Southern Australian Margin Research Articles

Quantifying Neogene plate-boundary controlled uplift and deformation of the southern Australian margin

Abstract Parts of the Australian continent, including the Otway Basin of the southern Australian margin, exhibit unusually high levels of neotectonic deformation for a so-called stable continental region. The onset of deformation in the Otway Basin is marked by a regional Miocene–Pliocene unconformity and inversion and exhumation of the Cretaceous–Cenozoic basin fill by up to c . 1 km. While it is generally agreed that this deformation is controlled by a mildly compressional intraplate stress field generated by the interaction of distant plate-boundary forces, it is less clear whether the present-day record of deformation manifested by seismicity is representative of the longer-term geological record of deformation. We present estimates of strain rates in the eastern Otway Basin since 10 Ma based on seismic moment release, geological observations, exhumation measurements and structural restorations. Our results demonstrate significant temporal variation in bulk crustal strain rates, from a peak of c . 2×10 −16 s −1 in the Miocene–Pliocene to c . 1.09×10 −17 s −1 at the present day, and indicate that the observed exhumation can be accounted for solely by crustal shortening. The Miocene–Pliocene peak in tectonic activity, along with the orthogonal alignment of inverted post-Miocene structures to measured and predicted maximum horizontal stress orientations, validates the notion that plate-boundary forces are capable of generating mild but appreciable deformation and uplift within continental interiors.

Seismic analysis of igneous systems in sedimentary basins and their impacts on hydrocarbon prospectivity: examples from the southern Australian margin

The increasing availability of 3D seismic data from sedimentary basins at volcanic and non-volcanic continental margins has provided fundamental new insights into both the storage and transport of magma in the continental crust. As global hydrocarbon exploration increasingly focuses on passive margin basins with evidence for past intrusive and extrusive igneous activity, constraining the distribution, timing and pathways of magmatism in these basins is essential to reduce exploration risk. Producing and prospective Australian passive margin basins where igneous systems have been identified include the Bight, Otway, Bass, Gippsland and Sorell basins of the southern margin. This paper reviews both the impacts of volcanic activity on sedimentary basin hydrocarbon prospectivity (e.g. advective heating, reservoir compartmentalisation and diagenesis), and the styles, distribution and timing of late Cretaceous–Recent extrusive and intrusive igneous activity along basins of the southern Australian margin, providing illustrative examples based on 2D and 3D seismic reflection data.

Salinity reflux and dolomitization of southern Australian slope sediments: the importance of low carbonate saturation levels

AbstractAnomalously saline waters in Ocean Drilling Program Holes 1127, 1129, 1130, 1131 and 1132, which penetrate southern Australian slope sediments, and isotopic analyses of large benthic foraminifera from southern Australian continental shelf sediments, indicate that Pleistocene–Holocene meso‐haline salinity reflux is occurring along the southern Australian margin. Ongoing dolomite formation is observed in slope sediments associated with marine waters commonly exceeding 50‰ salinity. A well‐flushed zone at the top of all holes contains pore waters with normal marine trace element contents, alkalinities and pH values. Dolomite precipitation occurs directly below the well‐flushed zone in two phases. Phase 1 is a nucleation stage associated with waters of relatively low pH (ca 7) caused by oxidation of H2S diffusing upward from below. This dolomite precipitates in sediments < 80 m below the sea floor and has δ13C values consistent with having formed from normal sea water (− 1‰ to + 1‰ Vienna Pee Dee Belemnite). The Sr content of Phase 1 dolomite indicates that precipitation can occur prior to substantial metastable carbonate dissolution (< 300 ppm in Holes 1129 and 1127). Dolomite nucleation is interpreted to occur because the system is undersaturated with respect to the less stable minerals aragonite and Mg‐calcite, which form more readily in normal ocean water. Phase 2 is a growth stage associated with the dissolution of metastable carbonate in the acidified sea water. Analysis of large dolomite rhombs demonstrates that at depths > 80 m below the sea floor, Phase 2 dolomite grows on dolomite cores precipitated during Phase 1. Phase 2 dolomite has δ13C values similar to those of the surrounding bulk carbonate and high Sr values relative to Phase 1 dolomite, consistent with having formed in waters affected by aragonite and calcite dissolution. The nucleation stage in this model (Phase 1) challenges the more commonly accepted paradigm that inhibition of dolomitization by sea water is overcome by effectively increasing the saturation state of dolomite in sea water.

Paleothermal and seismic constraints on late Miocene–Pliocene uplift and deformation in the Torquay sub-basin, southern Australian margin

The passive southern margin of the Australian continent contains a rich record of late Miocene–Pliocene neotectonic deformation and uplift that continues to the present day as witnessed by unusually high levels of seismicity for a so-called ‘stable continental region.’ To date, however, few studies have sought to estimate the magnitude of exhumation triggered by this deformation and uplift. Here we combine apatite fission track analysis (AFTA), apatite (U–Th)/He dating and vitrinite reflectance (VR) data from the Nerita-1 well in the Torquay sub-basin with seismic reflection data from this basin and the adjoining Otway Ranges to constrain the magnitude and driving mechanisms of exhumation in this part of the southern Australian margin. The Cenozoic succession in this basin has been deformed by folding and reverse faulting and contains a major, low-angle mid-Miocene unconformity that can be traced for distances of ∼1500 km along the margin. Paleothermal data from Nerita-1 show that the sub-mid-Miocene succession has been more deeply buried by ∼1 km of now missing section, and indicate that exhumation began between 10 and 5 Ma, in excellent agreement with stratigraphic constraints. Our estimates of removed section and higher than previous estimates based on extrapolation of seismic reflectors, but are corroborated by AFTA results from nearby wells. Seismic data show that late Miocene-onwards intraplate deformation in the Torquay-sub-basin and Otway Ranges has been accomplished by reverse-reactivation of normal faults within Cretaceous–early Paleogene syn-rift successions, resulting in folding of overlying post-rift late Paleogene–Neogene sediments. The probable cause of this deformation and uplift is increased levels of intraplate stress induced by enhanced coupling of the Indo-Australian and Pacific plates ∼10 Myr ago, and our results thus demonstrate the key role that plate boundary-controlled stress fields play in intraplate uplift and deformation.

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.

Continental margin compression: a comparison between compression in the Otway Basin of the southern Australian margin and the Rockall-Faroe area in the northeast Atlantic margin

There is growing recognition that many passive margins have undergone compressional deformation subsequent to continental breakup, including the southern Australian margin. This deformation commonly results in formation of domal anticlines with four-way dip closures that are attractive targets for hydrocarbon exploration, and many such structures host major hydrocarbon accumulations in the Otway and Gippsland basins; however, the driving mechanisms behind formation of these structures are not completely understood. We compare the history of post-breakup compression in the Otway Basin of the southern Australian margin, with that of the Rockall-Faroe area of the northeast Atlantic margin, which has been far more extensively studied with the aim of establishing a better understanding of the genesis and prospectivity of such structures. Both margins have experienced protracted Mesozoic rifting histories culminating in final continental separation in the Eocene, followed by distinct phases of compressional deformation and trap formation. Whilst the structural style of the anticlines in both margins is similar (mainly fault-propagation folds formed during tectonic inversion), the number, amplitude, and length of the structures in the northeast Atlantic margin are much higher than the southern Australian margin. We propose that compressional structures at both margins formed due to far-field stresses related to plate boundaries, but the magnitude of these stresses in the northeast Atlantic margin is likely to have been higher, and the strength of the lithosphere lower. In the northeast Atlantic margin, the presence of Early Cenozoic basalt lava flows may have also contributed to an increase in pore-fluid pressure in the underlying sediment making pre-existing faults more prone to reactivation.

Cenozoic post-breakup compressional deformation and exhumation of the southern Australian margin

We present results from a margin-wide analysis of the history of post-breakup Cenozoic compressional deformation and related exhumation along the passive southern margin of Australia, based on a regional synthesis of seismic, stratigraphic and thermochronological data. The Cenozoic sedimentary record of the southern margin contains regional unconformities of intra-Lutetian and late Miocene–Pliocene age, which coincide with reconfigurations of the boundaries of the Indo-Australian Plate. Seismic data show that post-breakup compressional deformation and sedimentary basin inversion—characterised by reactivation of syn-rift faults and folding of post-rift sediments—is pervasive from the Gulf St Vincent to Gippsland basins, and occurred almost continually since the early- to mid-Eocene. Inversion structures are absent from the Bight Basin, which we interpret to be the result of both the unsuitable orientation of faults for reactivation with respect to post-breakup stress fields, and the colder, stronger lithosphere that underlies that part of the margin. Compressional deformation along the southeastern margin has mainly been accommodated by reactivation of syn-rift faults, resulting in folds with varying ages and amplitudes in the post-rift Cenozoic succession. Many hydrocarbon fields in the Otway and Gippsland basins are located in these folds, the largest of which are often associated with substantial localised exhumation. Our results emphasise the importance of constraining the timing of Cenozoic compression and exhumation in defining hydrocarbon prospectivity of the southern margin.

The wave energy resource along Australia’s Southern margin

The Southern Australian margin is one of the most energetic regions in the world suitable for the extraction of wave energy for electricity generation. We have produced a data set in which the deep-water wave energy resource for the region is described by three representative deep-water wave states, equivalent to the 10th, 50th, and 90th percentiles of the deep-water wave energy flux, derived from archives of the USA National Oceanic and Atmospheric Administration (NOAA) WaveWatch III (NWW3) operational wave model. The Simulating WAves Nearshore (SWAN) wave model is then applied along the full Southern Australian margin to propagate these representative wave states into the near-shore region to quantify the effects of shallow water processes such as refraction, shoaling, and bottom friction. The wave energy incident on the 25-m isobath (∼30–50 kW/m) is approximately 35%–50% less than the World Energy Council estimates of offshore wave energy but is approximately 20% greater than the energy observed from long-term buoy deployments on the midshelf. The latter discrepancy is attributed to an overestimation of significant wave height along the Southern Australian margin by the NWW3 model. The near-shore model applied in this study adequately simulates the attenuation of wave heights across the continental shelf when compared with estimates of wave height attenuation obtained from the Topex satellite altimeter. The attenuation of wave energy across the continental shelf reduces the estimates of offshore wave energy as given by the World Energy Council; however the wave energy resource incident on the Southern Australian margin remains considerable. We estimate that if 10% of the incident near-shore energy in this region, which is an ambitious target when conversion efficiency is considered, were converted to electricity, approximately 130 TW h/yr (one-half of Australia’s total present-day electricity consumption) would be produced.

Impacts of Neogene-Recent compressional deformation and uplift on hydrocarbon prospectivity of the passive southern Australian margin

The passive southern margin of the Australian continent, which formed following Cretaceous–Palaeogene separation from Antarctica, contains a rich record of Neogene–Recent compressional deformation and uplift. This deformation and uplift is manifested by reversal of displacement along syn-rift extensional faults, folding of mid–late Cenozoic post-rift sediments, and regional unconformities that can be traced for distances of up to 1,500 km along the margin. Palaeothermal data from onshore and offshore exploration wells indicate that erosion associated with deformation and uplift locally exceeds 1 km in the eastern Otway Basin. Both neotectonic palaeostress trends inferred from these structures and present-day stress orientations are consistent with northwest–southeast directed compression controlled to first-order by plate boundary forces. The critical role of the relative timing of trap formation and source rock maturation in controlling hydrocarbon prospectivity in the southern Australian margin is investigated by comparing two structures that formed during Neogene–Recent deformation in the Otway Basin: the Minerva and Nerita anticlines. While the Minerva Anticline hosts a major gas field (558 BCF GIP), the Nerita Anticline was found to be dry. A combination of apatite fission track analysis (AFTA), vitrinite reflectance (VR) and present-day temperature data show that all units intersected in Minerva–1 are presently at their maximum post-depositional temperatures, and are presently mature for hydrocarbon generation. In contrast, similar data collected from the preserved section at Nerita–1 indicate cooling from maximum post-depositional temperatures prior to formation of the Nerita Anticline in the late Miocene. Based on regional AFTA data, the underlying early Cretaceous source rocks probably reached maximum palaeotemperatures and ceased hydrocarbon generation during mid-Cretaceous uplift. These results indicate that areas of the southern margin that were deformed during the Neogene–Recent have the greatest potential to trap hydrocarbons where potential source rocks are presently at their maximum post-depositional temperatures.

Interactions between continental breakup dynamics and large‐scale delta system evolution: Insights from the Cretaceous Ceduna delta system, Bight Basin, Southern Australian margin

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 show that the structural architecture of this delta system consists of two stacked 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 restoration suggests that the overall gravitational sliding behavior of the White Pointer delta wedge (∼45 km of seaward extension, i.e., ∼27%) 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 and 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‐Antarctic conjugate margins, in which the Antarctic margin corresponds to the upper plate and the Australian margin to the lower plate.

A marine geophysical study of the Wilkes Land rifted continental margin, Antarctica

SUMMARY The Wilkes Land margin of East Antarctica, conjugate to the southern Australian margin, is a non-volcanic rifted margin that formed during the Late Cretaceous. During 2000–01 and 2001–02, Geoscience Australia acquired ∼10 000 line km of seismic reflection and refraction, magnetic anomaly and gravity anomaly data over the margin. We have used the seismic data to estimate the sediment thickness along the margin. The data reveal a deep (>11 km) rift basin that contains over 9 km of sediments seaward of the Totten Glacier, western Wilkes Land. The limits of oceanic and continental crust that underlies the thick post-rift and the significantly thinner pre- and syn-rift sediments are equivocal. Seismic reflection data suggest a 30–100 km wide continent–ocean transition zone (COTZ) along the margin. The COTZ extends over 400 km seaward of the shelf break off the eastern Wilkes Land/Terre Ad´ elie sector

Long-wavelength tilting of the Australian continent since the Late Cretaceous

Global sea level and the pattern of marine inundation on the Australian continent are inconsistent. We quantify this inconsistency and show that it is partly due to a long wavelength, anomalous, downward tilting of the continent to the northeast by 300 m since the Eocene. This downward tilting occurred as Australia approached the subduction systems in South East Asia and is recorded by the progressive inundation of the northern margin of Australia. From the Oligocene to the Pliocene, the long wavelength trend of anomalous topography shows that the southern margin of Australia is characterized by relative subsidence. We quantify the anomalous topography of the Australian continent by computing the displacement needed to reconcile the interpreted pattern of marine incursion with a predicted topography in the presence of global sea level variations. On the southern margin, long wavelength subsidence was augmented by at least 250 m of shorter wavelength anomalous subsidence, consistent with the passage of the southern continental margin over a north–south elongated, 500 km wide, topographic anomaly approximately fixed with respect to the mantle. The present day reconstructed position of this depth anomaly is aligned with the Australian Antarctic Discordance and is consistent with the predicted passage of the Australian continent over a previously subducted slab. Both the long-wavelength continental tilting and smaller-scale paleo-topographic anomaly on the southern Australian margin may have been caused by subduction-generated dynamic topography. These new constraints on continental vertical motion are consistent with the hypothesis that mantle convection induced topography is of the same order of magnitude as global sea level change.

Carbon and oxygen isotope geochemistry of live (stained) benthic foraminifera from the Aleutian Margin and the Southern Australian Margin

Comparisons of ambient bottom-water geochemistry and stable isotopic values of the tests of living (stained) calcareous benthic foraminifera from the North Pacific (on the Aleutian Margin, water depth 1988 m) and Murray Canyons group in the Southern Indian Ocean (Australian Margin, water depths 2476 m and 1634 m) provide modern environmental analogs to calibrate paleoenvironmental assessments. Consistent with the hypothesis that microhabitat preferences influence foraminiferal isotopic values, benthic foraminifera from both margins were depleted in 13C with respect to bottom-water dissolved inorganic carbon (DIC). The carbon isotope values of deep infaunal foraminifera ( Chilostomella oolina, Globobulimina pacifica) showed greater differences from estimates of those of DIC than shallow benthic foraminifera ( Bulimina mexicana, Bolivinita quadrilatera, Pullenia bulloides). This study provides new isotopic and ecological information for B. quadrilatera. The mean Δ δ 13C value, defined as foraminiferal δ 13C values minus estimated ambient δ 13C values from the Aleutian Margin, is 0.97‰ higher for G. pacifica than the mean from the Murray Canyon. This difference may result either from genetic or biological differences between the populations or from differences in environmental isotopic influences (such as pore water differences) that were not accounted for in the equilibrium calculations. These analyses provide calibration information for the evaluation of bottom water conditions and circulation patterns of ancient oceans based on fossil foraminiferal geochemistry.

Comparing 1800–1600 Ma accretionary and basin processes in Australia and Laurentia: Possible geographic connections in Columbia

We focus on potential pinning points on the southern and eastern margins of Australia and the southern and western margins of Laurentia and interpret a convergent plate boundary along the southern margin of the proto-Australian continent during the Palaeo- to Mesoproterozoic (1800–1600 Ma). Specific geological events within the southern Australian margin can be correlated with events preserved along the Medicine Bow–Yavapai–Mazatzal Belt in southwestern Laurentia. These belts were characterised by southward continental growth with varying degrees of crustal reworking and accretion of juvenile material. In Australia crustal reworking appears to have been the dominant process from 1800 to 1670 Ma, with more juvenile material accreted at 1620 Ma. In contrast, the Southern Laurentian terranes were dominated by accretion of juvenile material from 1800 to 1620 Ma, although reworked continental crust was also accreted to the margin. In the interior of both continents basins developed with comparable histories from 1800 to 1670 Ma. Thereafter the evolution of the interior basins diverged, with sag-phase processes dominant in Northern Australia and a protracted depositional hiatus in Laurentia. The geologic evolution is consistent with palaeomagnetic data and provides further evidence for geographic links between Eastern Australia and Western Laurentia from 1800 to 1600 Ma.

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.

Genesis of Palimpsest Cool-Water Carbonate Sediment on the Continental Margin of Southern Australia

Abstract The continental margin of southern Australia is a site of extensive Quaternary cool-water carbonate sedimentation. Seafloor sediments are a mixture of Holocene biofragments and late Pleistocene relict and stranded particles. Relict sediments, produced during Marine Isotope Stages 3 and 4, are generally preserved as iron-stained intraclasts. Such grains in the Great Australian Bight are photozoan in character. They include symbiont-bearing benthic foraminifers and coralline algal particles, and indicate that the wide shelves were bathed in shallow, relatively warm, oligotrophic waters and floored by marine grasses, similar to many modern shallow inboard embayments in the region today. In contrast, east of the Great Australian Bight relict sediments are more heterozoan in character, with coralline algal and molluscan facies on the inner shelf and bryozoan sediments on the outer shelf. Stranded sediments, marooned during the sea-level rise associated with Marine Isotope Stage 2, are gray to buff in color and composed of both biofragments and intraclasts. These particles are mainly heterozoan in character throughout the region, with subtropical photozoan marine grassbank facies localized to portions of the inner Great Australian Bight. Stranded sediments are interpreted to have accumulated in an areally restricted photic zone and upon an overall narrow shelf covered in cool, nutrient-rich waters. Swell-driven seafloor sediment disturbance to depths of 100 m along the southern Australian margin in conjunction with reduced rates of sedimentation that characterize the cool-water carbonate realm, have resulted in mixing of sediment currently being produced with exhumed relict and stranded sediments generated during previous sea-level stands. The composition of these deposits and the nature of their pre-burial alteration are a function of sea-level history. Late Pleistocene relict sediments originally accumulated in low-energy, inboard environments but were modified in the surf zone during subsequent sea-level fluctuations. This resulted in the formation of intraclasts: abraded, infilled, and cemented biofragments of local origin. The widespread distribution of intraclasts and of composite grain assemblages derived from multiple sedimentary environments that display highly variable degrees of seafloor alteration may be signatures of modern cool-water carbonate assemblages worldwide. While such palimpsest sediments may complicate paleoenvironmental interpretation, their differentiation in the rock record can be utilized to discern subtleties in paleoceanography and sea-level history.

Cenozoic stratigraphic succession in southeastern Australia

Strata of Cenozoic age occur around the southern margin of Australia as thin and discontinuous outcrops, interpolated and fleshed out by economic exploration onshore and offshore. The neritic strata fall into four sequences or allostratigraphic packages of (I) Paleocene — Early Eocene, (II) Middle Eocene — Early Oligocene, (III) Late Oligocene — Middle Miocene, and (IV) Late Miocene — Holocene age: a four‐part pattern that can be seen also in the flanking pelagic and terrestrial realms including regolith deep weathering. Problems of correlation and age determination (predominantly biostratigraphic) have included biogeographical constraints (endemism in neritic molluscs and terrestrial palynomorphs, mid‐latitude assemblages in calcareous plankton), and slow progress in magnetostratigraphy and chemostratigraphy. Sequence I largely repeats the Cretaceous siliciclastic‐coal, marginal‐marine facies (carbonate‐poor, with marine and non‐marine palynomorphs and agglutinated foraminifers) punctuated by marine ingressions with microfaunas and sparse macrofaunas. Sequence II contains the first carbonates in the region since the Palaeozoic and the most extensive coals of the Cenozoic anywhere. Sequence III contains the most extensive neritic carbonates and the last major coals. Sequence IV is more strongly siliciclastic than the two preceding. Each of these four second‐order entities (107 years duration) comprises third‐order packages each with an unconformity and marine transgression. These packages hold true right along the southern Australian margin in the sense that the hiatuses and transgression do not display significant diachroneity at the relevant time‐scales (105–106 years). Recognised, delimited and correlated independently of the putatively global Exxon sequences, they are remarkably consistent with the latter, thereby providing a significant regional test. There are two widespread emphases on southern Australian geohistory and biohistory: (i) to regard the regional story as part of the global story of accreting continents, an expiring Tethys, and an episodically cooling planet; and (ii) a somewhat contrary emphasis, with the region being a special case of rapid longitudinal motion towards the equator. Both emphases are plausible with the former being the more heuristic. The stratigraphic record is strongly punctuated, the four sequences being separated by both tectonic and climatic events. Thus: the sequence I/II gap involved extensive plate‐tectonic reorganisation and a new spreading regime from ca 43 Ma, coevally with early growth of Antarctic ice; in the II/III gap, deformation in marginal basins is coeval with a global low in cooling, large ice sheet and falling sea‐level to ca 30 Ma; and the III/IV gap is marked by widespread cessation or contraction of stratal accumulation and withdrawal of thermophilic taxa coevally with the major expansion at ca 14 Ma of the Antarctic ice sheet, onset of intense canyon cutting, and plate‐wide basin inversion.

The eustatic and tectonic origin of Neogene unconformities from the Great Australian Bight

The Neogene succession recovered during ODP Leg 182 from the Great Australian Bight comprises unconformity-bounded, cool-water carbonates. A total of 15 hiatuses, each lasting ∼0.5 Myr or more, are identified or inferred primarily on the basis of planktonic foraminifer biostratigraphy from the basal Miocene to early Pleistocene. They are interpreted as local manifestations of major third-order boundaries at about 23.8 (H1), 22.3 (H2), 20.5 (H3), 18.7 (H4), 16.4 (H5), 14.8 (H6), 13.5 (H7), 11.5 (H8), 9.3 (H9), 7.0 (H10), 6.0 (H11), 4.5 (H12), 3.5 (H13), 2.5 (H14), and 1. 5 Ma (H15). The coincidence of these hiatuses with third-order global sequence boundaries suggests a eustatic control complicated by local tectonics. Three mega-hiatuses at about 15–16 Ma, 8–9 Ma and 1.5–2.5 Ma, each lasting >5 Myr at selected sites, are interpreted as being caused by large-scale slope failure during times of differential uplift/subsidence and sudden changes in relative sea level. These results provide evidence that the evolution of the southern Australian margin during the Neogene occurred in steps, controlled by a W–E stress field in the course of Australia’s northward drift and by changes in relative sea level that triggered sedimentation as well as sediment packaging by unconformities.

Paleoenvironmental Controls on Upper Eocene Biosiliceous Neritic Sediments, Southern Australia

During the late Eocene in southern Australia, biosiliceous spiculite and spongolite accumulated within shallow marine estuaries and embayments inboard of an open-ocean, low-energy, cool-water carbonate shelf. Estuarine and embayment waters were ideal for an otherwise deep-water sponge assemblage to colonize these paleoenvironments and outcompete all other organisms. Shallow, warm-water sponge growth was promoted by (1) a flat, humid, rainforested terrestrial hinterland with little surface runoff and high groundwater flow rates that led to an enhanced export of dissolved silica, low sedimentation rates, and oligotrophic inshore environments; (2) a paleogeography that protected silica-rich estuarine and embayment waters from dilution by ocean waters, and helped maintain calm conditions; (3) low siliciclastic sedimentation rates that enhanced substrate ventilation by frequent bioturbation, occasional storms, and diffusion; (4) muddy substrates and occasional turbidity that inhibited growth of phototrophic and calcareous sedentary epibenthic organisms; (5) substrates that lacked carbonate or organic matter (due to low productivity) and were consequently suboxic with high pH, a situation that drove extensive iron oxide-biogenic opal-kaolinite dissolution and clinoptilolite-verdine precipitation immediately below the sediment-water interface. Substrate ventilation helped cycle dissolved silica between substrate pore waters and overlying marine waters where the silica was fixed in epibenthic sponges. This silica cycle, combined with export of terrestrial silica, maintained high dissolved silica concentrations within embayments and estuaries. The result was late Eocene embayments-estuaries along 2000 km of the southern Australian margin that were both chemically and physically ideal for sponge colonization. The general controlling paleoecological variables of these late Eocene environments are applicable to both modern deep-sea sponge habitats and ancient shallow-water, biosiliceous sponge facies.

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.