Bureau Of Mineral Resources Research Articles

GOLD: Geophysical Setting of BIF-Hosted Gold Deposits at Tuckabianna, Western Australia

Tuckabianna Gold Mine is located about 25 km east of Cue, on the Mount Magnet-Meekatharra Shear Zone. The mine sequence consists largely of basalt, dolerite to gabbro, mafic schists, banded iron-formation (BIF), and intrusive quartz-feldspar porphyry dykes and sills. Gold mineralisation is hosted by the NE-striking, E-dipping BlFs with shears localising the ore zones. Laterite derived from the weathering of these BlFs may contain a significant gold resource. Downhole logging has shown that the BlFs typically have lower relative gamma activity and higher densities, resistivities and susceptibilities compared with the mafic units and porphyry. Petrophysical measurements of magnetic susceptibility, anisotropy of magnetic susceptibility and natural remanent magnetisation have characterised two BIF types, although no relationship between gold mineralisation and the BIF types has been conclusively demonstrated. Tuckabianna's regional geophysical dataset consists of Bureau of Mineral Resources (BMR) aeromagnetic and gravity data and high-resolution aeromagnetic and radiometric data. Of these, the most useful have been the high-resolution aeromagnetic data, because they clearly define the BlFs and structures which can localise gold mineralisation. Such lithological and structural detail cannot be recognised in either the BMR regional magnetic and gravity datasets or the high-resolution radiometric dataset. The Geoscan remote-sensing system has been flown over Tuckabianna. However, the laterite cover (1-21 m thick) has blanketed much of the geology and only some rock types and major lineaments can be identified from the data. Ground geophysical surveys have been dominated by the magnetic method, which has been extremely useful in delineating the BlFs and structural features, as has the gravity method. Electrical surveys, including magnetometric resistivity, induced polarisation (frequency and time-domain) and electromagnetic methods have also been used. These electrical techniques have not been completely successful in the direct detection of mineralised zones. However, the resistive nature of the BIF is such that these methods may be successfully used to locate the BIF and porphyry, in addition to delineating some structures.

Geology of the Kosciusko National Park, 1990

This map received the award in the Thematic Mapping category at the 1992 AIC Conference in Adelaide. The incorporation of supplementary information to assist the reader to understand the geology of the map differs from the Bureau of Mineral Resources (BMR)'s traditional product range. The map has proved to be a popular product and the harbinger for change within the organisation.

Tests of Australian aerial radiometric data for use in petroleum reconnaissance

Recon Exploration Pty. Ltd. has successfully completed initial testing of a new method for processing and interpretation of AGSO’s (Australian Geological Survey Organization, formerly Bureau of Mineral Resources) aerial gamma‐ray spectrometer data for petroleum exploration in the Canning Basin, Western Australia and the Otway Basin, Victoria. Count‐rate data for potassium and uranium were normalized to the thorium count rate for each sample to suppress unwanted effects of variations in surface lithology or soil type, soil moisture, vegetation cover, and counting geometry. The Canning Basin test area included five producing oil fields. All except one clearly exhibit significant and characteristic radiometric anomalies which include negative normalized potassium and more positive normalized uranium values. The Otway Basin test areas included PPL-1 commercial gas production which is associated with a group of significant radiometric anomalies similar to those in the Canning Basin. These results are similar to extensive ongoing tests in the U.S. and are explained in terms of well‐understood geological, geochemical, and geophysical models. Based on 69 wells in the three test areas, it is estimated that the chance of encountering hydrocarbons (economic production or shows) in wells within the radiometrically favorable zones is about 2.6 times greater than outside the favorable areas.

Assessment of Undiscovered Petroleum Resources by the Bureau of Mineral Resources, Australia

Abstract Petroleum resource assessment begins with geological study of sedimentary basins and subdivision of the sedimentary sequence and petroleum prospects into plays. Next, information about the plays, the area and geometry of their traps, and the sizes of their accumulations are compiled in a database within the AUSTPLAY program. Some data are sorted, plotted, and analyzed by the program. All data are examined by specialist geoscientists, who interpret the historic trends, select analogues, make quantitative models, and use subjective judgment to determine the information to be used in the assessment. AUSTPLAY simulates drilling each untested trap in each play. It estimates the size of each simulated discovery using estimates of the area of closure and of the resources per unit area of closure of each trap that are input, in order of drilling or discovery, as loglinear models. The program also uses other types of information, such as existence risk, success rate, the proportion of oil to oil and gas, ...

The Deep Velocity Structure Beneath the Gippsland Basin from Long-Offset Seismic Data

The Bureau of Mineral Resources (BMR) undertook a regional marine seismic survey of the Gippsland Basin in 1988?89. During this survey, recording stations were deployed onshore by the BMR and the Department of Earth Sciences, Monash University, to record long-offset wide-angle reflection and refraction data from the standard marine seismic reflection air-gun shots. The purpose of recording this complementary data was to obtain deep velocities and regional structure, to map variations of crustal thickness, and to link the marine data with earlier onshore results. Arrivals from offsets up to 230 km were recorded, thus sampling the total crustal thickness. It was necessary to enhance the data at large offsets by filtering and stacking because of the relatively low power of the source and noisy conditions during the survey. The acquisition method is logistically simple, and a large amount of additional data can be gathered for negligible extra cost to a survey. Velocities between 5.3 and 5.9 km/s were obtained for the basement and mid-crust. Lower crustal velocities were around 6.3 km/s, and upper mantle velocities were between 7.8 and 8.1 km/s. Sediment velocities between 1.8 and 4.5 km/s were obtained from sonobuoys. The velocity control constrains the identification of deep reflectors and allows accurate depth conversion of interpreted seismic reflection profiles. The deep basement geometry and crustal structure were interpreted along selected traverses from the long-offset data in conjunction with sonobuoy and reflection results. The sediments in the Central Deep reach a thickness of about 12 km. The total crustal thickness thins from about 36 km in Victoria, north of the basin, to about 23 km below the deepest sediments. The boundary between the upper and lower crust, where the velocity increases from about 5.9 to 6.3 km/s, correlates to a reflector which has been previously interpreted as a major regional detachment surface associated with basin formation.

New Magnetic and Gravity Images of Victoria, Australia

A new compilation of magnetic and gravity data for Victoria has enabled the production of images of these data bases. The magnetic image of Victoria has been produced by combining regional and detailed airborne surveys. All the 1:250 000 standard map sheets in Victoria (21 in total) have been flown by the Bureau of Mineral Resources (BMR) and the Geological Survey of Victoria (GSV), and the data have been gridded using a 150 m ´ 150 m cell size and merged together by linear adjustment to adjacent data sets. Detailed company and government airborne surveys, and marine surveys have been superimposed onto the regional data to produce a composite image of the total magnetic intensity of Victoria. The GSV has edited existing onshore gravity data, added new data, and converted the data base to the IGSN71 (Isogal 84) datum. A summary is given of the five methods used to convert the gravity data from the Potsdam datum (Isogal 65) to IGSN71 datum (Isogal 84). The resultant gravity data base has been gridded using a 300 m ´ 300 m cell size and imaged. The images of the magnetic and gravity data sets highlight a number of major geological features not previously apparent. The improved ability and increased computing power of modern imageprocessing software to handle and merge images of large, multiple data sets has substantially enhanced the contribution that regional geophysical surveys have made to the geological knowledge of Victoria.

The Magnetic Anomaly Map of Australia

The Bureau of Mineral Resources has been routinely acquiring airborne magnetic surveys over the land area of Australia since 1951 to record and map anomalies in the earth's magnetic field attributable to geological structures and lithologies. In forty years, over four million line kilometres of survey data have been flown, while the technology of survey practice has passed through various stages of development. About 83 per cent of the land area has now been covered with so-called reconnaissance surveys flown 150 m above terrain at line spacing between 1.5 and 3.2 km. Located profile data for these surveys have been gridded using a minimum curvature technique, to 15 second of arc (approximately 400 m) and, where necessary, micro-levelled. Data for much of the remaining areas ? particularly the inland sedimentary basins covered by surveys of lower specifications ? were obtained from digital data on an approximately 2-km grid (72 seconds of arc) published in 1976; these have also been interpolated to 15 second of arc. The data were first assembled for each of over five hundred 1:250 000 map sheets. The 1:250 000 sheets were linked by minimizing the discrepancies along their common boundaries (which were often also survey boundaries) and reducing remaining mis-ties through Laplacian smoothing to minimize the visibility of boundaries between surveys acquired separately. While the data quality varies with instrumentation and survey parameters, it is almost everywhere good enough to provide a useful synoptic view of rnagnetic anomaly patterns, which can be expected to give important new insights into geology and tectonics at a continental scale, and to provide a regional framework within which to interpret more local magnetic anomalies. The purpose of this short paper is to report the latest progress on compilation of the Magnetic Anomaly Map of Australia, which is scheduled for publication late in 1992.

Pixel Map Preparation using the HSV Colour Model

Digital images are an invaluable aid to the study of large spatial data-sets. With the advent of the National Geoscience Mapping Accord (NGMA) between the Bureau of Mineral Resources, Geology and G...

Vulcan Graben, Timor Sea; Regional Structure From a Magnetic Survey

In late 1989 the Australian Bureau of Mineral Resources (BMR) carried out a regional aeromagnetic survey by contract over the Vulcan Graben region. Elongate short-wavelength anomalies, with wavelengths of 3 to 12 km, amplitudes of 0.2 to 2.0 nT, and with generally northeast strikes, can be correlated for distances of up to 100 km on the basis of their shape and amplitude. Their source appears to be principally at depths of between 0.7 and 3.0 km. Some of the relatively larger amplitude and wavelength anomalies overlie major northwest-dipping normal faults which have been mapped by seismic. All of the elongate anomalies are thought to be due to structures or magnetic deposits associated either directly with NE-trending normal rift faults, or with faults in the post-rift sediments which are related to rift fault reactivation. ENE fault trends, which developed in the Neogene as a result of the collision with Timor, do not appear on the aeromagnetic data. We have mapped numerous northwest-trending faults. These have a spacing of approximately 10 to 25 km. They are thought to be strike-slip because some offset the major northeast-trending normal faults. The northwest trending faults may be due to the reactivation of Palaeozoic trends, the reactivation of Mesozoic transfer faults, or possibly some may be younger and independent of the earlier faults. Some of these faults have previously been mapped on seismic and correspond to seismic 'bad data' areas. These faults may have a major role in the entrapment of hydrocarbon accumulations in the Timor Sea, with most of the significant fields being close to a northwest-trending fault or its extension. In the Timor Sea, high resolution aeromagnetics have proven to be a relatively cheap and cost-effective tool which both complements and supplements the existing seismic data. It should prove equally useful in other areas with extensive seismic coverage. In frontier areas, it should rapidly define the geometry of the major tectonic elements of the basin, allowing for better positioning of seismic programs.

DEEP STRUCTURAL IMAGING FROM ONSHORE RECORDING OF A MARINE AIR-GUN SOURCE IN THE GIPPSLAND AND BASS BASINS

In 1988 and 1989, the Bureau of Mineral Resources (BMR) completed two regional seismic reflection surveys in the Gippsland and Bass Basins. Seismic arrivals from the routine air-gun shots fired during these surveys were recorded on land by BMR and the Department of Earth Sciences, Monash University. Individual analogue and digital recording stations were deployed in Victoria, Tasmania and Deal Island on the Bassian Rise. Long-offset wide-angle reflection and refraction data were obtained at these stations from traverses across both basins.The data quality was variable, depending on local site conditions, but useful arrivals were observed over 200 km away from the source on some lines. The close shot spacing, either 37.5 or 50 m, and the large number of shots, up to 5000 per traverse, provides the opportunity for stacking and other signal enhancement techniques in areas of poor data quality.The arrival times of the refracted events show significant delays corresponding to changes in basin sediment thickness. Preliminary results suggest no major asymmetry in the rifting process, which would require modification in the current models for rifting of the basins. Sediment and basement apparent velocities obtained from near-station records range from 4.8 to 5.1 km/s; below the deepest part of the basin, the basement apparent velocity is around 5.6 km/s. Deep crustal/upper mantle velocities of 7.2 km/s, and around 8 km/s, are also observed.These velocities, combined with the coincident reflection data provide critical constraints on models of basement geometry. The refraction and wide-angle reflection data can be used to derive the crustal structure associated with the basins and surrounding margins. These sections will complement deep reflection profiling to test and refine tectonic models to guide further exploration.

A NEW LOOK AT THE STRUCTURAL TRENDS IN THE ONSHORE OTWAY BASIN, VICTORIA, USING IMAGE PROCESSING OF GEOPHYSICAL DATA

Although the Otway Basin is oriented west-north-westerly, and previously recognised major structural elements follow a similar trend, other structural trends have been found on recently obtained geophysical data.In 1989, an aeromagnetic and radiometric survey of the onshore Otway Basin was completed for the Victorian Department of Industry and the Bureau of Mineral Resources, Geology and Geophysics. This survey, together with a recent gravity compilation by the Geological Survey of Victoria, enables analysis of magnetic and gravity data trends reflecting basement and intra-basin structure.The trend analysis was carried out using modern image processing techniques including simulation of real-time sun-angles of the magnetic and gravity data, and composite images of the radiometric data, to highlight lineaments. This technology enables integration of magnetic, gravity, radiometric and, potentially, seismic, Landsat, topography and bathymetry data for basin structure analysis.The magnetic, gravity and radiometric trend analysis was compared to an earlier Landsat study (Baker, 1980) and a previous seismic data compilation of the Otway Basin (Megallaa, 1986).The present study has revealed the significance of major early Palaeozoic north-south and east-north-east to easterly trends. The latter trends have not previously been identified or discussed in earlier basin reviews. There appears to be a difference between trends reflected in the radiometric and seismic data and trends apparent in the gravity and magnetic data. This could indicate a change in principal stress directions during the evolution of the basin. The shape of the northern margin of the basin appears to be controlled by major north-easterly structures.

High-Resolution Aeromagnetics as an Exploration Tool in the Timor Sea, Northwestern Australia

The Mesozoic Vulcan Graben in the Timor Sea, northwestern Australia, is one of Australia's most active oil exploration areas, with a number of discoveries made within the last five years. To further assist in understanding the structural setting of the region, the Australian Bureau of Mineral Resources (BMR) carried out a regional high-resolution aeromagnetic survey over the Vulcan Graben in late 1989. Three to twelve km wavelength, 0.2-2.0 nT amplitude magnetic anomalies have northeast strikes and correlate well with the position and orientation of the rift faults, as defined by seismic reflection data. Significantly, the magnetic trends closely match the fault trends at the prospective Valanginian unconformity horizon, where all of the major producing reservoirs are located. Numerous northwest-trending (strike-slip ) faults have also been mapped as zones of low anomaly, some of which correspond with offsets in the major northeast-trending anomalies. These faults, which are probably due to the reactivation of either Mesozoic transfer faults or Paleozoic normal faults, may have a major role in the entrapment of hydrocarbons in the Timor Sea, as most producing fields are located close to a northwest-trending fault or its extension. The northwest-trending faults have not previously been defined using seismic data. Thismore » survey shows that even in an area with extensive seismic coverage such as the Timor Sea, high-resolution aeromagnetics can provide important information that both complements and supplements the seismic data.« less

Seismic stratigraphy of the Raggatt Basin, southern Kerguelen Plateau: Tectonic and paleoceanographic implications

During Cenozoic and late Mesozoic time, sediment accumulated in the Raggatt Basin on the southern Kerguelen Plateau. We describe the seismic stratigraphy of the Raggatt Basin, utilizing multichannel seismic (MCS) data obtained by the Bureau of Mineral Resources, Geology and Geophysics (Australia) and the Institut de Physique du Globe de Strasbourg (France). Seven major seismic stratigraphic sequences in the Raggatt Basin overlie a basement complex of Early Cretaceous age. The underlying basement complex is characterized by two types of seismic images: acoustic basement and a layered basement which includes dipping reflectors. The seismic stratigraphic sequences include terrestrial and shallow-water units K1 and K2 of Early to Late Cretaceous age, which fill depressions in the basement; a thick unit, K3, of Late Cretaceous to Paleocene age, which is of mixed shallow-water and open-marine facies and in places has a mounded (carbonate) upper surface; a depression-filling pelagic deposit, P1, of Late Cretaceous to Eocene age; a thick pelagic unit, P2, which is mainly Eocene in age; and two post-Eocene pelagic sequences, PN1 and NQ1, which are relatively thin and more limited in areal extent than the underlying sequences. Highlights in the geologic history of the Raggatt Basin include formation and erosion of the basement complex in a subaerial or shallow-water environment in Early Cretaceous time; differential subsidence, probably thermal in origin, and development of carbonate mounds and a major western boundary transform fault in Late Cretaceous time; renewed subsidence near the Cretaceous-Tertiary boundary possibly related to formation of the Labuan Basin/Diamantina Zone by sea-floor spreading; and vigorous Antarctic Circumpolar Current activity beginning by Oligocene time, probably related to the breakup of the northern Kerguelen Plateau and Broken Ridge.

Australia's southern margin: a product of oblique extension

Recently developed detachment models of continental margin formation interpret the southern margin of Australia to have formed when the lower-plate Australian margin was pulled out from beneath the upper-plate Antarctic margin. Data now available and summarised in this paper, point very strongly to a generally NW-SE direction of initial continental extension for the southern margin, in contrast to the widely held picture of simple N-S rifting. The evidence for this extension direction comes from the analysis of deep Seismic data acquired by the Bureau of Mineral Resources in 1986 in the central Great Australian Bight (GAB), the gravity field of the GAB, Seismic and magnetic basement structures in the Eyre Sub-basin, Polda Trough, Ceduna Depocentre and Duntroon Basin and from the analysis of the magnetic seafloor spreading anomalies produced during the slow first phase of drifting between Australia and Antarctica.Further, it is now believed that the formation of the southern margin of Australia can be described in terms of three phases of continental extension (El to E3) and two phases of drifting (D1 and D2). In summary, these phases were as follows. 1.E1: approximately 300 km of Late Jurassic (?or older) to Early Cretaceous NW-SE-oriented extension in the GAB, with strike-slip motion in the nascent Otway Basin and along the Tasmanian margin.2.E2: 120 km of Early Cretaceous NNE-SSW-oriented extension which formed the basins of southeastern Australia (Otway, Bass, Gippsland) and which probably produced a structural overprinting in the GAB Basin.3.E3/D1: minor continental extension and the first 500 km of slow drift between Australia and Antarctica on an azimuth of 165°; wrenching on the Tasmanian margin.4.D2: 2600 km of fast drifting between Australia and Antarctica on a N-S azimuth.

AUSTRALIA'S DEEPWATER FRONTIER PETROLEUM BASINS AND PLAY TYPES

Vast areas of Australia's continental margin sedimentary basins lying seawards of the 200 m water depth line, or shelf edge, are under-explored for petroleum. Indeed, most are essentially unexplored. However, recent advances in drilling and production technology, as well as recent reconnaissance seismic, geochemical, geothermal and seabed sampling data collected by the Bureau of Mineral Resources' (BMR) Marine Division, may reduce the perceived economic risk of many of these deepwater basins relative to their shelf counterparts. Triassic reefs have been identified off the northern Exmouth Plateau and possibly in the deepwater Canning Basin, locally within a predicted oil window. In the deepwater North Perth Basin, major wrench structures have been identified. The deepwater areas of the Great Australian Bight and Otway Basin are actually the main depocentres of a major basin complex lying along the almost totally unexplored southern Australian continental margin. The Latrobe Group in the outer Gippsland Basin is likely to have similar geology to the well explored and productive shelf basin, but remains untested. The Queensland and Townsville troughs, in deepwater off northeast Australia, contain many significant structures typical of unbreached rift systems.All these areas have been risked relative to each other and their prospectivity assessed. The most attractive frontier areas in terms of relative risk may be the Otway and North Perth basins. The highest potential may occur in the deepwater rift troughs off northeast Australia, although the relative risk is very high. Triassic reefs of the Northwest Shelf may have the best prospectivity in the shorter term, given that they are known from drilling in a region with proven source potential and a substantial exploration infrastructure.

Environmental magnetism in Australia and Papua New Guinea

Paleomagnetic and rock magnetic research in Australia has for some time been among the best in the world, mainly through the work of groups at the Australian National University (ANU), the Bureau of Mineral Resources (BMR) and the Commonwealth Scientific and Industrial Research Organization (CSIRO). Environmental magnetism is a relatively new field, but one that is developing rapidly in Australia, as elsewhere. This report outlines both past and present research and suggests some future directions.Environmental magnetism in the Australian region began as an adjunct to paleomagnetic studies of lake sediments in the Highlands of Papua New Guinea (PNG) [Thompson and Oldfield, 1978; Oldfield et al, 1980] and in Western Victoria [Barton and Polach, 1980]. In both cases, magnetic susceptibility and some remanence measurements were used for core correlation.

Crustal structure and formation of a magnetic quiet zone in the Australian southern margin

The crustal structure of the magnetic quiet zone outboard of the Otway Basin of the Australian southern margin is interpreted for the whole crustal section using fully processed multichannel seismic reflection data, seismic refraction data and dredge data collected by the Australian Bureau of Mineral Resources (BMR) vessel Rig Seismic, and extrapolation of well control. The quiet zone is interpreted to consist of relatively thin (10 km), faulted and extended continental crust. The upper and lower crust are interpreted to have been extended separately by listric but approximately subplanar faulting, and planar normal faulting, respectively, during a final mid-Cretaceous extensional phase prior to sea-floor spreading. This brittle extension in both cases is calculated to be, on average, less than about 40%. A discrepancy occurs between the observed crustal thickness and that implied by fault geometry because the interpreted lower crustal thickness is approximately 7 km and the observed faulting cannot account for a thinning of the crust to less than 25 km thickness from an initial continental crustal thickness of around 35 km. Subsidence calculations are, however, consistent with the interpreted crustal thicknesses. These observations require previous crustal thinning, other than by the observed faulting, and argue for two distinct phases in the formation of the continental margin and quiet zone. A model is presented for the mid-Cretaceous formation of the continental margin quiet zone of the Otway Basin. This model involves a second stage rifting of an older (first stage) onshore rift. In the model the ‘lower plate’ of the first stage onshore rift is separated from the upper plate by simple shear, possibly accompanied by a component of pure shear within the lower plate. The lower plate is then subjected to subsidence and overlying sedimentation. This is followed by a second stage of rifting involving separate extension and thinning of the original lower plate and subsequent sediments prior to sea-floor spreading. The extensional faults interpreted from seismic reflection data collected in this study are thought to provide the mechanism. The second stage rifting is modelled as finally forming the ‘lower plate’ continental passive margin and continental magnetic quiet zone. Towards the end of the second stage rifting, high angle normal faults are interpreted to cut the whole crust on the continental quiet zone. They are believed to be associated with earliest thermal subsidence in that region after the onset of sea-floor spreading. These faults are not observed in seismic reflection data over the continental slope. The onset of oceanic crust occurs at a ridge at the southern end of the transect. The ridge corresponds to a free-air gravity high and the beginning of long wavelength magnetic anomalies with amplitudes of approximately 100 nT. The characteristic two-stage rifting model derived here appears to be also valid for the Exmouth Plateau and may have implications for some other magnetic quiet zones and continental margins.

Seismic reflection and refraction profiling across the Arunta Block and the Ngalia and Amadeus Basins

In order to investigate the tectonic evolution of the Arunta Block and the Ngalia and Amadeus Basins, a regional north‐south seismic reflection line 420 km long from the Northern Arunta Province to the southern part of the Amadeus Basin, and an east‐west refraction profile over 400 km within the Arunta Block, were recorded by the Bureau of Mineral Resources in 1985. The most significant basement features observed in the reflection data are prominent bands of northerly dipping reflections originating from beneath the Northern Arunta Province and the Ngalia Basin at times of between 4 and 10 s. In this region, reflected energy with frequencies as high as 100 Hz is present at two‐way times of 5–6 s, implying that the rocks have high Q to depths of at least 18 km. The character of the reflections changes markedly with varying frequency, which suggests that they arise by interference phenomena, probably associated with laterally varying lamellar structures. Deep crustal features on the reflection profiles from...

Anomalous geochemical signals from phosphatic Middle Cambrian rocks in the southern Georgina Basin, Australia

ABSTRACTMiddle Cambrian rocks of the Georgina Basin contain both phosphatic and organic‐rich sediments, the former often occurring as commercially viable phosphate deposits, and the latter as minor oil shale occurrences. Both kinds of rocks occur in the Hay River Formation penetrated by Bureau of Mineral Resources (BMR) stratigraphic corehole Tobermory 14. Detailed sampling and analysis of this core has been undertaken to assess the chemostratigraphy of the Hay River Formation; to examine the possible use, as a palaeoceanographic tool, of changes in δ13C values of carbonates and kerogens; and to gain an understanding of phosphate deposition in this part of the basin.The Upper Hay River Formation was deposited in a normal marine environment characterized by oxic bottom conditions. It contains several episodes of significant trace metal enrichment which appear to indicate periodic replacement of the water body. The Lower Hay River Formation, however, was deposited in an abnormal marine environment low in sulphate and Mo and enriched in P. Its carbonate phase has positive δ13C values 1–1.5%0 enriched over the accepted Cambrian marine carbonate value of ‐0.57 ± 0.17%0. The very low sulphate concentration, Mo‐depletion and spiking of carbonate δ18O values suggests there was an influx of deeper ocean water of sufficient volume to maintain prolonged stable anoxic bottom waters. These data support a previous suggestion that phosphogenesis in the Georgina Basin was related to ‘Oceanic Anoxic Events'.

Deep crustal reflections in Australia 1957-1973--II. Crustal models

In a companion paper, we described experiments conducted by the Australian Bureau of Mineral Resources (BMR) between 1957 and 1973 to record deep crustal seismic reflections. At 24 of the 46 sites investigated, events were recorded at times up to 10-13 s. At some sites, only one or two shots were recorded; where more comprehensive investigations were made, the events were consistent with deep reflections, though they lacked continuity in many cases. The results were sufficiently encouraging that we have now analysed the events on the hypothesis that they represented deep crustal reflectors, including the Moho. In 1976, BMR introduced digital recording, and successfully carried out several long deep crustal profiles since then. This paper analyses only the recordings made with analogue equipment prior to 1973. In a few places, the reflection structure could be compared with nearby refraction traverse results; some events correlated well with first or second-order discontinuities in the refraction velocity-depth curves, while for others there was no correspondence. It is difficult to generalize as to which types of refractors give rise to reflections. The refraction surveys show that the Moho is not always a boundary with a sharp change in velocity across it. In some areas in Queensland the deepest reflector correlates well with the refracting Moho. For reflection shots not near refraction surveys, we converted travel times to depths using simplified velocity models based on refraction surveys in similar geological environments, and on isostatic balance considerations using gravity and elevation data. In central Australia, if the deepest reflector is the Moho, a relatively thin and light crust is implied. However comparison with refraction models makes it seem probable that a heavier, thicker crust could exist, with the Moho being deeper than the deepest reflector. Also in the Tasman Geosyncline, it is probable that the Moho is deeper than the deepest reflector.