An analysis of the gravity field in two largely Precambrian metamorphic rock areas of Australia, one in the southwestern and the other in the central part, indicates that the regional Bouguer anomalies may be explained by models of the crust and upper mantle consistent with the other geophysical and geological observations. In the southwestern part, the longer wavelength component of the anomalies is consistent with a crust which has been determined by seismic measurements to be greater than normal in thickness and density in the west owing to the presence of a high-velocity basal layer that thins out eastwards. The shorter wave-length components show excellent correlation with the near-surface geology. In central Australia, where no comparable seismic measurements have been made, the Bouguer anomaly field, which is dominated by large amplitude (up to 150 mGal) and long wavelength (about 150 to 200 km) components, is interpreted in terms of thickness and structure of a two-layer crust. The derived crustal model is based on the concept of folding and faulting that involves the entire crust and upper mantIe, and is compatible with the broad aspects of surface geology and structure. The crustal upwarps in the model, with the intermediate and Mohorovicic discontinuities at depths as shallow as 15 and 27 km, are associated with the Arunta and Musgrave Blocks, where deep crustal rocks are exposed against large thrust faults. The crustal downwarps, with the discontinuities as deep as 31 and 43 km, are associated with basins containing substantial thicknesses of sediments. Depths to these discontinuities are in agreement with those estimated from the only two isolated deep seismic reflection probes in the basin areas.

All the uranium deposits of the Alligator Rivers Uranium Field of Australia, which between them contain a quarter of the worlds reasonably assured uranium resources, are strata-bound within the Cahill Formation. Three of the four economic deposits are more or less conformable within a lower member near the base of the formation, which contains carbonate and carbonaceous rocks. The formation forms a poorly exposed but continuous folded belt, 5 km or more wide, over an area of about 15 000 square kilometres. Exposure is sparse, the rocks are generally deeply weathered, and knowledge of the stratigraphy of the unit is based largely on the results of shallow stratigraphic drilling. Quartzo-feldspathic and micaceous metasediments are the dominant rock types in the Cahill Formation as a whole, and they have been metamorphosed to the amphibolite grade (staurolite-almandine subfacies). The unit appears to unconformably overlie the Archaean-Lower Proterozoic granite-gneiss-migmatite Nanambu Complex and the Lower Proterozoic psammo-pelitic Mount Partridge Formation, and is overlain, in places unconformably, by the Fisher Creek Siltstone. It grades into the migmatite-gneiss-granite terrain of the Nimbuwah Complex in the north-east. The formation is a facies equivalent of the Koolpin Formation (host to uranium mineralization in the South Alligator Valley uranium field) to the south, and, in places, was separated from it during deposition by a basement high of Mount Partridge Formation. The carbonate-carbonaceous sequence was probably deposited under shallow near-shore shelf conditions, along with considerable admixed terrigenous material. The upper part of the formation represents a period of transgression over the shelf. Uranium was concentrated under reducing conditions which developed locally in many places within the carbonate shelf. Subsequent concentration and reconstitution took place a number of times, the main period being during 1800 m.y. regional metamorphism and deformation. Mobilization of uranium away from the carbonate-carbonaceous sequence took place only under high temperature and pressure conditions such as those attending formation of the Nimbuwah Complex, where the metal was relocated in favourable low pressure structures.

Skylab spacecraft stereoscopic photography of the Alice Springs and Snowy Mountains regions of Australia was studied by conventional photogeological techniques to assess its usefulness in geological mapping. In the arid Alice Springs region, which has well exposed sedimentary rocks and relatively simple structures, broad rock units can be differentiated and correlated, and rock trends, joints and folds interpreted with the same accuracy as that shown on the 1:500 000 scale geological map of the region. The distribution of Cainozoic travertine and other surficial materials can be interpreted with sufficient reliability to allow updating of 1:250 000 scale geological maps. In the more humid Snowy Mountains region, where the geology-to-morphology relationships are complex and varied, little lithological information can be obtained: only Tertiary volcanic rocks and alluvium can be identified and outlined with confidence. The Skylab photographs proved more useful for structural interpretations: faults, lineaments and joint trends can be detected. Several circular structures can be related to features of igneous origin. Statistical analysis of linear features revealed a direct relationship between known structural trends and linear features annotated on low resolution Skylab photographs.

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.

Small nodules and tabular bodies of practically pure micro-crystalline magnesite up to a few centimetres across occur in late Cainozoic calcrete around Gosses Bluff. The calcrete underlies an area of about 15 km^2 and the magnesite constitutes somewhat less than 1 per cent of the outcrop. The bedrock in the area of calcrete is impact breccia of the Gosses Bluff structure. The superficial weathered products of these breccias constituted a suitable host rock for the calcrete because of their inherent permeability and their position in topographically low areas. The source of magnesium carbonate was most likely in the Late Proterozoic and Palaeozoic dolomites and magnesian limestones exposed to the north in the MacDonnell Ranges. Petrographic evidence from the random samples of calcrete collected at Gosses Bluff suggests that the history of the deposit commenced with invasion of magnesium carbonate rich waters and formation of small bodies, probably of a metastable hydrate of magnesite which later reverted to the stable phase. A period of sub-aerial dessication ensued and caused disintegration of the nodules and production of cracks and voids. Phreatic water enriched in calcium carbonate invaded the deposit; voids were lined with fibrous calcite, smaller cracks were completely filled, and at the same time the magnesite was replaced, in part, by calcite. The larger voids were subsequently filled by calcrete which consisted of a mixture of disintegrated breccia fragments, chiefly quartz, feldspar and quartzite grains, with the addition of a minor proportion of magnesite grains incorporated from the nodules; and an abundant matrix of calcite. There is scant reference to magnesite in calcretes elsewhere in Australia, possibly because magnesite is not readily distinguished in some fine-grained calcretes.

Rb-Sr isotopic data from total rock and mineral samples provide a broader geochronological framework than has hitherto been available for the Proterozoic rocks of the Tennant Creek Block. The oldest documented event is the amphibolite-facies metamorphism at 1920 ± 60 m.y. of possible basement rocks from the BMR 3 area, to the west-southwest of Tennant Creek township. The lower grade metamorphic rocks of the Warramunga Group were deposited before the major deformation episode which is inferred to have occurred at about 1810 m.y. Certain of the units within the Warramunga Group, of which the Bernborough Volcanics are an example; underwent isotopic resetting of total-rock systems during subsequent deformation. Granite ages range from 1797 m.y. for a phase of the Tennant Creek Granite to less than 1500 m.y. for the Cabbage Gum and Gosse River East Granites. Dated lamprophyre and porphyry samples yield ages of about 1660 m.y. and 1760 m.y., respectively. Muscovite separates from the Juno, Warrego, Golden Forty, and Nobles Nob mines suggest a common origin for the ore deposits about 1810 m.y. ago.

Isotopic ages of Proterozoic acid volcanics and associated granitic intrusions in the northeast and southwest part of The Granites-Tanami region are reported and discussed. In the northwest, within The Granites-Tanami Block, the Mount Winnecke Formation, which includes acid volcanics that were probably erupted underwater, is intruded and weakly metamorphosed by the high-level Winnecke Granophyre. The acid volcanics and intrusives have similar major element compositions, and this, together with the field evidence, indicates that they are probably comagmatic. Rb-Sr total rock and mineral data for the Winnecke Granophyre yield an isochron indicating an age of 1802 ± 15 m.y. and an initial Sr87/Sr86 of 0.7074 ± 0.0036. Isotopic data for the lavas of the Mount Winnecke Formation give a preferred age of 1808 ± 15 m.y. and an initial Sr87 /Sr86 of 0.7052 ± 0.0038. In the southwest, at the western extremity of the Arunta Block, another acid volcanic suite, belonging to the Pollock Hills Formation, is intruded and weakly metamorphosed by the Mount Webb Granite. As in the northeast, the volcanics and intrusives have similar major element compositions, and are considered to be comagmatic. Combining Rb-Sr isotopic data from both the volcanics and intrusives yields an isochron indicating an age of 1526 ± 25 m.y. and an initial Sr87/Sr86 of 0.711 ± 0.004. These rocks are appreciably younger than those in the northeast. The acid igneous rocks in the southwest also postdate three other newly dated granitic units in The Granites-Tanami Block - the Lewis Granite, 1720 ± 8 m.y.; the Slatey Creek Granite, 1770 ± 55 m.y.; and The Granites Granite, 1780 ± 24 m.y. - and are younger than the Gardiner Sandstone, the basal unit of the Birrindudu Group, which unconformably overlies The Granites-Tanami Block. K-Ar and Rb-Sr ages determined on glauconite from the Gardiner Sandstone show little consistency, due evidently to partial loss of radiogenic daughter products, but a preferred K-Ar age of 1560 ± 20 m.y. is considered a reasonable minimum estimate for the age of sedimentation and diagenesis in the unit.

A recently discovered Templetonian (Middle Cambrian) trilobite fauna, with affinity to that of the Beetle Creek Formation of western Queensland, is reported from pebbles derived from the Elcho Island Formation (Wessel Group) on Elcho Island, in the Arafura Basin, northern Australia. Consequently, a previously determined isotopic age of 790 m.y., on glauconite from the Elcho Island Formation, is now clearly much greater than the age of deposition of the formation, and the age of the occurrence of Skolithos at the base of the Wessel Group (Buckingham Bay Sandstone) can be reconsidered as Early or early Middle Cambrian, rather than late Proterozoic. Regional correlation of the Buckingham Bay Sandstone and Raiwalla Shale of the Arafura Basin with the Bukalara Sandstone and Cox Formation of the McArthur River region is reiterated on the basis of rock types and presence of the trace fossil Skolithos.

Nettletons concept of density profiling can be utilised to give useful estimates of the bulk density of topographic features. These estimates can be used to infer the composition of such topography, or to assist in the interpretation of local gravity anomalies. Two methods that facilitate multiple density profiling over elongate topography are presented. One is a simulation reduction method utilising the two-dimensional line integral formula of Talwani, Worzel and Landisman (1959). It enables data from any detailed gravity traverse crossing an elongate topographic feature at right angles to be automatically reduced by computer to a set of multiple density Bouguer profiles. From these profiles, the bulk density of the topographic feature can be estimated by visual correlation. The other is a graphical method of converting a set of multiple density Bouguer profiles directly to point density estimates, without the need for visual correlation. Both methods are theoretically exact for the ideal case. A visual correlation determination of 2.85 ± 0.05 g cm^-3 is demonstrated for a traverse crossing the 300 m high Harts Range, Northern Territory, and three point determinations of 2.97,2.97, and 2.99 g c^-3, for a traverse crossing the 100 m high Fraser Range, Western Australia.

Carbonaceous clays in a stratigraphic borehole near Napperby homestead, in the southern part of the Northern Territory, have yielded a microfloral assemblage comprising over thirty form-species of pollen. The assemblage can be referred to the middle Eocene Proteacidites confragosus. Zonule on the basis of the presence of the nominate species, and is the most inland of any Australian Tertiary flora described. The Napperby sediments may be correlated, on a palynological basis, with the upper part of the Eyre Formation of northeastern South Australia; they are older than the vertebrate-bearing Waite Formation of the adjoining Alcoota Sheet area. A high frequency of pollen from aquatic and marsh-loving angiosperms and the presence of dinoflagellate cysts indicates deposition under lacustrine conditions. The presence of Nothofagus and podocarpaceous pollen types in significant amounts suggests that a humid climate prevailed, although seasonal aridity cannot be ruled out.