Southern Australian Continental Margin Research Articles

Stratigraphic architecture of the Cenozoic Dugong Supersequence: implications for the late post-breakup development of the Eucla Basin, southern Australian continental margin

ABSTRACTThis study presents an appraisal of the Middle Eocene–Quaternary Dugong Supersequence of the Eucla Basin, offshore southern Australia. It combines details of the rock record with seismic-stratigraphical information, and the resulting stratigraphic framework provides constraints on the nature of the late post-breakup development of the southern Australian continental margin. It is well established that the onshore-to-mid-shelf succession comprises a predominantly aggrading-to-prograding unconformity-bounded succession of carbonate platform deposits; however, our analysis of the outer shelf–upper slope section challenges the widely held view that this shelf-margin wedge represents a distally steepened prograding carbonate ramp primarily modulated by global eustasy. Instead, our results show that the Middle Eocene–Quaternary succession is punctuated by a series of unconformities that reflect a persistent tectonic instability and differential vertical movements throughout the late post-breakup period, the genesis of which is most closely related to tectonic events. Moreover, the upper slope clinoform succession was constructed and shaped predominantly by alongslope processes, and four different contourite drift types are recognised based on their seismic-stratigraphic expression: elongate mounded drift (Quaternary); infill drift (Pliocene); plastered drift (Oligocene); and separated drift (Middle–Upper Eocene). The Quaternary drift – herein termed the ‘Eyre Terrace Drift’ – is a spectacular basin-scale deposit, over 500 m thick and traced for up to 200 km along the upper slope Eyre Terrace. Upslope-migrating sediment waves are associated with this drift. Key sedimentary attributes consistent with a contourite origin include fine-grained sediment, multi-scale gradational bed contacts and pervasive bioturbation. There is also evidence of episodic downslope mass-movement processes ranging from the large-scale Late Neogene Slide, which extends downslope for 15–20 km, to sporadic slumped beds and turbidites recovered in boreholes. The interaction of alongslope and downslope processes indicates a more dynamic sedimentary setting than previously assumed along the outer margin of the Eucla Basin.

Shallow water masses and their connectivity along the southern Australian continental margin

Four water masses are identified and described using hydrographic, nutrient and stable isotope data for the top 1000 m water depth of the southern Australian continental margin, from Cape Leeuwin to Tasmania. Three are identified from previous literature on the southeast Indian Ocean: Subtropical Surface Water (STSW), Tasmanian Subantarctic Mode Water (TSAMW) and Tasmanian Intermediate Water (TIW), and one is newly identified and named: South Australian Basin Central Water (SABCW). STSW (0–250 m) is transported east by the Leeuwin Current System and is modified by heating and evaporation along the subtropical continental shelf. SABCW (250–400 m) and TSAMW (400–650 m) form southwest of Tasmania from deep winter mixing in the region: SABCW at the Subtropical Front and TSAMW north of the Subantarctic Front. TIW (>650 m) forms southwest of Tasmania from mixing of warm, saline Antarctic Intermediate Water from the Tasman Sea and cool, fresh Antarctic Intermediate Water from the Antarctic Circumpolar Current. SABCW, TSAMW and TIW are transported west along the slope by the Flinders Current System, here defined as the western slope-trapped Flinders Current, Tasman Outflow and equatorward Sverdrup transport. Water mass distributions on the slope identify the interface between subantarctic water from the Southern Ocean and subtropical water transported by the Leeuwin Current System. This interface is ~300 m during winter and ~250 m during summer, but can be 150 m during summer in upwelling regions and off western Tasmania. Furthermore, stable isotope data of the water masses south and west of Australia show connectivity between the Subantarctic Zone, the southern Australian margin and the western Australian margin.

Thermal weakening localizes intraplate deformation along the southern Australian continental margin

The controls on seismicity and fault reactivation in stable intraplate crust located far from active plate boundaries are poorly understood. The southern Australian continental margin has been undergoing mild levels of deformation over the past ~10 Myr, manifested today by high levels of seismicity for a stable intraplate region. However, this deformation is partitioned, with zones of abundant neotectonic faults with evidence for Pliocene–Quaternary displacement, enhanced relief (up to 1–2 km) and high seismicity (numerous M > 5 earthquakes) such as the Flinders Ranges adjoining areas of little neotectonic activity, subdued topography and low levels of seismicity such as the Murray Basin and Nullarbor Plain. Here we present a new compilation of 192 heat flow data for the southern Australian margin. Variations in heat flow correlate well with variations in neotectonic and seismic activity, with regions of deformation corresponding to elevated heat flows of up to ~90 mW m − 2 . We propose that the southern Australian margin provides the best evidence to-date that active intraplate deformation may be localized and controlled by the thermal properties of the crust and upper mantle.

Methanogenesis and sulfate reduction in marine sediments: A new model

A number of studies have shown that methanogens are active in the presence of sulfate under some conditions. This phenomenon is especially exemplified in carbonate sediments of the southern Australian continental margin. Three sites cored during Ocean Drilling Program (ODP) Leg 182 in the Great Australian Bight have high concentrations of microbially-generated methane and hydrogen sulfide throughout almost 500 m of sediments. In these cores, the sulfate-reducing and methanogenic zones overlap completely; that is, the usual sulfate-methane transition zone is absent. Amino acid racemization data show that the gassy sediments consist of younger carbonates than the low-gas sites. High concentrations of the reduced gases also occur in two ODP sites on the margin of the Bahamas platform, both of which have similar sedimentary conditions to those of the high-gas sites of Leg 182. Co-generation of these reduced gases results from an unusual combination of conditions, including: (1) a thick Quaternary sequence of iron-poor carbonate sediments, (2) a sub-seafloor brine, and (3) moderate amounts of organic carbon. The probable explanation for the co-generation of hydrogen sulfide and methane in all these sites, as well as in other reported environments, is that methanogens are utilizing non-competitive substrates to produce methane within the sulfate-reducing zone. Taken together, these results form the basis of a new model for sulfate reduction and methanogenesis in marine sediments. The biogeochemical end-members of the model are: (1) minimal sulfate reduction, (2) complete sulfate reduction followed by methanogenesis, and (3) overlapping sulfate reduction and methanogenesis with no transition zone.

Early Diagenesis of Carbonates on a Cool-Water Carbonate Shelf, Southern Australia

Abstract The southern Australian continental margin is an immense cool-water carbonate factory. The high-energy open shelf is blanketed by heterozoan carbonates of both Holocene and late Pleistocene age. Three distinct grain types have been identified and radiocarbon dated. These include (1) relict grains, highly abraded iron-stained intraclasts formed during the intermediate sea-level stands of Marine Isotope Stages 3 and 4, (2) stranded grains, gray and buff-colored abraded biofragments and intraclasts marooned during the sea-level rise associated with Marine Isotope Stage 2, and (3) Holocene grains, unaltered biofragments formed during Marine Isotope Stage 1. Optical, mineralogical, and chemical characterization of grains of each age has revealed pathways of early diagenesis in this realm. Both calcitic and aragonite biogenic grains undergo dissolution. While dissolution of calcitic components is minor and incomplete (relict sediments are predominantly Mg calcite), aragonitic skeletons undergo significant dissolution within ~ 30,000 years. In contrast to early dissolution of aragonite, micritic calcite cement, with ~ 12 mol% MgCO3, is a significant diagenetic phase in these sediments, infilling many stranded and most relict skeletons. Similar cement precipitates between grains, forming small numbers of cemented grain aggregates in most samples. Microprobe and ICP-MS analysis demonstrates Fe oxides, Mn oxides, and associated clay minerals discolor stranded and relict grains, by coating grain surfaces and infesting micropores. Such coatings begin to form on Holocene grains during or soon after formation. The oxides scavenge trace elements including V, Ti, Cr, and REE, altering the elemental compositions of biogenic carbonate.

A classification of wave generation characteristics during large wave events on the Southern Australian margin

The Southern Australian continental margin is exposed to some of the largest waves of the global ocean, having been generated by Southern Ocean extra-tropical storms. Waverider buoy data from four buoys which span the full 3000 km length of the Southern Australian continental margin, supplemented with 10 years of numerical model output from the WAVEWATCH III wave model, have been used to describe the directional wave climate of the region. The buoy records include two records at the western end of the margin (a 10-year record from Rottnest Island, and an 8-year record from Cape Naturaliste); a 6-year record from Cape de Couedic in the central portion of the margin; and a record spanning 20 years from Cape Sorell, at the southeastern end of the margin. Cape Sorell exhibits the steepest waves, largest mean significant wave heights, and most westerly wave directions, indicating increased proximity to Southern Ocean wave generating storms. Rottnest exhibits the least steep, smallest mean significant wave heights, and most southerly wave directions, reflecting the greater distance from the Southern Ocean generating storms. All sites demonstrate highly energetic extreme events, particularly at Cape Sorell, and a predominance of swell over wind sea states. Spatial coherence between simultaneous buoy records show that wave events are easily tracked in the wave record as they propagate eastwards along the Southern Australian margin. Maximum correlation between records indicates that wave events at Cape Sorell lag those at Rottnest by approximately 80 h, and at Cape de Couedic by approximately 7 h. The speed of propagation of events between records is therefore consistent with the propagation speeds of extra-tropical cyclones along the Southern Australian margin. Synoptic typing of six-hourly mean sea level pressure (MSLP) patterns obtained from the NCEP–NCAR reanalysis has been carried out to determine the main synoptic types associated with large wave events at the Cape Sorell waverider buoy site. Significant differences occur in wave characteristics between synoptic types. At Cape Sorell, wave event magnitude shows a significant correlation to the intensity (as measured by the depth) of the forcing cyclone, with larger wave events occurring during more intense storms in the Southern Ocean. At the other buoy sites, the magnitude of wave events shows little correlation to the intensity of the storm, but is more closely related to the location of the storm, with larger events occurring when storms are closer to the Australian continent. These relationships are of importance for determining how the wave climate might change in this region (and how Australia's coastline might respond) in response to changes in atmospheric forcing which are currently being observed.

Nature of the continent–ocean transition zone along the southern Australian continental margin: a comparison of the Naturaliste Plateau, SW Australia, and the central Great Australian Bight sectors

AbstractWe 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.

Origin of the Late Neogene Roe Plains and their calcarenite veneer: implications for sedimentology and tectonics in the Great Australian Bight

The Roe Calcarenite, a 2 – 3 m-thick, soft, quartzose molluscan sand of grainstone to rudstone texture, is a critical unit for deciphering the geodynamic and sea-level history of the southern Australian continental margin. Biostratigraphic and Sr-isotope analysis of molluscs and brachiopods confirms that the unit is Late Pliocene. Amino acid racemisation analyses indicate a minimum age of Early Pleistocene. The general depositional environment was a shallow illuminated shoreface to the inner shelf with the seafloor probably covered by seagrass, much like the modern seafloor offshore the Roe Plains today, but perhaps somewhat warmer. The calcarenite lies on an interpreted marine erosion surface cut into Upper Oligocene to Middle Miocene Eucla Group cool-water carbonates. Such planation, which affected all of the inner shelf, took place throughout the Early Pliocene due to the combination of uplift via basin inversion and eustatic highstand. The process, which led to ∼85 km of cliff retreat in ∼3 million years, is interpreted to be a variant on the shaved shelf process operating on the shelf today. The Roe Calcarenite is envisaged as the last of many calcarenites deposited during small-scale highstands that were eroded during subsequent transgressions. It is preserved because the Roe Plains were uplifted immediately after deposition, part of a widespread Plio-Pleistocene boundary tectonic event. It has been continuously exposed since uplift and subject to arid-zone pedogenic diagenesis. This succession is a relatively quiescent example of uplift, erosion and deposition related to basin inversion. It was much less intense than coeval events further east in the St Vincent, Otway and Gippsland Basins. Together, these Late Neogene tectonic-sedimentary packages illustrate the spectrum of stratigraphic successions that might be expected from basin inversion along an otherwise passive continental margin.

A review of the Cenozoic stratigraphy and glacial history of the Lambert Graben—Prydz Bay region, East Antarctica

The Cenozoic glacial history of East Antarctica is recorded in part by the stratigraphy of the Prydz Bay—Lambert Graben region. The glacigene strata and associated erosion surfaces record at least 10 intervals of glacial advance (with accompanying erosion and sediment compaction), and more than 17 intervals of glacial retreat (enabling open marine deposition in Prydz Bay and the Lambert Graben). The number of glacial advances and retreats is considerably less than would be expected from Milankovitch frequencies due to the incomplete stratigraphic record. Large advances of the Lambert Glacier caused progradation of the continental shelf edge. At times of extreme glacial retreat, marine conditions reached > 450 km inland from the modern ice shelf edge. This review presents a partial reconstruction of Cenozoic glacial extent within Prydz Bay and the Lambert Graben that can be compared to eustatic sea-level records from the southern Australian continental margin.

Middle and Late Eocene Great Australian Bight lithobiostratigraphy and stepwise evolution of the southern Australian continental margin

Middle to Upper Eocene carbonates recovered at ODP Leg 182 sites from the Great Australian Bight comprise three packages: (i) a 43–40 Ma quartzose limestone and packstone package; (ii) a 39–37 Ma wackestone package with ooze; and (iii) an Upper Eocene fine‐grained wackestone package mainly 36.5–35 Ma in age. Bounding these sediment packages are unconformities inferred to have occurred at ca 43, ca 39, ca 37 and ca 34 Ma, coinciding with four major third‐order sequence boundaries. Rapid changes in sea‐floor spreading probably caused these unconformities, while a stable high sea‐level between spreading pulses led to sediment packaging from offshore to coastal basins. The regional sea‐level changes in these Middle to Late Eocene times, when glacioeustatic influence was minimal, were mainly driven by plate tectonics. Biofacies indicate a rapid subsidence, faster offshore than nearshore, during the first phase of accelerated sea‐floor spreading between Australia and Antarctica around 43 Ma. Most significant environmental changes affecting sedimentation along the southern Australian margin at that time were the influx of warm waters from the Indian Ocean and the initial development of a stratified water mass in the deepening and widening Australo‐Antarctic Gulf. The warm (surface) water supported plankton from the subtropics and migratory and endemic larger benthos including foraminifers,while the fertile deeper waters were sites of chert accumulation. The bloom of carbonate‐generating bryozoans contributed to widespread carbonate sedimentation that progressed towards the southeastern margin ∼4 million years later. Subsequent carbonate packaging and unconformities, reflecting a stepwise accelerated sea‐floor spreading pattern, persisted into the terminal Eocene before the onset of global glaciation.

Hydrogen sulfide–hydrates and saline fluids in the continental margin of South Australia

During the drilling of the southern Australian continental margin (Leg 182 of the Ocean Drilling Program), fluids with unusually high salinities (to 106‰) were encountered in Miocene to Pleistocene sediments. At three sites (1127, 1129, and 1131), high contents of H2S (to 15%), CH4 (50%), and CO2 (70%) were also encountered. These levels of H2S are the highest yet reported during the history of either the Deep Sea Drilling Project or the Ocean Drilling Program. The high concentrations of H2S and CH4 are associated with anomalous Na+/Cl− ratios in the pore waters. Although hydrates were not recovered, and despite the shallow water depth of these sites (200–400 m) and relative warm bottom water temperatures (11–14 °C), we believe that these sites possess disseminated H2S-dominated hydrates. This contention is supported by calculations using the measured gas concentrations and temperatures of the cores, and depths of recovery. High concentrations of H2S necessary for the formation of hydrates under these conditions were provided by the abundant SO42− caused by the high salinities of the pore fluids, and the high concentrations of organic material. One hypothesis for the origin of these fluids is that they were formed on the adjacent continental shelf during previous lowstands of sea level and were forced into the sediments under the influence of hydrostatic head.

Holocene carbonate sedimentation on the west Eucla Shelf, Great Australian Bight: a shaved shelf

The southern continental margin of Australia is a cool-water carbonate sedimentary province located in a high-energy, swell-dominated oceanographic setting. A vibrocore transect of 14C-dated sediments across the centre of the Eucla Shelf is the first record of Holocene shelf deposition in the Great Australian Bight. Much of the seafloor shallower than 70 m water depth, the base of wave abrasion, is bare Cenozoic limestone, in some places encrusted by (?) Late Pleistocene, coral-rich, limestone that is cemented by high-magnesium calcite (12 mole% MgCO 3). The areally extensive, 100 km-wide, hard, bored substrate supports an epibiota of coralline algae, minor bryozoans and soft algae or is covered by patches of Holocene sediment up to 1.5 m thick; generally a basal bivalve lag (< 3 ka) overlain by quartzose-bioclastic palimpsest sand. This pattern of active carbonate production but little accretion on the wave-swept mid- to inner-shelf is similar to that on other parts of the southern Australian continental margin. The term shaved shelf is proposed for this style of carbonate platform, formed by alternating periods of sediment accretion, cementation and erosion. The palimpsest sand is typically rich in bivalves, coralline algae and locally, detrital dolomite. Outer shelf Holocene sediment, below the base of wave abrasion but inboard of the shelf edge, is a metre-thick unit of fine, microbioclastic muddy sand with minor delicate bryozoans overlying a 9–13 ka rhodolith gravel. Some of this outer shelf sediment appears to have been resedimented. The shelf edge is a sandy and rocky seafloor with active bryozoan growth and sediment production. The Holocene sediments are enriched in coralline algal particles and conspicuous large foraminifers (cf. Marginopora) and depleted in bryozoans, as compared to coeval deposits on the Lacepede and Otway shelves off southeastern Australia. These differences are interpreted to reflect warmer waters of the Leeuwin Current and prevalent downwelling in this area as opposed to the general upwelling and colder waters in the east.

Slope morphology adjacent to the cool‐water carbonate shelf of South Australia: GLORIA and Seabeam imaging

A combined GLORIA, Seabeam and high‐resolution seismic survey was conducted over a selected area of the southern Australian continental margin. The survey focused on the continental slope immediately seaward of the wide cool‐water carbonate shelf, in a region dissected by a suite of large submarine canyons. Examination of the data underscores the significance of mass‐wasting processes in the development of slope morphology. Slump scars and obvious sediment slides are virtually ubiquitous, in many cases being sourced immediately below the shelf break and connecting downslope with erosional channels. The channels themselves possess what appear to be degraded levee banks suggesting possible spill‐over of turbidity currents which themselves may have been triggered by the upper slope slides. The age of these large‐scale sediment movements remains uncertain, however, and it is currently unclear whether or not they relate to specific sea‐levels. A pair of east‐trending scarp‐like features dominates the mid‐slope...

Origin of a cool‐water, Oligo‐Miocene deep shelf limestone, Eucla Platform, southern Australia

ABSTRACTThe Abrakurrie Limestone is an areally extensive, bryozoan‐rich unit within the Eucla Platform, a Tertiary carbonate shelf which caps the central part of the southern Australian continental margin. The onshore portion, the topic of this study, has been exposed since middle Miocene time and lies beneath the Nullarbor Plain. The rocks are fine‐sand‐ to granule‐sized calcarenites, composed of bryozoans, bivalves, benthic foraminifera and echinoids with lesser numbers of brachiopods, solitary corals and serpulids. They conspicuously lack significant numbers of planktonic foraminifera and coralline algae. Most bryozoan remains are from delicate branching cyclostomes although delicate branching, robust branching, foliose, fenestrate, multilaminar encrusting and free‐living cheilostomes are variably abundant in specific units. The poorly lithified sequence is punctuated by well‐cemented layers with erosional tops, which are interpreted as hardgrounds.The limestone is interpreted as a cool‐water, deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the Eucla Platform. A model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation. The shelf is envisaged as partitioned by the depth of the zone of wave abrasion. Sediments were produced throughout, but accumulated only below this depth. When the seafloor was above this depth it was an environment of cementation and erosion.The vertical sequence correlates in a general way with the global sea‐level model for the mid‐Cenozoic. While accumulation rates for southern Australian carbonates are similar to rates of cool‐water carbonate deposition elsewhere (c. 2.5 cm kyr‐1), the rate of Abrakurrie accumulation is much less, suggesting that significant time periods are represented by the hardgrounds.

AUSTRALIA&amp;apos;S DEEPWATER FRONTIER PETROLEUM BASINS AND PLAY TYPES

Vast areas of Australia's continental margin sedimentary basins lying seawards of the 200 m water depth line, or shelf edge, are under-explored for petroleum. Indeed, most are essentially unexplored. However, recent advances in drilling and production technology, as well as recent reconnaissance seismic, geochemical, geothermal and seabed sampling data collected by the Bureau of Mineral Resources' (BMR) Marine Division, may reduce the perceived economic risk of many of these deepwater basins relative to their shelf counterparts. Triassic reefs have been identified off the northern Exmouth Plateau and possibly in the deepwater Canning Basin, locally within a predicted oil window. In the deepwater North Perth Basin, major wrench structures have been identified. The deepwater areas of the Great Australian Bight and Otway Basin are actually the main depocentres of a major basin complex lying along the almost totally unexplored southern Australian continental margin. The Latrobe Group in the outer Gippsland Basin is likely to have similar geology to the well explored and productive shelf basin, but remains untested. The Queensland and Townsville troughs, in deepwater off northeast Australia, contain many significant structures typical of unbreached rift systems.All these areas have been risked relative to each other and their prospectivity assessed. The most attractive frontier areas in terms of relative risk may be the Otway and North Perth basins. The highest potential may occur in the deepwater rift troughs off northeast Australia, although the relative risk is very high. Triassic reefs of the Northwest Shelf may have the best prospectivity in the shorter term, given that they are known from drilling in a region with proven source potential and a substantial exploration infrastructure.

Maturation Modeling in Otway Basin, Australia

The Otway basin is a Jurassic to Pliocene sedimentary basin formed on the southern Australian continental margin. Its formation is associated with rifting and breakup of the Australian and Antarctic plates. Lithospheric cooling and contraction have probably produced post-breakup subsidence. Either lithospheric stretching or deep crustal metamorphism may have produced pre-breakup subsidence. These mechanisms have identifiable thermal histories. Organic diagenesis (specifically the reflectance of vitrinite in oil) is empirically determined by the thermal and depositional history of an organic sediment. Thus, the stages of hydrocarbon maturity of Otway basin sediments can be modeled. Depositional history is determined from geohistory analysis and thermal history depends on the subsidence mechanism applied to the basin. A paleo-heat-flow history derived from the deep crustal metamorphism model of subsidence produces a maturation profile with depth that is consistent with observed vitrinite reflectance data, although organic diagenesis modeling is relatively insensitive to precise details of thermal history. Depositional and maturation history modeling for the present day, 20 Ma ago, 40 Ma ago, and 60 Ma ago is applied to a se smic profile across the southern Australian continental shelf in the Otway basin as a demonstration of the projection backward in time of sedimentation and organic diagenesis.

DEPOSITIONAL ENVIRONMENTS AND GEOTECTONIC FRAMEWORK: SOUTHERN AUSTRALIAN CONTINENTAL MARGIN

Three principal phases occurred in the development of the basins of the southern Australian continental margin: epi-continental, marginal continental and oceanic. These correspond generally to the phases of margin development proposed by Falvey (1974): pre-rift, rift valley, and post-breakup; but tectonic and depositional transitions are not necessarily contemporaneous.Prior to the Upper Cretaceous, the region of the present day southern Australian margin lay well within the Eastern Gondwanaland continent, essentially barred from deep ocean basins. During the Upper Cretaceous the series of epicontinental basins was increasingly subjected to marine breakthroughs. Thus marine ingressive horizons were deposited along an incipient rift valley between the primitive Indian Ocean and Tasman Sea. Rift valley subsidence, possibly related to deep crustal metamorphism, was most significant on the flanks of the rift zone. Further marine influence during the Paleocene ('infra-breakup') and early Eocene corresponded to the onset of seafloor spreading between Australia and Antarctica. The neo-breakup phase is dominated by shelf and plateau subsidence and spreading ridge development, with topography influencing ocean current. The changing palaeogeography can be accurately illustrated by computer-derived reconstructions based on quantitative sea-floor spreading data. Quantitative thermal uplift/subsidence models can be used to estimate post-breakup water depth of the subsiding ocean basin and the continental margin. A complex pattern of transgressive continental deposition and submarine erosion diminished with the gradual widening of the Southern Ocean and the establishment of circumpolar ocean current paths. Oceanic basins dominated the margin through the Neogene.