A new rock unit, the Bullara Limestone, is proposed for a Late Oligocene bioclastic limestone, which is probably restricted to the subsurface of Rough Range. The Bullara Limestone is a lateral equivalent of the lower part of the Mandu Calcarenite, and contains a Tertiary lower e stage larger foraminiferal fauna and a Zone N.3 planktic fauna.

Seismic reflection profiles over the southwestern margin of the Exmouth Plateau, which have been tentatively tied to the Northwest Shelf stratigraphic sequence, indicate that rocks of probable Permian to Early Tertiary age form bedrock on the lower continental slope and probably crop out in places. Water depths range from 1400 to 4000 m. It is surmised that the rocks present include pre-Late Jurassic fluvial, deltaic and shallow marine sandstone, siltstone and shale; Late Jurassic to Neocomian deltaic sandstone, siltstone and shale; mid Cretaceous shallow marine shale; Late Cretaceous shelf limestone and Cainozoic foraminiferal ooze. The Exmouth Plateau is cut by normal faults which are predominantly downthrown oceanwards and trend north-northeasterly across most of the area, except near the southwestern margin where they trend northwesterly. Faulting occurred primarily in the Middle to Late Jurassic and led to formation of the north-western and northern margins by seafloor spreading. A major anticline, which parallels the southwestern margin, has one limb truncated by the lower continental slope, probably resulting in older rocks cropping out in places. Deep Sea Drilling Project results on the Cuvier Abyssal Plain indicate that the southwestern margin formed in the Late Cretaceous, either as a transcurrent fault associated with seafloor spreading or by collapse along normal faults. Sampling of outcrops along the southwestern margin would enable the sequence to be dated more reliably and this would permit the theories of geological evolution and favourable petroleum potential of the Exmouth Plateau to be tested.

The Arunta Block is the mass of Precambrian basement rocks in the southern part of the Northern Territory of Australia. It comprises an early Proterozoic (or older) discontinuous sequence of sedimentary and volcanic rocks that were multiply deformed and initially metamorphosed 1800-1700 m.y. ago, and numerous granite masses that intruded the metamorphic rocks from 1700 to 1000 m.y. The metavolcanic rocks are concentrated in the lower part of the sequence, and the sedimentary rocks become more mature and better differentiated towards the top of the sequence. One carbonatite and a mantle-derived intrusion of kimberlitic affinity are located near a major crustal lineament in the east of the Block. Mineral occurrences as presently known are small and in general uneconomic; only one small mine was operating in 1976. The occurrences can be grouped into the following types: I - Stratabound: copper-lead-zinc in metasediments in the lower and middle parts of the sequence; 2 - Pegmatitic: mainly copper, tin, tungsten, and tantalum derived from granite, and mica in pegmatites formed by partial melting of meta-sediments; 3 - Metasomatic: tungsten, molybdenum, and minor copper in calc-silicate rocks adjacent to granite; 4 - Hydrothermal: gold in a zone of late Palaeozoic deformation and retrogressive metamorphism, and fluorite-barite veins in zones of late Palaeozoic warping; 5 - Magmatic: very minor copper, nickel, and chromium, in mafic and ultramafic rocks; and 6 - Weathering: manganese and uranium in superficial Phanerozoic rocks. The mineral occurrences are areaIIy distributed in two zones which are directly related to the distribution of major rock-types in the Block. The stratabound occurrences in the lower part of the sequence, magmatic occurrences, mica pegmatites, and hydrothermal gold deposits are located in the southern part of the Block; the stratabound occurrences in the middle part of the sequence, metal-bearing pegmatites, metasomatic occurrences, and hydrothermal fluorite-barite veins are located in the north. In terms of future prospects, stratabound base-metal lodes in the lower part of the sequence, metasomatic tungsten, and superficial uranium are the most likely candidates for economic success. Diamonds, rare earth elements, and niobium, as yet undiscovered, are possibilities along or near the major lineament in the east of the Block. The Arunta Block shows marked geological resemblances to The Granites-Tanami, Tennant Creek, and Willyama Blocks in Australia, and to the Precambrian rocks of the Baltic and East African regions. All these regions are economically mineralized to some degree, and this, together with its own mineralization, suggests that the Arunta Block holds some potential for economic deposits. How much is a matter for further exploration and assessment.

In 1970 a large deposit of ferromanganese nodules was discovered on the floor of the Indian Ocean southwest of Cape Leeuwin by the research vessel USNS Eltanin. This discovery, which was based largely on bottom photographs from about 20 stations, was discussed by Frakes (1975) and Kennett and Watkins (1975, 1976). The photographs suggest that the deposit spreads, nearly continuously, over 900 000km^2, and cores showed that the nodules are essentially confined to the sediment surface. Kennett and Watkins (op. cit.) pointed to the abundance of ripple and scour marks and current-formed lineations on the present surface, and of extensive disconformities in the cores, as evidence of strong present and past bottom currents in the region. They suggested that the current action had resulted in very low sedimentation rates, which had allowed the nodule field, named by them (1976) the 'Southeast Indian Ocean Manganese Pavement', to develop. In early 1976 the authors used the research vessel HMAS Diamantina for a 10-day cruise in the region to sample the nodules in order to study their chemistry and mineralogy. During the cruise 9 stations were occupied, 8 of them successfully (Figure 1), and about 2000 nodules were recovered from the sea bed. The apparatus used was a light box dredge on the ships hydrowire, which had a breaking strain of about one tonne. Although an attempt was made to reoccupy Eltanin photographic stations, it should be noted that positioning was by celestial navigation, so errors of up to 10 km are possible.

A preliminary report on the manganese nodule field southwest of Western Australia published in this Journal recently (Frakes, Exon and Granath, 1977) quoted chemical analyses which were carried out on air-dried material. Significantly higher metal values have been recorded in some later analyses done on nodules dried at 105°C. Tests have shown that the ground, air-dried material retains considerable moisture, which accounts for the higher metal values of the later analyses. The average water content (after drying at 105°C) has been determined at 16 percent. The relevant chemical data now available on this material are summarised in the accompanying table: in this table metal values (by atomic absorption spectrophotometry) have been recalculated assuming a moisture content of 16 percent.

In this paper we present the first detailed geological information about the margins of the Exmouth and Wallaby Plateaus. New single-channel seismic profiles helped to select sampling targets, which were related to the established seismic stratigraphy. We obtained pre-Quaternary rocks, mainly by dredging, from 29 stations on the Exmouth Plateau, and from 12 stations on the Wallaby Plateau and the Cuvier Abyssal Plain. The results have led to major changes in our understanding of the northern margin of the Exmouth Plateau, which behaved more as part of the offshore Canning Basin than of the Carnarvon Basin. Middle and Late Triassic paralic, detrital sediments are generally overlain by an average of 2500 m of Early Jurassic shallow-water carbonates and mid-Jurassic coal measures and ferruginous sediments. The coal-measure sequence (lithofacies association A) includes carbonaceous silty claystones with seams of an immature sub-bituminous vitreous coal and coaly mudstone, carbonaceous quartz siltstones and very fine sandstones, medium to coarse-grained quartz arenites, and pyrite concretions. The barren ferruginous sediments (B) consist of brown clayey ironstones, sandy ironstones, ferruginous concretions, and ironstones breccias. The Liassic transgressive shallow-water carbonates (C) are a heterogenous group of micritic limestones, biocalcarenites, very coarse crinoidal biosparites, calcareous quartz arenites, recrystallised sparry limestones and dolosparites. On the northern margin of the, Wombat Plateau (a sub-plateau of the Exmouth), at least 300 m of Late Triassic to earliest Jurassic flows of early-rift alkali rhyolite and undersaturated trachyte s.l. (213-192 m.y.) underlie the early Jurassic carbonate sequence. Above the main unconformity, which corresponds to the Callovian breakup of this margin, Early Cretaceous shallow-marine claystones (facies D) indicate the formation of a juvenile ocean. The mature ocean stage is indicated by a condensed sequence of pelagic foraminiferal nanno chalks of Aptian, Albian and Tertiary age (facies E). On the northwestern Exmouth Plateau we sampled Aptian and Miocene chalks. On the southern margin Mesozoic sandstone and shale, latest Jurassic or earliest Cretaceous marine shale, and Cainozoic chalks were recovered. On the eastern and southern Wallaby Plateau, and on the Sonne Ridge which extends northward into the Cuvier Abyssal Plain, the layered sequence beneath the main Neocomian unconformity consists of interbedded weathered tholeiitic and differentiated alkali basalts, tuffs, basalt breccias and thick volcaniclastic sandstones and conglomerates. A minimum mid-Cretaceous age (K/ Ar age: 89 m.y.) was determined for a somewhat altered basalt from the southern Wallaby Plateau. This suggests that intense volcanism and associated deposition of volcaniclastic debris flows formed the plateau, during or after the Neocomian breakup of this region. Therefore, the plateau appears to have no petroleum potential. Quaternary cores in the central Exmouth Plateau contain methane in very small amounts; the u13C isotope results (-14 to -40%) and the absence of higher hydrocarbons tend to downgrade the petroleum potential of this part of the plateau. Manganese nodules from the southern and eastern margins of the Wallaby Plateau have combined Cu + Ni + Co values of about 0.76%, and are of no commercial interest.

Two Archaean cratons are exposed in the Precambrian shield of Western Australia: the Pilbara in the north and the Yilgarn in the south. They are separated by the Capricorn Orogen. Seismic recordings of quarry blasts in the north of the West Australian shield indicate that the upper crustal rocks have seismic velocities of 6.0 km s^-1, and are probably of acid to intermediate chemical composition. They overlie a lower crust, presumed to be granulitic, which has a seismic velocity of 6.4 km s^-1 at 13 km depth in the north of the Pilbara Craton and 16 km in the north of the Yilgarn Craton. The northern Yilgarn has a third layer, with a seismic velocity of about 7.0 km s^-1, at the base of the crust. This may represent a higher grade (eclogite?) phase, or it may indicate injection of basic material into the base of the crust. The crust-mantle boundary at the base of the Pilbara Craton and Capricorn Orogen appears to be transitional - it shows velocity gradients rather than a first-order velocity discontinuity. Within the Pilbara Craton, the crust is about 28 km thick in the north and 33 km in the south. South of the Pilbara Craton, the crust thickens under the Capricorn Orogenic Belt, and again under the northern Yilgarn Craton where it is 52 km thick. The form of the structures in the zone of thickening cannot be determined uniquely from the present data. Three models incorporating faults or monoclines and a wedge of 7.0 km s^-1 material at the base of the crust have been derived. The different crustal thicknesses of the cratons suggest that they formed separately and were then tilted towards the Orogenic Belt.

The Granites-Tanami region links the mainly Proterozoic areas of northwestern and central Australia. It is made up of two main tectonic units: The Granites-Tanami Block, which consists of metasediments and metavolcanics of the Lower Proterozoic Tanami Complex, younger Lower Proterozoic sedimentary and acid volcanic rocks, and 1820-1710 m.y. old granites; and the Birrindudu Basin, which contains the mainly clastic sedimentary rocks of the Carpentarian Birrindudu Group, dated at about 1560 m.y., and the Adelaidean Redcliff Pound Group, which was probably deposited less than 1000 m.y. ago, and their possible stratigraphic equivalents. The Proterozoic rocks are overlain by Early Cambrian Antrim Plateau Volcanics and younger Palaeozoic non-marine sediments. Five major phases of tectonic activity, ranging in age from Lower Proterozoic (1960 m.y.) to early Carboniferous (Alice Springs Orogeny), can be recognised. The Proterozoic and Palaeozoic rocks are correlated with similar rocks in the Kimberley region to the northwest, the Victoria River region to the north, the Arunta Block and Amadeus Basin to the south, the Tennant Creek region to the east, and the Canning Basin to the west. The best substantiated correlations are the Tanami Complex with the Halls Creek Group of the Kimberley region, and the Redcliff Pound Group with the Heavitree Quartzite and Bitter Springs Formation of the Amadeus Basin. The latter correlation indicates that when these units were deposited the Birrindudu and Amadeus Basins were interconnected.

Parallel shear, generated by sliding tongue and grooved boards past each other with clay on top produced, initially - in addition to the commonly recognised conjugate strike slip faults - a set of gently plunging folds at 30-45° to the wrench zone. Essentially vertical Riedel shears may subsequently become part of reverse drag structures as rotation of the clay, in response to the shearing couple, introduces a tensional component. A single right-lateral movement in the model has reproduced the orientation of the major faults and folds within the Fitzroy graben of the Canning Basin. Major features such as the offshore depression on the southern margin of the Leveque shelf and part of the Halls Creek mobile zone may have been produced, as simulated in the model, by major dextral wrenching and, in the case of the Leveque shelf, by later subsidence of basement rocks. Absorption of lateral movement by the Halls Creek mobile zone may account for the less deformed, more platform-like northeast Canning Basin.