A collection of palaeontological papers 1972 (bulletin 150)

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.

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.

Three hundred and sixty-five surface and near-surface seabed samples provide the basis for an assessment of regional lithofacies variations on the Tasmanian shelf and in eastern and western Bass Strait. Quartz-rich sands with variable amounts of shell debris occur on the innermost shelf and on the rises flanking the central Bass Strait basin. They are essentially modern deposits derived in the main from Pleistocene near shore sand bodies reworked and transported landwards during the Holocene marine transgression. Muddy sediments of the middle shelf off eastern Tasmania and in central Bass Strait are sites of present-day sedimentation, but they are likely to form only a thin veneer, and include coarse material probably reworked from the Pleistocene and early Holocene substrate. Extensive areas of the middle and outer shelf, particularly off southern and western Tasmania, are floored by dominantly relict bryozoan sands and gravels. Fine-grained and shelly, slightly quartzose sands in areas of the middle shelf consist of relict sediment, and sediment from the late Holocene transgressive marine sand sheet, in about equal proportions. Four main suites of heavy minerals are present in the surface sediments. Provenance relationships with sources in the adjacent hinterland suggest that little offshore sediment transport parallel to the coastline has taken place. Rare grains of cassiterite were identified in marine sediments lying off the tin-producing areas of northeastern Tasmania, but 10 ppm Sn was the maximum value recorded in the geochemical analyses. Some phosphatisation of relict limestone gravels on the middle and outer shelf off northwestern Tasmania has taken place, but the highest recorded whole-rock analysis was 3.6 percent Pi>0. Density of sample stations in this part of the shelf is low.

In January 1970, after a two-year period of inactivity, Ulawun volcano, on the north coast of New Britain, erupted. Ulawun is a stratovolcano built of basaltic and low-silica andesite lava flows and pyroclastic rocks, most of which were erupted from a central vent; many of the lavas were partly fragmented during flow, and some unsorted elastics mav have been deposited by nuees ardentes.

This Bulletin describes magnetic surveys of the Savage River and Long Plains iron deposits in northwest Tasmania made by the Bureau of Mineral Resources, Geology and Geophysics between 1957 and 1962. The results of the surveys are shown as vertical magnetic field contours. The Bulletin includes a discussion of the theoretical magnetic anomaly due to an infinitely long dipping vein of infinite depth extent, and gives an interpretation for the magnetic anomalies observed on a selection of traverses. The drilling completed at Savage River and Long Plains as at May 1964 is summarised and compared with magnetic profiles. Recommendations for additional drilling are made, particularly in areas where drilling has not been done.

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).

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.

Conodonts of Ludlovian-Gedinnian age from the Yass Basin of New South Wales are described. The fauna consists of 61 species referable to 24 genera of which one, Coryssognathus, is new. Four conodont assemblage zones are recognized and correlated with classic Ludlovian-Gedinnian zones of Europe; two faunas, one of probable latialatus Zone age and the other of probable woschmidti Zone age, have also been recovered. Reworked Ludlovian conodonts from Devonian conglomerates at the top of the Yass succession are also described.