The southern Australian margin is unique as it is the only known passive margin that formed over and orthogonal to a Mesozoic subducted slab in the mantle. The tectonic subsidence pattern observed along the southern Australian margin primarily reflects the extensional processes that were associated with the development of the divergent continental margins of Australia and Antarctica, coupled with Cretaceous mantle dynamics and the influence of intra-plate stress on the Australian plate in the Late Tertiary.

This cross agency report, highlights the areas of the central NSW continental slope prone to sediment mass wasting over time. It includes the critical factors which contribute to slope failure including basement geometry, angle of slope and thickness of overlying sediments. Evidence of slope failure are observed through: surficial tension cracks; creep features; faulting; redistribution of sediments, multiple relict slides on the sea floor and erosional surface scars.

This abstract describes the pre-breakup structures and history of the conjugate margins of southwest Australia (Naturaliste Plateau) and East Antarctica (Bruce Rise) and the subsequent formation of oceanic crust.

Legacy product - no abstract available

The interpretation of two regional seismic reflection profiles and the construction of a balanced cross section through the southern Australian margin (Bight Basin) are designed to analyze the influence of the Australia-Antarctica continental breakup process on the kinematic evolution of the Cretaceous Ceduna delta system. The data shows that the structural architecture of this delta system consists of two stacked sub-delta systems. The lower White Pointer delta system (Late Albian-Santonian) is an unstable tectonic wedge, regionally detached seaward above Late Albian ductile shales. Sequential restorations suggest that the overall gravitational sliding behavior of the White Pointer delta wedge (~45 km of seaward extension, i.e., ~25%) is partially balanced by the tectonic denudation of the subcontinental mantle. We are able to estimate the horizontal stretching rate of the mantle exhumation between ~2 km Ma-1and 5 km Ma-1. The associated uplift of the distal part of the margin and associated flexural subsidence in the proximal part of the basin are partially responsible for the decrease of the gravitational sliding of the White Pointer delta system. Lithospheric failure occurs at ~84 Ma through the rapid exhumation of the mantle. The upper Hammerhead delta system (Late Santonian-Maastrichtian) forms a stable tectonic wedge developed during initial, slow seafloor spreading and sag basin evolution of the Australian side margin. Lateral variation of basin slope (related to the geometry of the underlying White Pointer delta wedge) is associated with distal raft tectonic structures sustained by high sedimentation rates. Finally, we propose a conceptual low-angle detachment fault model for the evolution of the Australian-Antarctica conjugate margins, in which the Antarctica margin corresponds to the upper plate and the Australian margin to the lower plate.

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.

Legacy product - no abstract available

Many aspects of the evolution and overall architecture of the Australian southern rifted margin are consistent with current models for the development of non-volcanic rifted margins. However, when examined in detail, several key features of the southern margin provide useful points of comparison with the Atlantic and Alpine Tethyan margins from which these models derive. Extensive petroleum industry and government seismic and geophysical data sets have enabled detailed mapping of the basins of the southern margin and an improved understanding of its tectonostratigraphic evolution. Australia's southern rifted continental margin extends for over 4000 km, from the structurally complex margin south of the Naturaliste Plateau in the west, to the transform plate boundary adjacent to the South Tasman Rise in the east. The margin contains a series of Middle Jurassic to Cenozoic basins-the Bight, Otway, Sorell, Gippsland and Bass basins, and smaller depocentres on the South Tasman Rise (STR). These basins, and the architecture of the margin, evolved through repeated episodes of extension and thermal subsidence leading up to, and following, the commencement of sea-floor spreading between Australia and Antarctica. Break-up took place diachronously along the margin, commencing in the west at ~83 Ma and concluding in the east at ~ 34 Ma. In general, break-up was not accompanied by significant magmatism and the margin is classified as 'non-volcanic' (or magma-poor). Initial NW-SE ultra-slow to slow seafloor spreading (latest Santonian-Early Eocene), followed by N-S directed fast spreading (Middle Eocene-present), resulted in: (1) an E-W oriented obliquely- to normally-rifted marginal segment extending from the westernmost Bight Basin to the central Otway Basin; (2) an approximately N-S oriented transform continental margin in the east (western Tasmania-STR), and (3) a transitional zone between those end-members (southern Otway-Sorell basins).

A comprehensive black and white teachers' guide reviews the history and development of the United Nation's Convention on the Law of the Sea and its application to Australian marine jurisdiction. Case studies on the North West Shelf, Antarctica and Orange Roughy are included. 108 page booklet with student activities and suggested answers. Suitable for secondary geography and science students Years 8-12.

Since the publication in 1967 of the monograph on the marine geology of the Timor Sea,1 the Bureau of Mineral Resources has initiated a program of systematic reconnaissance geological surveys of the continental shelf. The results of this work are being published in the BMR Bulletin series accompanied by 1:1 000 000 lithofacies maps of the shelf sediments. Three sheets (Rowley Shoals, W.A.2; Scott Reef, W.A.2 ; and Arafura Sea, N.T.8 ) have been printed by early 1974, and work on two further sheets covering part of the east Australian continental shelf is well advanced. Users of the map should refer to Bulletin 83 (GeoCat # 163) to assist in interpretation. For instance, wide areas of the shelf are non-depositional, or even subject to erosion, and therefore the variations in lithology portrayed are not exclusively the result of variations in the modern depositional regime. Also the map does not distinguish sediments which are relics of earlier regimes from modern ones; however, some information of the distribution of these older sediments can be obtained from Bulletin 83 (GeoCat # 163) and inferred from a study of the gravel content in relation to the bathymetry.