D53/B1-91 Vertical scale: 10

G51/B1-86 Vertical scale: 2000

I50/B1-52 Vertical scale: 10

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.

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.

The new genus Quasicyclammina is described from the Lagaip Beds of upper Paleocene age in the Wabag area, Papua New Guinea; it is placed in the subfamily Cyclammininae of the family Lituolidae. This new genus has an agglutinated test wall, slightly asymmetrical test, short internal longitudinal partitions, and an asymmetrical interiomarginal aperture. Three species are described, Q. breviseptum sp. nov., Q. compressa sp. nov. and Q. inflata sp. nov., separated on the number of the chambers In the outer whorl and on the maximum diameter/thickness ratio. A new species of the genus Thalmannammina, T. anfracta, is also described.

This report announces the discovery of a diverse vertebrate fauna from exposures of the Namba Formation in the southern Frome Embayment (Tarkarooloo Basin), South Australia. The fluvio-Iacustrine Namba Formation can be divided into two informal members based on regional lithological changes. The lower member bears Balcombian-Batesfordian (medial Miocene) pollen floras representing subtropical rainforest and adjacent savanna habitats. The top of the lower member yields the Pinpa Fauna of aquatic and terrestrial vertebrates including fish, turtles, crocodiles, two genera of dasyurids and seven genera of diprotodontan marsupials and a platanistid porpoise. The base of the upper member contains a similar vertebrate fauna (Ericmas Fauna) but includes a platypus and, significantly, diprotodontid marsupials which are the dominant large mammals in the contemporaneous Ngapakaldi Fauna of the Lake Eyre basin.

In January 1974 aerial observations were made of the very severe flooding which occurred in the low-gradient plains country adjacent to the southern Gulf of Carpentaria. Floodwaters were derived from two sources, the extensive river channel network throughout the plains mostly carried silty water which originated from hinterland regions, while the plains surfaces were flooded with clear water derived from local rainstorms. Runoff of clear storm water caused active erosion around peripheries of planar interfluves on the clay plains. There was generally no flood- water erosion or deposition in the sandy plains. Flooding was extensive on tidal mudflat areas, particularly behind beach ridge remnants which inhibited runoff. Despite the severity of flooding, the overall lack of erosion and overbank flooding suggests that landforms of the fluviatile plains were developed under conditions of greater runoff than prevail during normal wet-season flooding. The major landform features were probably developed In the Late Pleistocene and have not been substantially modified since then.

Hot emissions of mainly sulphur dioxide and carbon dioxide took place from a mound in Koranga open cut, near Wau, following a landslide at the end of May, 1967. Rocks of the Holocene volcano, Koranga, are exposed in the open cut. The emissions lasted about three months, and ceased on 13 August after another landslide removed the active mound. During the period of activity, recorded temperatures ranged up to 680°C; no anomalous seismic or tilt phenomena were recorded. The cause of the activity is not known, but it is thought that the high temperatures and gases may have been the result of the spontaneous combustion of reactive sulphides and carbonaceous material present in the altered rocks of Koranga volcano.