Southern Australian Margin Research Articles

Pleistocene coastal stratigraphy, sea‐level highstands and neotectonism of the southern Australian passive continental margin—a review

AbstractLocated in a tectonically stable, intraplate setting and in the far field of former ice sheets, the southern Australian passive continental margin represents a globally important region for quantifying sea level during Pleistocene highstands. The stratigraphical record of the southern Australian margin is dominated by geographically extensive coastal barrier, estuarine and peritidal landforms of bioclastic, temperate carbonate affinity, a function of regional aridity. Siliciclastic successions dominate the eastern Australian coastline. Despite the generally high level of tectonic stability, regional contrasts are evident in the elevation of Pleistocene shoreline deposits that largely relate to pre‐Quaternary geotectonic setting. The most complete record of Pleistocene highstand deposits occurs in the Murray Basin–Coorong Coastal Plain region, and indicates that interglacial sea‐level highstands, for at least the past 11 interglacials, did not deviate by more than 6 m from present sea‐level. The Pleistocene marginal marine record of southern Australia is dominantly represented by coastal successions deposited during marine oxygen isotope stages 5e and 7, equating with relative sea‐level maxima of 2–6 m and greater than 1 m above present sea‐level (APSL) respectively. Intertidal facies of Late Pleistocene interstadial successions rarely occur above present sea level. The most reliable record of glacio‐eustatic (ice equivalent) sea level is registered on coastlines developed on stable Precambrian cratons such as the Gawler Craton (Eyre Peninsula), where a consistent elevation of 2 m APSL is registered for over 500 km for the last interglacial shoreline (125 ka). Copyright © 2002 John Wiley & Sons, Ltd.

OLIGO-MIOCENE CANYONS IN THE GAMBIER SUB-BASIN, SOUTHERN AUSTRALIA—DEEPWATER ANALOGUES FOR PETROLEUM EXPLORATION

The Gambier Sub-basin lies on the southern Australian passive continental margin that formed during continental breakup and seafloor spreading between the Australian and Antarctic plates. In addition to the numerous modern submarine canyons reported on the southern Australian margin, three palaeo-canyon systems have been identified within the Gambier Limestone of the South Australian Gambier Sub-basin. Favourable environmental conditions during the Oligocene and Early Miocene led to deposition of the Gambier Limestone, a widespread, prograding extra-tropical carbonate platform. A world-wide glacio-eustatic sea level fall in the Early Oligocene exposed the shelf in the Gambier Subbasin, causing widespread erosion and minor fluvial incision on the shelf and subsequent formation of nick points at the shelf edge. During the following marine transgression later in the Oligocene, the shelf was inundated and the nick points provided conduits for erosive turbidity currents to enlarge the canyons to the spectacular dimensions observed on seismic data. No less than 20 successive canyon cut and fill events ranging from Late Oligocene to Middle Miocene have been observed and mapped on seismic data across the shelf in the Gambier Sub-basin. The thick, dominantly fine-grained carbonate sheet logically represents a potential regional seal to underlying clastic reservoirs. However, the possibility exists for carbonate reservoir sands to be present within the palaeo-canyons, sealed by surrounding fine-grained carbonates. Although no hydrocarbons have yet been identified in the carbonates of the Gambier Sub-basin, the canyons provide an analogue useful for establishing the scale, internal architecture and geometry of canyon fill systems.

Palaeoceanographic significance of recent foraminiferal biofacies on the southern shelf of Western Australia: a preliminary study

The southern shelf of Western Australia lies close to the Subtropical Convergence, in a region strongly influenced by the warm Leeuwin Current from the north and the cold, more massive Western Australian Current from the south. Fresh Holocene and relict Pleistocene foraminiferal specimens are mixed in dredged sediment samples, similar in composition to those from other parts of the southern Australian margin. The Holocene planktonic assemblages are dominated in the west by subtropical forms ( Globigerinoides trilobus s.l., Globorotalia menardii and Neogloboquadrina dutertrei) and in the east by the temperate species Globorotalia inflata. Three Holocene benthic assemblages are distributed from the inner shelf to the upper slope, also showing a strong longitudinal gradation from west to the east. The change from warm-water assemblages to temperate assemblages is progressive and continuous, but the southwest corner is marked by the southerly limit of some larger benthic taxa including Heterostegina, and the area off Esperance is the furthermost extent of an abundant, though patchy, living Amphisorus– Marginopora association. This W → E gradation indicates that the Leeuwin Current has played a key role in influencing the distribution of foraminifera as from at least the last interglacial, because this pattern exists not only in the Holocene but also in the relict foraminiferal biofacies.

Oceanographic implications of recent planktonic foraminifera along the southern Australian margin

The distribution of planktonic foraminifera along the southern Australian margin is strongly gradational from warmer assemblages in the west to more temperate assemblages in the east. This pattern follows a decreasing temperature gradient generated by the warm Leeuwin Current which flows southward and then eastward on the southern margin. The abundances of tropical and subtropical species including Globorotalia menardii and Globigerinoides trilobus s.l. decrease rapidly after rounding the south-western corner, while the temperate index species Globorotalia inflata increases and subsequently dominates the fauna towards the east, especially on the Lincoln and Lacepede shelves of South Australia. This W–E gradation is also observed in the relict Pleistocene assemblages, indicating that the Leeuwin Current has been influential since at least the last interglacial time. The interaction between the warm and saline water from the Great Australian Bight and cold water from the south produced an assemblage in deeper parts of the Bight with abundant small species such as Globigerina falconensis, Globotur-borotalita rubescens, Neogloboquadrina pachyderma and Turborotalita quinqueloba.

Synergetic influence of water masses and Kangaroo Island barrier on foraminiferal distribution, Lincoln and Lacepede shelves, South Australia: A synthesis

The mid-latitude, cool-water, high-energy Lincoln and Lacepede Shelves along the southern Australian margin are covered with mixed Holocene and Pleistocene sediments. Foraminiferal specimens of Recent and Pleistocene age from dredge samples are strongly mixed. Eight assemblages in two major groups A and B were recognised from the inner shelf to the upper slope, and they appear to be depth-related. However, some are typical of localised environments such as an upwelling zone (assemblage A2) or areas strongly affected by a high accumulation of relict specimens indicative of previous lagoonal deposition (assemblages A3 and A4). Sediment starvation due to strong wave abrasion is largely responsible for the preservation of this mixed biofacies. The interaction between shelf waters (the Great Australian Bight Current), waters from the Spencer and St. Vincent Gulfs, oceanic waters and waters from the River Murray, is suggested as the main factor controlling the overall distribution pattern of foraminifera. A warmer, nearly oligotrophic condition on the Lincoln Shelf, signalling a warmer regime, is indicated by a higher abundance of warmwater planktonic species and by a higher faunal diversity. On the Lacepede Shelf, the diversity is lower and warm-water forms are less common, implying a cooler or mesotrophic condition. These differences may be due to the position of Kangaroo Island which blocks the passage of the shelf water flowing to the southeast, thereby creating two different water regimes. This barrier would have been more significant during the late Pleistocene, when sealevel was low, the shelves were narrower and a lagoonal environment was present as indicated by different relict faunas on these two shelves.

Eocene and younger biostratigraphy and lithofacies of the Cascade Seamount, East Tasman Plateau, southwest Pacific Ocean

Six dredgings, five taken during fishing operations, have recovered sedimentary rocks from the volcanic framework of Cascade Seamount, which is built on thinned continental crust on the East Tasman Plateau off eastern Tasmania. The top of the original volcano is now about 600 m below sea‐level. Planktonic foraminifers and supplementary calcareous nannofossils yielded Late Eocene‐Early Oligocene, mid‐Late Oligocene, Early/Middle Miocene, approximately Late Miocene and Quaternary ages. The history of the seamount is more complex than for most seamounts built on oceanic crust and may involve several intervals of volcanism, burrowing or dissolution and cavity infilling. Some of the volcanism was subaerial and some submarine. Older rocks include conglomerate, volcaniclastic sandstone, and a variety of interstitial sparry calcite, formed under a shallow‐water, high‐energy regime, whereas later sediments are mainly oozes. The oldest sedimentary rocks are Late Eocene, shallow water, fully marine with calcareous algae and abundant echinoid debris. Two phases, Chiloguembelina‐dominated and Globigerinatheka‐dominated, can be differentiated. Warm waters prevailed in the Late Eocene and in the Middle Miocene, cooler conditions between and since, consistent with other indications around the southern Australian margin. Post‐depositional alteration consists of phosphatisation and development of ferromanganese crusts.

Foraminiferal biostratigraphy and depositional environments of the mid‐Cenozoic Abrakurrie Limestone, Eucla Basin, southern Australia

The Abrakurrie Limestone from the central part of the southern Australian margin contains abundant bryozoan, molluscan and other calcareous skeletons including many poorly preserved foraminifers. Limited planktonic and benthic foraminiferal data suggest that the limestone was deposited during the Late Oligocene and Early Miocene. It is believed to have been deposited at about 28–21 Ma, as the local manifestation of eustatic second‐order supercycle TB1. The limestone accumulated mainly in water depths around 100 m, for its benthic fauna was largely composed of mid to outer shelf taxa, particularly the cibicidids and eponidids. There was a major faunal change between the infauna‐dominated assemblage in the middle, and probably also lower, Abrakurrie Limestone and the epifauna‐dominated assemblage in the upper member. Amphistegina lessonii made its first but rare and episodic appearance in the lower member, and the species became abundant and widespread only in the upper member. Likewise, such species as Elphidium spp. and Pararotalia mackayi, which prefer a warmer water habitat, become more common in the upper member. Therefore, a well‐lit, warmer, oligotrophic environment is inferred for the upper Abrakurrie Limestone, while the infauna‐rich assemblages from the middle and lower members indicate relatively cooler, mesotrophic conditions similar to those which were prevailing in other parts of southern Australia. The scarcity of the plankton together with a low benthic diversity may indicate that the Eucla Basin was a swell‐dominated, embayed shelf during the deposition of the Abrakurrie Limestone.

Modern bryozoan assemblages and distribution on the cool‐water Lacepede Shelf, southern Australian margin

Modern and relict sediments on the cool‐water Lacepede Shelf are dominated by fragments of bryozoans, molluscs and foraminifers. This study evaluates the distribution of modern bryozoan sediment on the Lacepede Shelf based on 88 species from 16 localities using cluster analysis and principal component analysis. Six bryozoan faunal assemblages, corresponding roughly to physiographic position on the shelf, are recognized objectively. Assemblages are robust and generally defined by the changing diversity and relative abundance of the bryozoan fauna across the shelf. Slope assemblages can be differentiated from shelf assemblages based on presence or absence of a few key species. Lateral variation along the three transects is associated with: (i) differences in shelf geometry (shelf width, depth of shelf edge and angle of the slope); (ii) previous influx of continental sediments; and possibly (iii) presence of upwelling and long‐shelf currents. Data were also evaluated to determine the level of detail required to provide the optimal information, with the most efficiency, from diverse bryozoan faunas. Presence‐absence data for species prove nearly as effective in determining bryozoan assemblage distributions as analyses that incorporate taphonomic and abundance information. In addition, similar distribution patterns were observed using as few as 12 key species. The distribution of bryozoan species provides additional insight into the composition of carbonate skeletal particles on the Lacepede Shelf and the ecology of the diverse bryozoan fauna. These data are contributing to our understanding of the development of cool‐water carbonates.

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...

AUSTRALIA'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.

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.

STRUCTURE AND DEVELOPMENT OF THE SOUTHERN MARGIN OF AUSTRALIA

Off the southern coast of Australia, exploration results and deep-water reconnaissance seismic data support the concept of an aseismic Atlantic-type continental margin. Characteristic is a sedimentary wedge which extends from the shelf to the abyssal plains and includes crustal elements of continental and oceanic origin.Oceanward, a continuous level of diffractions ascribed to the top of oceanic crust can be observed on seismic records, steeply dipping beneath the continental rise towards a smooth, flat, often faulted reflector which is correlated with top Precambrian or Palaeozoic continental basement.The sedimentary wedge which overlies the block-faulted and collapsed continental basement is subdivided by unconformities into: (a) a continental Lower Cretaceous unit and a fluviodeltaic unit of Upper Cretaceous-Danian age which are taken to represent rift valley stages of deposition controlled by extensional tectonics and (b) a post-breakup sequence of Tertiary units representing regional collapse and out-building of the shelf. The Upper Cretaceous sequence is missing along much of the continental edge where Tertiary sediments appear to rest directly on the Lower Cretaceous unit.Our interpretation suggests that a prolonged period of uplift took place along the axis of the rift valley prior to continental break-up. On the basis of palaeomagnetic data and biostratigraphic analysis the breakup phase started in the Upper Paleocene.From the continent outward several structural zones can commonly be recognised: (a) a zone of shallow basement with a thin Lower Cretaceous cover normally faulted and overlain by thin gently dipping Tertiary beds, (b) a zone of faulted and landwards tilted basement blocks and Lower Cretaceous sediments overlain (sometimes with clear unconformity) by thick Upper Cretaceous sediments, (c) a zone of thick, moderately deformed Tertiary sediments whose axis of deposition is generally offset to the south of the Upper Cretaceous basinal axis, (d) a zone of rotational faults and associated toe thrusts affecting the Cretaceous sediments and apparently related to the time of margin collapse, (e) an area of little disturbed Cretaceous and Tertiary sediments overlying continental basement. This zone extends into the "magnetic quiet zone" which is therefore believed to be, at least in part, a collapsed portion of the continental margin adjacent to oceanic crust.The interpretation of the geological evolution of the southern Australian margin based on the stratigraphic and structural data presently available can be related to current theoretical models on continental margin development.