Deep-seismic reflection data have provided information on the crustal architecture of several highly mineralised regions within the Archaean northeastern Yilgarn Craton, Western Australia. These seismic data are characterised by several prominent features and include 1) a change in the thickness of the crust across the northeastern Yilgarn Craton, 2) subdivision of the crust into three broad layers, 3) a prominent east dip to the majority of the reflections, and 4) the identification of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones divide the region into four terranes and are surprisingly similar in geometry. In the hangingwalls of the shears, there is evidence of a marked increase in deformation adjacent to the shear zone. This region is underlain by another low-angle shear zone at depth. Major orogenic Au deposits in the northeastern Yilgarn are spatially associated with major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class Au deposits plus many other smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed as the Laverton structure. We infer that the complex deformed region within the hangingwall and underlain by a low-angle shear zone forms a wedge-shaped trap with upward and/or sub-horizontal moving fluids being focused into the apex of the wedge.

Results from an audit of 32 petroleum exploration wells in the Bass Basin have shown that approximately half of the wells in the basin were invalid tests due to off-structure drilling or mis-interpretation. Of the remaining wells, primary reasons for failure were lack of effective seal, timing, trap validity, lack of access to mature source rocks or reservoir problems. In parts of the basin the regional seal (Demons Bluff Shale) has undergone a period of structural inversion during the late Tertiary resulting in seal breach. Anticlinal closures of Eocene age were particularly affected, while structures located on fault-bounded basement highs were less affected, and provide the only fields within the basin. In the Yolla and White Ibis fields, access to mature source rocks was provided by large-displacement, non-sealing faults, that linked the upper EVG reservoirs with deeper source rocks. Traps without this conduit have as yet been unsuccessful. Sandy units within the Eastern View Group in the Pelican Trough are tight reservoirs that have good porosity but poor permeability. This is due to diagenetic effects that prohibited the creation of secondary porosity and permeability. Although identified risks within the basin can be minimised, the key to successful exploration will be finding traps that were in-place prior to the generation of hydrocarbons, but did not undergo significant Tertiary inversion.

In 1992, ferromanganese nodules and crusts on the sea bed of the Christmas Island region south of Java (~10?S) were investigated. The region consists of flat plains covered in red clay, abyssal hills, volcanic ridges and isolated sea mounts. Ferromanganese nodules were sought at 28 free-fall grab stations in water depths of 4700-5900 m, and were recovered from 23 deployments at nine stations. Nodules were most commonly recovered near the base of seamounts. Nodules are small and not very abundant; some are rough and others smooth. They are of low grade, averaging 9.6 % Fe, 19.7% Mn, 0.51% Ni, 0.49% Cu and 0.12% Co. The majority contain vernadite as the dominant Mn-bearing phase, with birnessite and jacobsite very commonly present, and todorokite commonly present. The nodules are dominantly hydrogenetic. Ferromanganese crusts were recovered from eight dredge hauls on the flanks of five of nine dredged seamounts, in water depths of 1450-3700 m. The crusts were laid down in several phases and are up to 10 cm thick, with estimated average deposition rates of 1-1.5 mm/m.y.. Significant compositional variations occur in one thick crust examined in detail. The crusts are generally of low grade, averaging 13.9% Fe, 16.2% Mn, 0.35% Ni, 0.11% Cu and 0.44% Co. The majority contain vernadite as the dominant Mn-bearing phase, with birnessite and jacobsite common, and todorokite sometimes present. The crusts are also dominantly hydrogenetic. The highest Co grades are ~0.8% in the shallowest water depths, suggesting that any future exploration should be conducted in depths of about 500-1500 m, near the oxygen minimum zone.

Existing age constraints for geological events in the Tanami Block come predominantly from U-Pb geochronology of i) detrital zircons in sediments, and ii) magmatic zircons in granitoids. These constraints have been used together with observed and inferred geological relationships to help constrain timing of stratigraphy, magmatism, deformation, metamorphism and Aumineralisation (e.g. Vandenburg et al., 2001). Ongoing GA/NTGS zircon geochronology is continuing to refine our understanding of the stratigraphy and magmatic history of the Tanami, with attendant implications for tectonic evolution. In this regard it is noteworthy that detrital zircon ages of ~1815 Ma from the Killi Killi formation require either (or both) a revision of existing stratigraphy, or that the so-called Tanami Orogenic Event significantly post-dates ~1815 Ma, in contrast to previous estimates of ~1845 - 1830 Ma. However, detrital and magmatic zircons can provide no direct constraints on timing of deformation, metamorphism and Au-mineralisation, and consequently our current understanding of these processes in the Tanami region is relatively poor, despite being critical to predictive exploration models.

Overall, the cruise met its objectives of studying rift and drift sedimentation, and obtaining cores for palaeo-oceanography. The east Tasmanian seismic program was completely successful. The planned sampling program was somewhat curtailed by bad weather, equipment failures and other factors. It was least successful off east Tasmania. A total of about 1300 km of 8-fold multichannel seismic data were acquired along 8 transects across the east Tasmanian margin. The quality of the seismic profiles was excellent, with good resolution and penetration, given the bad weather and the limitations of the acquisition system. The seismic source comprised 2 GI airguns (each 45/105 cu. in. capacity) giving a penetration of 2-2.5 s twt (2.5-3 km) in places. The seismic profiles indicate a structurally complex margin with rugged basement relief that includes large-scale horst/graben structures and volcanic intrusions. The sedimentary section on the continental slope is at least 1.5 s twt thick in some graben and includes Campanian-Paleocene early sag-phase deposits, which are 0.5-1.0 s twt thick. Regional compressive tectonism in the Late Paleocene-Early Eocene has produced widespread inversion (folding/faulting) in this succession. A wedge of Neogene shallow-water carbonates underlies the continental shelf. It shows seaward progradation and attains a maximum thickness of ~700 m beneath the shelf edge. Oceanic basement (?Campanian) adjacent to the margin lies at a depth of 7.0-7.5 s twt. The continental rise and Tasman Abyssal Plain in this zone are underlain by 1.5-2.0 s twt of post-breakup sedimentary section. The East Tasman Saddle is underlain by `transitional? basement and contains a sedimentary section of similar thickness. During the sampling program 58 of 86 stations were successful: 38 gravity cores (21 successful), 4 piston cores (3), 16 dredges (7) and 28 grabs (28). Total core recovery was 81.4 metres from the 16 successful cores taken in soft sediments, an average recovery of about 5 metres. The fairly low success rate with the gravity corer can be ascribed to problems with foram sand east of Tasmania, and shelly sand in Storm Bay. The low success rate with the dredge was related to the lightness of the gear. The deployment of the heavy piston corer for the first time on Franklin was successful. However, we did not attempt to piston core in deep water. East of Tasmania we recovered 8 gravity cores, and 7 dredge hauls. Deepwater dredging and coring were surprisingly unsuccessful. The upper slope stations, designed to sample older rocks, were reasonably successful. From these results and some existing information, general conclusions can be drawn about changes along the margin with increasing water depth. The shelf and upper slope wedge of Neogene, seaward-prograding sediments was sampled out to 1640 m. The sediments recovered include muddy sand, clayey sandstone with siliceous nodules, siliceous sandstone and calcarenite. The calcarenite is presumably part of the Middle Miocene shelf limestone sequence that is widespread off St Helens. Somewhat deeper on the upper slope, basement outcrops occur in steep slopes: granite, arkose, metasediments, conglomerate, quartz sandstone and gritty mudstone. The granites are probably from Devonian batholiths like those onshore up the east coast. Volcanic rocks and conglomerate form a basement block in deeper water on a ridge off northeast Tasmania at ~3750m. Deepwater outcrop ridges support manganese nodules and crusts. Nannofossil oozes cling to the slope, particularly in local basins, and are ubiquitous in deep water. The East Australian Current apparently winnows many of the oozes to form a blanket of foram sand.

Current geological mapping by the Northern Territory Geological Survey is leading to a much better understanding of the surface geology of the Territory. Less well understood is the geometry of the Northern Territory in the third dimension, although this has been predicted by the construction of cross sections (e.g. on recent 1:250 000 geological maps). At shallow depths, the cross sections can be constrained by drilling results, if available, but deeper levels can only be examined by geophysical techniques such as seismic reflection or magnetotelluric profiling, or by modelling of potential field data. Text of paper presented at the NTGS AGES 2002 Workshop, Alice Springs, 26-27 March 2002.

Proceedings of papers presented at an industry workshop held in Perth, 20 June 2002. Edited by K.F. Cassidy

Magnetic, gamma-ray and gravity data sets provide vital information for mineral and petroleum explorers as well as researchers studying the geology of the Australian continent. Commonwealth and State and Territory governments have devoted considerable resources to acquiring these data sets and making them available to encourage exploration. Geoscience Australia's geophysical databases contain data acquired by governments, and this report summarises coverages over Australia of these data. On the occasion of the centenary issue of Preview, it is worth reflecting on the advances in the coverage of publicly available magnetic, gamma-ray and gravity data over Australia since the first edition of Preview in February 1986. Since then the areas and resolution of coverages have increased dramatically. Quality of the data through better acquisition and processing techniques has also improved, and new types of data sets added to the explorers' supplies.

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