Abstract We document the interpretation of three crustal sections from coincident deep seismic reflection, gravity and magnetic data acquired on Australia's southern margin: one section from the Naturaliste Plateau and the Diamantina Zone; and two in the Great Australian Bight (GAB). Interpretations are based on an integrated study of deep multichannel seismic, gravity and magnetic data, together with sparse sonobuoy and dredging information. All interpreted sections of the margin show a transition from thinned continental crust, through a wide continent ocean transition zone (COTZ). In the GAB the transition is to slow sea-floor spreading oceanic crust that dates from breakup in the Campanian ( c . 83 Ma); in the Naturaliste–Diamantina margin the earliest oceanic crust is undated. The COTZ on these margins is geologically and geophysically complex, but interpretation of all data, including dredge hauls, is consistent with the presence of a mixture of modified continental lower crust, breakup related volcanics and exhumed continental mantle. Serpentinized detachment faults are not well imaged, but have been inferred from high-amplitude magnetic signatures interpreted to arise from magnetite associated with the hydration of peridotites. Alternative models for the structure of the COTZ, involving either mafic underplating or aborted sea-floor spreading, have been explored, but are considered unlikely on this margin. Similarity in the final architecture of these margins has major implications for the nature of rifting in the Southern Rift System, and may point to the entire 4000 km-long system being non-volcanic in character. Second-order differences in geometry and morphology of the two areas studied are unlikely to be a function of strain rate. Instead, they probably reflect complexities owing to the multiple tectonic events that occurred during final Gondwanide fragmentation. The most dramatic of these is the impact of hotspot activity in the Kerguelen Plateau, which commenced some 50 Ma prior to final breakup in that sector.

Coals of Cretaceous age are preserved within the fill of several Australian sedimentary basins. Presently, Cretaceous coal is mined in only one area, although numerous coalfields have been active over the past century. Cretaceous organic facies in the subsurface of the Gippsland Basin, offshore southeast Australia, are thought to have sourced the major oil and gas accumulations of that area. Cretaceous coal-bearing basins in Australia fall into four groups: interior basins, notably the Eromanga Basin where the greatest occurrence of coal is in the Cenomanian Winton Formation; east coast basins, including the Laura Basin (Late Jurassic to Early Cretaceous Battle Camp Formation), Styx Basin (Albiah to Cenomanian Styx Coal Measures), Stanwell Outlier (Albian Stanwell Coal Measures), and Maryborough Basin (Albian Burrum Coal Measures); south coast basins, notably the Great Australian Bight, Otway Basin (Otway and Sherbrook Groups), Bass Basin, (Otway and Eastern View Groups), and Gippsland Basin (Strzelecki and Latrobe Groups), where coal is known from throughout the Cretaceous system; west coast basins, notably the Perth Basin, which contains minor, Early Cretaceous coal. The major control on the formation and distribution of Cretaceous coal in Australia was the development of widespread, rapidly-subsiding lowland environments during passive margin breakup between Australia and Antarctica, and between Australia and Lord Howe Rise. The widespread stratigraphic distribution of coal resources suggests that fluctuating climate and evolving vegetational communities did not fundamentally affect coal development. The role of eustatic sea-level changes is difficult to assess at present. Within individual basins, structural regime and the distribution of depositional systems also played an active role in controlling coal distribution.

Abstract The continental shelves of SE Australia from the eastern Great Australian Bight to southern New South Wales are situated within a tectonically stable, passive continental margin. The shelves began to develop during Late Cretaceous time and preserve a rich but punctuated record of shallow-marine sedimentation from the Late Paleocene to the present day. The southern margin is the world's largest modern, cool-water temperate carbonate sediment province, a function of the limited terrigenous-clastic sediment being delivered to the coast in response to continental-scale aridity, endorheic drainage and low topographical relief. With the exception of the South Australian gulfs and the protected portions of Bass Strait, the entire shelf sector of SE Australia is a high-energy, storm- and swell-dominated, microtidal province. Along the southern margin, the dominant sedimentary constituents in the modern shelf sediments are skeletal carbonate bioclasts derived from marine invertebrates that include molluscs, foraminifers, bryozoans, coralline algae and echinoids, variably intermixed with quartz sand and heavy minerals (Heterozoan Association). Sediments of Pleistocene and Holocene age are compositionally similar to their modern equivalents, and all show varying degrees of reworking and incorporation of older bioclasts to form the palimpsest sedimentary successions that characterize the entire region. The southern New South Wales sector of the Eastern Australian Shelf is dominated by siliciclastic sediments but preserves relict molluscan-rich carbonate sediments of glacial-age lowstand origin in the outer-shelf region in present water depths exceeding 100 m. The Quaternary stratigraphical record of the SE Australian continental shelves is dominated by successions that formed in the last glacial cycle, but older, inner-shelf successions have been documented from geographically restricted regions such as the South Australian gulfs, Roe and Coorong coastal plains, and the Bass Strait Islands. The outer-shelf successions are not in hydrodynamic equilibrium with modern shelf processes, as reflected in the paucity of mud, and are either largely relict (New South Wales) or dominated by complex modern bryozoan associations.

Abstract We present a synthesis based on the interpretation of two pairs of deep seismic reflection crustal sections within the Southern Rift System (SRS) separating Australia and Antarctica. One pair of sections is from the conjugate margins between the Great Australian Bight (GAB) and Wilkes Land, in the central sector of the SRS, which broke up in the Campanian. The second pair of conjugate sections is located approximately 400 km further east, between the Otway Basin and Terre Adélie, which probably broke up in Maastrichtian time. Interpretations are based on an integrated synthesis of deep multi-channel seismic, gravity and magnetic data, together with sparse sonobuoy and dredging information, and the conjugate sections are presented with the oceanic crust removed beyond the continent–ocean boundary (COB). At first order, both conjugate pairs show a transition from thinned continental crust, through a wide and internally complex continent–ocean transition zone (COTZ), which shows features in common with magma-poor rifted margins worldwide, such as basement ridges interpreted as exhumed subcontinental mantle. In the central GAB sector, the COTZ is symmetric around the point of break-up and displays a pair of mantle ridges, one on each margin, outboard of which lies a deep-water rift basin. Break-up has occurred in the centre of this basin in this sector of the SRS. In contrast, the Terre Adélie margin is nearly 600 km wide and shows an abandoned crustal megaboudin, the Adélie Rift Block. This block is underlain by interpreted middle crust, and appears to have a mantle ridge structure inboard, as well as an outboard exhumed mantle complex from which mylonitized harzburgite has been dredged. The conjugate margin of the Beachport Sub-basin is relatively narrow ( c. 100 km wide) and does not appear to contain an exhumed mantle ridge, as observed along strike in the GAB. These observations from a single rift spreading compartment show that radically different break-up symmetries and margin architectures can result from an essentially symmetric rifting process involving multiple, paired detachment systems. This indicates the need for caution in interpreting causative mechanisms of rifting from limited conjugate sections in other rifts. We speculate that the underlying crustal composition, rheology and structural preconditioning play a significant role in partitioning strain during the transition to break-up.