The northern fall of the Central Range, the largest unexplored area in New Guinea (Fig. 1), separates the swampy Sepik Plain in the north from the high dissected plateau forming the backbone of New Guinea to the south. The whole region is rugged and covered by tropical rain forest; it is almost uninhabited, and as there are few tracks, the long meandering southern tributaries of the Sepik River provide the only practicable access. The South Sepik region occupies a small segment of the fundamental break separating the stable Australian continental block from the oceanic crust to the north. This break, which is marked in the South Sepik region by the Lagaip Fault Zone, has had a profound effect on sedimentation in the region throughout the geological record: shelf-type sediments were laid down on the continental block, while geosynclinal sediments were being deposited to the north. The oldest rocks are Middle and Upper Triassic in age and include a widespread and distinctive volcanic unit called the Kana Volcanics. They are succeeded unconformably by a thick sequence of black pyritic shale (Lagaip Beds), which was laid down south of the Lagaip Fault Zone during the Jurassic and Cretaceous.

Following the discovery of large phosphate deposits in the eastern part of the Georgina Basin in 1966 by Broken Hill South Limited, the Bureau of Mineral Resources embarked on a detailed stratigraphical and palaeontological study of the Cambrian sediments of the area in 1967. Particular attention was given to the phosphatic part of the section, but new information on the associated Cambrian units was also gained. In 1967, F. de Keyser, J. H. Shergold, C. G. Gatehouse, R. Thieme, and C. Murray (Geological Survey of Queensland) mapped the Burke River Outlier, and in 1968 de Keyser and Thieme mapped the Cambrian of the northeastern corner of the Barkly Tableland. In 1969 de Keyser and P. J. Cook completed the mapping of the known phosphogenic areas in Queensland when they mapped the eastern margin of the Georgina Basin in the Mount Isa/Urandangi area. Associated palaeontological, petrological, and geochemical studies were also carried out.

The Stairway Sandstone was studied in the field in 1962 and 1963 during the reconnaissance geological mapping of the Amadeus Basin. In 1964 rather more detailed work was carried out: sections were measured and cross-bedding determined throughout the Amadeus Basin, and the phosphorites were examined in some detail because of their possible economic significance. In 1965 and 1966 surface and subsurface material was studied in detail to determine the provenance, environment of deposition, and palaeography of the formation. This work was done partly in the laboratories of the Bureau of Mineral Resources, and partly in the Department of Geology of the Australian National University, Canberra (under the sponsorship of the Bureau of Mineral Resources).

The Georgina Basin is a large Palaeozoic sedimentary basin extending from northwestern Queensland into the Northern Territory. Field parties from the Bureau of Mineral Resources mapped some of the Queensland part of the basin in the early 1950s, and the remainder in the period 1957-65; officers of the Geological Survey of Queensland participated in most of the Queensland surveys. Since 1957, petroleum exploration companies have made geological, gravity, aeromagnetic, and seismic surveys in selected areas, and the Bureau of Mineral Resources has carried out aeromagnetic and reconnaissance gravity surveys over most of the basin, and seismic surveys in Queensland and in small areas in the Northern Territory. Sixteen petroleum exploration wells have been drilled, but no commercial deposits of oil or gas were found; three deep stratigraphic holes were drilled by the Bureau, which also drilled an aggregate of about 3000 m in shallow scout holes. The Georgina Basin covers an area of about 325,000 sq km and extends in a belt trending northwest from latitude 25°S in northwestern Queensland to about latitude 18°S in the Northern Territory. The basin is about 1000 km long and about 500 km wide. The southwestern, western, northern, and eastern margins are defined by Precambrian outcrops, but the northwestern and southwestern margins are concealed by Mesozoic sediments. The position of the northwestern margin has not been resolved, but the southeastern margin has been defined approximately by gravity, seismic, and aeromagnetic surveys and by limited stratigraphic drilling.

This report presents the results of geochemical investigations in the Mount Isa district, northwest Queensland. Samples, mainly from cores, represent Group 2 Shales (comprising Kennedy Siltstone and Spear Siltstone), Urquhart Shale, Native Bee Siltstone, greenstones, and local basic igneous rocks. These have been used to study element distributions in mineralized and unmineralized localities. It was found that primary element dispersions are associated with the 1100 Cu orebody, but not with the Ag-Pb-Zn bodies. This, together with the different modes of occurrence of the orebodies, suggests that the mineralization at Mount Isa took place in two separate events. The Ag-Pb-Zn deposits are considered to be syngenetic whereas the Cu deposits appear to be, in part at least, epigenetic. The chemical evidence suggests strongly that much of the Cu in the silica dolomite bodies was derived from the underlying greenstones. In addition, an attempt has been made to differentiate the Urquhart Shale from the other units on the basis of chemical composition. Of the elements analysed, Ca appears to be the most diagnostic and it may be possible to define the upper limit of the Urquhart Shale using this element.

Collection of Palaeontological Papers, 1983.

The Australian Calibration Line (ACL) , with a total gravity interval of 3 Gal, was established during 1970 between Laiagam in Papua New Guinea and Hobart in Tasmania. During 1973 the Australian Bureau of Mineral Resources and the USSR Geodesy and Cartography Survey made joint observations along the full length of the ACL. Measurements made with eight Soviet GAG-2 gravity meters established a gravity scale for Australia to an accuracy of 2.5 parts in 105. This scale and a datum of 979 671.86 mGal for Sydney A were adopted for Australia in 1973. The Soviet scale established for the ACL appears to be within 1 part in 104 of both the IGSN71 scale established for the Western Pacific Calibration Line by absolute determinations, pendulum measurements and international gravity meter comparisons, and the scale established for the Soviet Calibration Line by OVM pendulums. The Soviet scale for the ACL defines a milligal which is 1.5 parts in 104 larger than that defined by IGSN71 values for the ACL, and 5 parts in 104 larger than the 1965 Mean Australian Milligal that was used as an Australian milligal standard between 1965 and 1973. Both of these scales are partly based on Cambridge pendulum measurements made in Australia during 1950-51. These measurements are now thought to have been incorrect in scale. LaCoste, Romberg gravity meters have been used during six surveys along the whole or part of the ACL. The LaCoste observations have been reduced using the Soviet ACL scale and the new datum for Sydney A. The most probable values for airport gravity stations, calculated from the LaCoste results, have a precision of better than 0.01 mGal and are consistent to within experimental error with values calculated from the GAG-2 results. LaCoste observations reduced using the Soviet ACL scale give more accurate values for the gravity differences of the main intra-city ties and calibration ranges along the ACL.

Special demagnetising apparatus was constructed to study the stability of several samples of basic igneous rocks from three localities in eastern Australia, particular emphasis being placed on the reliability of the directions of NRM. The direction of primary magnetisation acquired when the rocks first cooled was determined for samples at all three sites. Mesozoic dolerite from Red Hill Dyke in southern Tasmania has little or no secondary magnetisation and the mean direction of NRM is representative of the Jurassic in Tasmania. There is no evidence of systematic error due to stress or shape, and therefore the direction of NRM is a reliable estimate of the direction of the geomagnetic field at the time of intrusion. Devonian Nethercote basalt from southern New South Wales can be divided into two distinct groups, one in which the NRM is completely unaffected in either direction or intensity by demagnetisation in peak alternating fields of up to 1000 oersteds, and the other in which secondary magnetisation completely masks any primary magnetisation that may be present. Tertiary basalts from southern New South Wales show a wide range of stability. The NRM consists of primary TRM and varying proportionate amounts of secondary magnetisation, which is almost certainly viscous and which was probably acquired in the present Earth's field. The stability shown by the three rock types makes it more probable that previous palaeomagnetic results, which span a long period from Devonian to Tertiary, form a reliable record of the geomagnetic field in Australia.The general effects of alternating demagnetising fields are also discussed and a comparison made between the theoretical predictions and the data obtained.

This Bulletin presents the results of a marine geological survey carried out by BMR in the Arafura Sea in 1969 as part of a program of regional geological reconnaissance mapping of the Australian continental shelf. It is a continuation of work in the Timor Sea and northwest shelf (van Andel, Veevers, 1967; Jones, 1968, 1970). The area surveyed is the northern Australian continental shelf between longitudes 130° and 136°E and between latitudes 8° and 12°S (Fig. 1), an area of about 240 000 km-. From 2 to 25 May the Japanese research submersible Yomiuri and its mothership, the converted deepsea tug Yamato, were made available. The major part of the survey lasted from 21 September to 6 December 1969, using the chartered oil-rig supply vessel San Pedro Sound as a platform.

A collection of palaeontological papers 1972 (bulletin 150)