A short article describing the outcomes of the Tasman Frontier Petroleum Industry Workshop held at Geoscience Australia on 8 and 9 March 2012.

Uraninite dating of the Kintyre deposit and Rossing South prospect using electron probe chemical U-Th-Pb technique.

AGSO's northwest Australian margin project (NWAM) aims to provide a high level understanding of the geological framework of the entire northwest margin of the continent, with particular emphasis on the crustal and basin architecture. The following studies are currently addressing these objectives: 1) ArcView GIS, 2) Potential field and bathymetric grids (2nd version), 3) Regional deep seismic re-interpretation, and 4) Ocean-bottom seismograph velocity models.

Joint Release of the National ASTER geoscience maps at IGC The ASTER (Advanced Spaceborne Thermal Emission and Reflectance Radiometer) Geoscience Maps are the first public, web-accessible, continent-scale product release from the ASTER Global Mapping data archive. The collaborative Australian ASTER Initiative represents a successful multi-agency endeavour, led by the Western Australian Centre of Excellence for 3D Mineral Mapping (C3DMM) at CSIRO, Geoscience Australia and the State and Territory government geological surveys of Australia, along with other national and international collaborators. National ASTER geoscience map These geoscience maps are released in GIS format as 1:1M map-sheet tiles, from 3,000 ASTER scenes of 60x60km. Each scene was cross-calibrated and validated using independent Hyperion satellite imagery. The new ASTER geoscience products range in their application from local to continental scales, and their uses include mapping of soils for agricultural and environmental management, such as estimating soil loss, dust management and water catchment modelling. They will also be useful for resource exploration, showing host rock, alteration and regolith mineralogy and providing new mineral information at high spatial resolution (30m pixel). This information is not currently available from other pre-competitive geoscience data.

In a previous paper densities of crustal layers were inferred from seismic refraction surveys in Australia and surrounding marine areas. These indicated substantial variations in crustal mass. As the free-air gravity field does not show anomalies corresponding to these, it is inferred that compensating mass variations must occur in the upper mantle. Sub-crustal mantle densities, inferred from Pn velocities, in general do not provide the required mantle mass distribution; however in some parts of the continent observed increases in seismic velocity at depths of 60 to 100 km suggest density changes which would lead to approximate compensation at about 130 km depth, corresponding to the top of a low-velocity layer suggested by surface wave studies. Marine crustal masses are reasonably close to a common value, but the wide variation of Pn velocities implies a corresponding variation of densities which would counteract this compensation if they persisted in depth. It is suggested that the Pn velocities represent comparatively thin layers, and that deeper density changes occur so that compensation takes place at 80 to 100 km, at the top of the sub-oceanic asthenosphere. The West Australian shield has the highest crustal mass, and also the highest Pn velocity, which implies a further relative increase in mass with depth. If the sub-shield mantle is assumed to consist of refractory peridotite with a relatively low density corresponding to its Pn velocity, the discrepancy in crustal mass with respect to the other areas is reduced with increasing depth, but is still not eliminated at depths less than about 160 km. This suggests that the sub-shield mantle above this depth has enough strength to support the differential pressure associated with the excess crustal mass. This conclusion is in accordance with other evidence, suggesting that sub-shield or sub-continental mantle differs from sub-oceanic mantle to depths of several hundreds of kilometres, and hence that the return flow compensating for plate-tectonic motions cannot take place at depths of 100 to 200 km, as often supposed.

Environmental and urban geology is an expanding field in which the government geological surveys are inevitably becoming more involved. Throughout the world, pressures for urban development and the exploitation of resources are coinciding with a growing awareness of threats to the environment. Phillip (1976) has recently reviewed the status of environmental geology in Australia, and indicated its development particularly in relation to land-use planning. Several papers in the 25th International Geological Congress held at Sydney in August 1976, described geological contributions to environmental management in Australia. These papers included reports of the urban geology of Sydney (Burgess, in press); the environmental geology of the Western Australian coastal limestone (Gordon, 1976); landslides affecting urban development in Tasmania (Knights and Matthews, 1976); engineering geology for subdivisional planning in South Australia (Selby, 1976); environmental geological mapping for land-use planning in Queensland (Hofmann, 1976); geological aspects of the pollution of coastal lagoons in New South Wales (Albani and Brown, 1976); and quarry reclamation problems in Victoria (Bowen, 1976). Interest aroused at the Congress led to a workshop meeting of government survey geologists concerned with environmental and urban geology being held in Canberra, 15-17 November 1976. Participants included W. S. Chesnut and I. Wallace (New South Wales Geological Survey); K. G. Bowen and J. L. Neilson (Victorian Geological Survey); G. W. Hofmann (Geological Survey of Queensland); J. Selby (South Australian Geological Survey); P. C. Stevenson (Geological Survey of Tasmania); E. G. Wilson, G. Jacobson, P. D. Hohnen, P. H. Vanden Broek, G. A. M. Henderson and R. C. M. Goldsmith (BMR); J. C. Braybrooke (SMEC); and other Commonwealth government personnel. At the meeting, government activity in environmental and urban geology was reviewed; abstracts of these reviews are given below, and illustrate the wide range of investigations in this field currently being undertaken by the geological surveys. The meeting included discussion of specific problems encountered in environmental geology investigations (see below); and a field inspection of the urban geology of Tuggeranong, A.C.T.

In 1976 a coloured 1:5 000 000 Gravity Map of Australia was published by BMR. At that stage the systematic reconnaissance gravity coverage of Australia, initiated by BMR in 1959, was complete, and preliminary gravity values were available from marine coverage of the continental shelf and margins. The map was based on approximately 260 000 gravity observations obtained by various organisations at a cost of about $12 000 000 from 300 surveys. It was produced using a CDC Cyber 76 computer system, Calcomp plotters and cartographic techniques. The computer processing phase took about four months and the cartography about three months. The coloured 1:25 000 000 Bouguer and free-air anomaly maps contained in this issue were also produced from the same data bank.

The earliest known gravity measurements in Australia were made by French expeditions in 1819 and 1824, using pendulums at Sydney. Later in the 19th century, further pendulum measurements with an accuracy of about 10 mGal were made at various capital cities by observers from Britain, Bavaria, Austria-Hungary, Russia and Italy. A very early gravity meter was designed and constructed at Sydney University during the last decade of the century, but was used only experimentally. Reasonably accurate gravity meter surveys started about 1947. The Cambridge pendulums were used in 1950-51 to establish a national network of 59 stations with an accuracy of about 0.8 mGal; this was supplemented between 1950 and 1959 by gravity meter and pendulum measurements made as part of international surveys, which also helped to relate the Australian datum to the international network. Meanwhile, surveys, mainly for geophysical prospecting, were made by Government authorities, universities, and private companies; some of these surveys covered extensive areas and enabled compilation of a preliminary Bouguer anomaly map in 1959. Early marine gravity surveys included observations in nearby oceans with Vening Meinesz submarine pendulums, gravity measurements on offshore islands and reefs, and from 1956, underwater gravity meter surveys on the continental shelf. Two factors stimulated regional gravity coverage in 1959 - firstly the Petroleum Search Subsidy Act, which ensured that exploration data from subsidized surveys were publicly available, and secondly, use of helicopters for reconnaissance gravity coverage of the continent, which was completed in 1973. Gravity data at sea were obtained mainly from reconnaissance marine geophysical surveys carried out under contract to BMR between 1965 and 1973, but include also traverses by international survey vessels and exploration companies. Gravity meter calibration ranges were established in the main cities in 1960-61. Gravity values at the Cambridge pendulum stations were revised in 1962 using all relevant data, to establish a more accurate control network with standard errors ranging from 0.2 to more than 0.4 mGal for compilation of data from many surveys. These values were superseded by the Isogal Project of 1964-67, which gave values with standard errors of 0.1-0.2 mGal, in which several gravity meters were transported by aircraft along transcontinental east-west traverses. Values at the eastern ends of the traverses, forming part of the Australian Calibration Line (ACL), were established by a US Air Force survey in 1965. The accuracy of the ACL was significantly improved by Soviet pendulum measurements in 1972-74, and joint Soviet-Australian gravity meter measurements in 1973 - a precision of about 0.01 mGal being achieved.

The Australian gravity field is analysed to determine whether the basement differs between regions of exposed basement and of covered basement (sedimentary basins). When the thickness of cover is allowed for, there is no systematic change in gravity variability from exposed to covered basement regions and the pattern of density differences is inferred to be similar in the two basement types. With one exception the trends of the gravity anomalies are continuous from covered to exposed basement regions; therefore geological structure and rock formations in the basement are considered to be continuous between the two kinds of regions. After making allowance for altitude and the effects of cover beds, the covered basement is calculated to have gravity anomalies that average 5 mGal less than the exposed basement. This gravity difference is probably caused by the two kinds of basement not being isostatically balanced relative to each other, rather than the basements having a different average density. No major differences between exposed and covered basement are apparent. It is likely that the two basement types have a similar history of formation, and consequently similar mineral potential.

In eastern Australia free-air anomalies and altitudes averaged over 1° x 1° areas show a positive correlation of +0.059 ± .004 mGal. m^-1. lsostatic compensation of topography and crustal masses is thought to be mainly at the base of the crust, but partly deeper in the mantle, and to be almost complete for 1° x 1° areas. In central and western Australia free-air anomalies and altitudes have a zero or slightly negative correlation with a larger scatter; isostatic compensation is thought to be complete only for 3° x 3° areas and to be predominantly at the base of the crust. The deeper compensation in eastern Australia is thought to be related in some way to the youth of the crust and the presence of a low velocity zone at about 130 km. Elsewhere in the world Phanerozoic areas have a positive free-air anomaly altitude correlation, and Precambrian areas a negative correlation; so the different modes of isostatic compensation found in Australia may apply to crust of similar age elsewhere in the world. In Australia the residual 1° x 1° area anomalies found by removing altitude, long wavelength and sedimentary effects generally have an amplitude of 30 mGal and a wavelength of 6°; they are thought to be due to isostatically compensated variations in crustal density. Using these anomalies, variations in crustal thickness are predicted.