Magnetotelluric techniques have been used to investigate structural trends in the McArthur Basin . Observations were made at 34 sites, extending 450 km across the Wearyan Shelf, the Batten Fault Zone, and the Bauhinia Shelf. For sites on the Wearyan Shelf, the orthogonal components of resistivity are generally similar, suggesting continuous horizontal strata and uniform basement depths. However, lateral changes in resistivity, evident on the Bauhinia Shelf, become extreme in the Batten Trough. For sites near the Emu Fault, the two components diverge at long periods, indicating a major change in structure with a pronounced vertical contact. Resistivities associated with the Tawallah Group appear distinct enough to show that no appreciable thickness of McArthur Group can be present east of the Emu Fault. The data are consistent with geological models based on the assumption that the Batten Trough formed as a syndepositional graben with rapid changes in depositional thickness at the boundary faults.

A U-Pb zircon age for acid volcanics in the Bernborough Formation of the Warramunga Group indicates a depositional age of between 1819 m.y. and 1849 m.y. If a previously postulated correlation is correct, Division II rocks of the Arunta Block are also this age. The new data show unequivocally that amphibolite-facies rocks west-southwest of Tennant Creek are basement to the Warramunga Group and not merely its higher-grade equivalents.

The middle Miocene macropodoids of the Riversleigh, Bullock Creek, and Kangaroo Well local faunas are the oldest recorded assemblages of kangaroos. Five new species are described here. One, Bulungamaya delicata n. gen. and sp., is placed with Wabularoo naughtoni Archer, 1979 in the new potoroid subfamily Bulungamayinae. Gumardee pascua Ii n. gen. and sp. is placed within the Potoroinae. Balbaroo camfieldensis n. gen. and sp. and Balbaroo gregoriensis n. sp. are placed in a new macropodid subfamily, the Balbarinae. Galanarla tessel/ala n. gen. and sp. is placed in the Macropodidae , but cannot as yet be assigned to a particular subfamily. All the new species are from the Riversleigh local fauna, except B. camfieldensis from the Bullock Creek local fauna. The previously known Kangaroo Well macropodid appears to be referable to the genus Balbaroo. The Bulungamayinae is represented by forms which, although related to potoroines, have bulbous premolars and lophodont molars. The Balbarinae includes generally plesiomorphic forms that may have been ancestral to other macropodids. None of the macropodoid genera described here are known from more recent deposits. This suggests that a radical transition in the dominant kinds of macropodoids took place between middle and late Miocene times.

Deep seismic refraction and vertical reflection recordings have been made in the southern McArthur Basin, over the Bauhinia Shelf and Batten Trough between Daly Waters and the H .Y.C. mineral deposit, and over the Wearyan Shelf between Borroloola and Westmoreland. In the Batten Trough, McArthur Group rocks have a velocity of 5.81 km/s; no velocity contrast was detected between them and basement. Over the Bauhinia Shelf, 100--200 m of Cainozoic, Mesozoic, and perhaps Cambrian sediments, overlie ?Roper Group sediments (P-wave velocity, V = 4.6 km/s). Below these are probable Tawallah Group rocks (V = 5.8-5.9 km/s). Magnetotelluric measurements define a resistivity contrast, possibly basement, at 6-9 km depth. On the Wearyan Shelf at Borroloola, 370 m of Roper Group (V = 3.58 km/s) overlies 2.9 km of Tawallah Group (V = 5.55 km/s). At Robinson River, 650 m of Cainozoic, thin McArthur Group, and, perhaps, upper units of Tawallah Group (V = 4.81 km/s) were detected. Tawallah Group rocks (V = 5.44 km/s) crop out northwest of Robinson River and are about 2.8 km thick. Basement velocity is 6.04 km/s. Between Robinson River and Westmoreland, basement is 3.5-2.7 km deep. At Westmoreland the McArthur Basin sequence thins against the Murphy Ridge . A layer 260 m thick (V = 3.50 km/s) lies on top of a 2.4 km thick layer (V = 5.44 km/s), and the basement velocity of 5.99 km/s increases to 6.06 km/s towards Robinson River. At mid-crustal depths velocities are 5.9--6.9 km/s , and in the lower crust, to depths of 43 km in the west and 40 km in the east, 6.8-7.5 km/s. Below this, a velocity gradient is interpreted until upper mantle velocities are reached at 43-53 km depth in the west (V = 7.5-8.4 km/s) and 44 km in the east (V = 7.9 km/s). Generally, the crustal structure of the North Australian Craton is characterised by high lower-crustal velocities, broad velocity gradients, and thick crust, which probably evolved from an Archaean continental crust during Proterozoic tectonism.

Boron-bearing kornerupine occurs in phlogopite-albite gneisses of the Arunta Block, southwest of Mount Baldwin and in the western Harts Range. At the second locality, idioblastic sapphirine occurs with the kornerupine, and excellent crystals of both minerals have been collected. Most of the larger kornerupine crystals from the Mount Baldwin locality are , in fact , pseudomorphs of fine-grained phyllosilicates after kornerupine .

Rb-Sr data reveal a long and complex Proterozoic history for the Arunta Inlier of central Australia. The first event was a widespread, dominantly granulite-facies episode, known as the Strangways Event, at 1790 ± 35 Ma. Five relatively precise ages from metamorphic and granitic rocks give an estimate between 1650 and 1700 Ma for widespread deformation and metamorphism during the Aileron Event. Two granites in the northern Arunta Inlier were probably emplaced at about 1500 m.y. Deformation and low-grade metamorphism during the Anmatjira Event are dated by generally imprecise isochrons at about 1400 Ma. Four isochrons, documenting both primary events and resetting, are in accord with an age of 900-1050 Ma for the Ormiston Event. The Rb-Sr data do not define the time of initial crust formation in the Arunta Inlier.

Sapphirine is a common mineral in silica-undersaturated rocks within the Mid-Proterozoic granulites of the Arunta Block in the northern Strangways Range, central Australia, and reactions involving sapphirine illustrate three episodes in the metamorphic evolution of the region. Prograde sapphirine-orthoclase-ortho-pyroxene assemblages occur as relics from the granulite stage (estimated P-T conditions 8 ± I kb, 850-920°C). In an early pervasive regional hydration, orthopyroxene-orthoclase-spinel assemblages reacted to form sapphirine-phlogopite or cordierite-phlogopite assemblages. As the rocks cooled from the abnormally high geothermal gradient to a normal geothermal gradient, fine-grained intergrowths of orthopyroxene and sillimanite formed by reaction between sapphirine and cordierite.

The Poodyea Formation, comprising boulder conglomerate, cross-stratified sandy conglomerate and pebbly sandstone, crops out as narrow linear and sinuous belts of small outliers within and to the south of the Toko Range, in the Georgina Basin, western Queensland and Northern Territory. The outliers are confined to present-day narrow valleys in the Carlo Sandstone, corresponding closely to either the Toko Syncline axis or cross-cutting structures, and form low linear ridges over plains of Mithaka Formation. The Poodyea Formation is considered to be of probable Tertiary age, and represents a river-channel deposit derived from reworking of parts of the Mesozoic, Palaeozoic, and Proterozoic sequences. In the past, outcrops of the formation have been included in Palaeozoic units. The formation is discussed and defined.