

  • Coastal communities in Australia are particularly exposed to coincident natural hazards, whereby tropical cyclones and extra-tropical storms cause damage to infrastructure and shorelines from severe wind, flood and storm surge. Because the climatic drivers of severe storms are stronger under certain conditions (e.g. during La Ni±a periods for tropical cyclones), these events can repeatedly impact the coast over periods of weeks to months. Historically, major episodes of beach erosion along southeast Australia have occurred during every decade over the last century, with the most severe in 1974 resulting from two extra-tropical storms in two months. <p>While the process of beach erosion is well understood in general terms, the response of a specific sector of coast to clustered storms may not be. For effective coastal management, this site specific knowledge becomes essential. Here we present a framework for integrating coastal geomorphology and coastal engineering approaches to model shoreline response to clustered storms at a spatial scale that can directly inform management agencies. We focus on two case study areas in southeast Australia, the beaches of the Adelaide metropolitan coast (South Australia) and Old Bar beach (central New South Wales) where erosion is a management priority. <p>For each site we adopt the coastal sediment compartment as the functional management unit, mapped for the Australian continent at multiple spatial scales, and use sub-surface information (boreholes, ground penetrating radar profiles) to estimate sediment volumes in the upper beach to foredune. These data are then used to inform shoreline response modelling linked to an event time series (observed and hind cast) as a separate project component. Future work includes assessment of `at-risk infrastructure at each site. This paper is a contribution to the Bushfire and Natural Hazard Cooperative Research Centre project Storm surge: Resilience to clustered disaster events on the coast.

  • The Perth Basin is localised by reactivation of Neoproterozoic shear zones on the western margin of the Archaean Yilgarn Craton in Western Australia. While Ordovician to Silurian sandstones were deposited in the northern Perth Basin, the earliest sediments elsewhere are Middle Carboniferous to Permian in age. A sinistral transtensional regime, during which the main architecture of the basin was established, developed during NE-SW extension between Greater India and Western Australia in the Permo-Triassic. NW-SE shortening with continued NE-SW extension resulted in sinistral transpression in the late-Early to Middle Triassic. Sag-phase sedimentation in the LateTriassic followed this oblique rifting event. An analogy may be made between the Perth Basin and the Permo-Carboniferous to Jurassic Karoo basins in southern and central Africa and Madagascar. Deposition of the Karoo sequence took place within pull-apart and transtensional basins resulting from sinistral reactivation of basement shear zones. The Indian Gondwana Supergroup, and an equivalent sequence in Antarctica, were deposited within normal fault-bounded graben. The Late Paleozoic to Early Mesozoic formation of the Perth Basin, the Karoo basins of Africa and Madagascar, and the Gondwana basins of India was due to intraplate stress resulting from convergence along the Panthalassa margin of Gondwanaland. Late-Early to Middle Triassic compressional events in all basins mark terminal collision along the Panthalassa margin.

  • High quality refraction and wide-angle reflection seismic data recorded by ocean-bottom seismographs (OBSs) deployed by the Australian Geological Survey Organisation along the 700 km long transect in the Carnarvon Basin effectively supplement results obtained by means of the conventional reflection technology. Velocity information can now be derived from both CDP (nearvertical reflection) and OBS (refraction/wide-angle reflection) data. Generally, CDP-derived average velocities are lower than OBS-derived velocities and this deviation increases with depth: from ~0.1 km/s at 8 s two way time (TWT) to 0.8-1.6 km/s at 16 s TWT. If the CDP-derived velocities are used to depth convert reflection data, then depth to these TWTs would be underestimated by 0.4 to 6.4-12.8 km respectively. Some local anomalies (at ~6s TWT CDP-derived velocities may be more than 0.1 km/s higher than the OBS-derived velocities) distort this general trend. These would result in ~0.3 km local overestimates of the depth equivalent of 6s TWT. Co-analysis of the interval velocity field reconstructed from the travel time-based interpretation of the OBS data and the conventional reflection image of the crust in some cases shows their poor correlation.

  • The almost complete absence of basement outcrop or surface expression of mineralisation is the prime impediment to mineral exploration in the Gawler Craton of South Australia, and large areas elsewhere in Australia (Fig. 1). To explore in these covered terranes, we need high quality regional information, plus tools and techniques that allow us to better utilise the limited direct geological information from buried rocks. Analysis of potential-field data is one of the most common and cost-effective ways of inferring hidden geology. There are high quality aeromagnetic data available over most of the actively explored areas of Australia, and it is hoped that more closely-spaced ground gravity measurements, together with improvements in airborne gravity and airborne gravity gradiometer methods, will in the future raise the standard of gravity coverage to that of the magnetics.