Central Great Australian Bight Research Articles

Calcinean sponges (Porifera: Calcarea) from the shelf edge of the Great Australian Bight

Abstract The deep-water regions of eastern and central Great Australian Bight (GAB) are subject to hydrocarbon exploration; consequently, there are efforts to study their biodiversity. Australia has ~200 species of Calcarea, but none has been described taxonomically from the GAB, and most are known down to only 200 m depth. A characterization of deep benthic communities was performed in the GAB at depths ranging from 200 to 3000 m. Of sponge species collected, 16% were members of Calcarea, and 11 were members of subclass Calcinea, which is the focus group of this work. Morphological and molecular tools were used for identification. Calcinean sponges were not found deeper than 400 m. The most diverse genus was Ascoleucetta, which had four species. Nine species and two genera are new to science: Clathrina williamsi sp. nov., Leucascus boomerang sp. nov., Ascoleucetta gabensis sp. nov., Ascoleucetta globularis sp. nov., Ascoleucetta parallela sp. nov., Ascoleucetta plana sp. nov., Leucetta conspicua sp. nov., Robspongia vociva gen. nov., sp. nov., and Rappya australiensis gen. nov., sp. nov. Calcareous sponges represented 1.8% of overall sponge biomass, and site 181 had the highest species richness. Our results reveal a unique community of calcareous sponges hidden within the GAB, suggesting high endemism.

Spatio-temporal variability in the demersal fish assemblage on the outer continental shelf of the Great Australian Bight

We examined spatial and temporal variations in the demersal fish assemblage on the continental shelf of the central Great Australian Bight to understand how the assemblage is affected by both fishing and environmental gradients. Data from the Great Australian Bight Trawl Sector (1988–2018) and fishery-independent trawl surveys (2005–2009, 2011, 2015, 2018) were used for the analyses. The independent survey data were used to analyse trends in overall species composition and abundances, while the commercial fishery data were used to extend the time series for the key commercial species, Deepwater Flathead (Platycephalus conatus) and Bight Redfish (Centroberyx gerrardi). The demersal fish assemblage was dominated by four commercial species: Deepwater Flathead, Bight Redfish, Ocean Jacket (Nelusetta ayraud), and Latchet (Pterygotrigla polyommata); and one by-catch species: Wide Stingaree (Urolophus expansus). Assemblage composition varied between day/night and along an east-west gradient. Survey abundance and commercial catch-per-unit-effort of several species declined at the end of the time series. Survey abundance was low in 2011, 2015, and 2018 for Bight Redfish and in 2015 and 2018 for Deepwater Flathead. Assemblage composition and catch rates of some species recorded in 2011, 2015, and 2018 were distinct from previous years, but the differences appear to reflect the longer gaps between these surveys and the combined effects of historical fishing pressure and environmental variability. Recent downward trends in the abundance indices of target species, as well as long-term changes in the assemblage, demonstrate the need for continued fishery-independent monitoring. The relative importance of fishing pressure, environmental variability, and other human activities in driving these changes warrant further investigation.

Large scale seabed processes in a deep cool water carbonate ramp system: A case study of the Great Australian Bight

Among all the modern cool water carbonate margins in the world, the Great Australian Bight (GAB) presents several unique characteristics: a near complete absence of terrigenous input, highly energetic swell conditions producing vigorous off-shelf sedimentary transport to the upper continental slope, and overall high-sedimentation rates during the Pleistocene. All these conditions promoted biota and facies that are more typical of a highly productive carbonate margin. High-resolution bathymetric data collected from marine voyages between 2015 and 2017 revealed that the continental slopes of the central GAB are characterised by numerous channels and canyons as well as a large number of sedimentary mass-wasting features including kilometre scale submarine landslides and slumps. The morphometric and structural analysis of these seabed features across the continental slopes suggest that the three key controlling factors are: deep-rooted faults breaching the seabed, increasing slope gradient at the beginning of the lower continental slope, and channel wall erosion. Pulses of rapid sedimentary accumulation on the upper continental slopes, as well as differential uplift and subsidence combined with sea-level changes, appear to be the main trigger mechanisms for mass wasting.The GAB is the largest cool-water carbonate ramp system in the world. As such, the scale of sedimentary erosion and transport through channels and canyons caused by mass wasting identified in this work represents a major mechanism of carbon transfer to the deep oceans. Whilst high carbonate sedimentation during the Pleistocene in the GAB could be an important CO2 sink in the global ocean, high sedimentation on the outer shelf and continental slope could also increase mass wasting which could act as an effective mechanism for carbon transfer to the deep oceans. However, continental slopes in the Great Australian Bight have, until recently, remained poorly studied.

Benthic biogeographic patterns in the southern Australian deep sea: Do historical museum records accord with recent systematic, but spatially limited, survey data?

AimTo document biogeographic patterns in the deepwater benthic epifauna and demersal fishes of southern Australia, and determine whether museum records and systematic survey data provide matching results.LocationSouthern Australian (32–44oS) continental slope (200–3,000 m deep).TaxonMarine benthic fauna (Arthropoda, Bryozoa, Cnidaria, Echinodermata, Mollusca, Porifera, Sipuncula, and fishes).MethodsAll available electronic records of fauna from the above taxa and ≥200 m depth off the southern Australian coastline, regardless of organism size, were collated from Australian museums and checked for geographic and taxonomic consistency. These records were then split into 40 geographic segments of roughly equal numbers, with each segment then treated as a sample in multivariate analyses of assemblage composition. Data from a recent (2015) systematic beam trawl survey along five north–south transects in the central Great Australian Bight were also included for comparison.Main conclusionsThe systematic survey data grouped with the associated geographic segments despite differences in sampling technique (single gear compared to multiple gears), with subsequent differences in taxonomic biases, and the use of a 25 mm mesh, which would undersample some smaller organisms present in the museum data. Thus, the museum data and the survey data provided the same results for the central Great Australian Bight at the level of the whole assemblage. The main biogeographic break occurred off southeastern Tasmania, with a second substantial break occurring at around the border between New South Wales and Victoria. This indicates the potential for unused museum data to describe biogeographic patterns over regional spatial scales, especially in the deep sea where the expense of collecting new data is relatively high.

Micronekton assemblages and bioregional setting of the Great Australian Bight: A temperate northern boundary current system

A multi-disciplinary pelagic ecosystem study of the Great Australian Bight (GAB) examined the micronekton communities and bioregionalisation in this area that harbours significant predator populations. Micronekton community structure and biomass differed between the eastern and central regions of the GAB. Regional differences were mainly driven by relatively high abundance of krill (Nematoscelis megalops, Nyctiphanes australis and Euphausia similis) in east GAB and high abundance of Maurolicus australis and chaetognaths in central GAB. Central GAB was also characterised by relatively high abundance of gelatinous zooplankton and small-bodied cephalopods. Lanternfishes were used as a model pelagic group to investigate the bioregional setting of the GAB. The analyses indicated that the oceanic habitats of the GAB had bioregional affinities with the subtropical Indian Ocean. However, over the continental slope, the east GAB had bioregional affinities with the Subtropical Convergence and grouped with western and southern Tasmania and the southern Tasman Sea. Lanternfish assemblage groups were compared against existing regionalisations that are based on physicochemical variables and the Delphi Method. No single existing scheme suitably reflected lanternfish assemblage distributions, but parsimonious boundaries from existing schema were identified. This study demonstrates that the central GAB, where oil-and-gas exploration occurs alongside Marine Protected Area management, needs to be considered as a separate bioregion to the east GAB for the purposes of identifying conservation values and monitoring requirements.

Observations of vertical turbulent nitrate flux during summer in the Great Australian Bight

To assist with developing an understanding of the nutrient supply dynamics in the Great Australian Bight (GAB) the vertical turbulent nitrate flux was calculated at stations along two cross-shelf transects using direct measurements of turbulence and nitrate concentrations. Coincident hydrographic profiling conducted at the onset of the summertime upwelling season revealed upwelling onto the shelf in the eastern GAB (eGAB) and downwelling at the shelf slope in the central GAB (cGAB). Under these conditions, examination of the strength of competing vertical temperature and salinity gradients using the Turner angle suggested a high potential for double diffusive convective mixing. However, corresponding estimates of the mixing efficiency derived from microstructure profiling indicated turbulence was the main mixing process driving vertical fluxes across the region. The average upward nitrate flux between the surface mixed layer depth(MLD) and the base of the euphotic layer (92 m depth) was O(10−6−10−5) mmol N m−2 s−1 in the cGAB, with the peak flux observed at the upper-slope station (~ 400 m total water depth). In the eGAB, the magnitude of flux estimates at the shelf-slope and offshore stations were similar to those measured in the cGAB. An enhanced nitrate flux O(10−4) mmol N m−2 s−1 was observed on the shelf in the eGAB (~ 100 m total water depth) as the result of enhanced turbulence and an increased vertical nitrate gradient. Application of Redfield stoichiometry suggests vertical turbulent nitrate fluxes play a significant role in supporting up to 50% of the primary production in the GAB. We hypothesize that mixing processes resulting from interactions between upwelled and downwelled water masses and new water masses formed on the shelf during summer play an important role in maintaining the bands of subsurface chlorophyll a maxima observed below the MLD at the nitricline during periods of strong stratification. The snapshot provided by this study, emphasizes the need to better understand the variability and influence of mixing processes on nutrient supply and productivity across the GAB marine ecosystem.

Characterising the invertebrate megafaunal assemblages of a deep-sea (200–3000 m) frontier region for oil and gas exploration: the Great Australian Bight, Australia

The first systematic benthic survey in the deep sea (200–3000 m) of Australia’s Great Australian Bight (GAB) was undertaken in 2015 to characterise the invertebrate megafauna, and to inform the selection of indicators and metrics for ecological monitoring ahead of oil and gas exploration. The survey yielded more than 629 species of invertebrate megafauna; of the 376 species with distributional data, 92 (25%) were undescribed and 77 were new records for Australian waters. The families and genera present were all known to occur in the deep sea and many species had been previously recorded in Australia and worldwide; faunal composition was broadly typical for temperate deep-sea regions. The highest diversities (>80 putative species, or Operational Taxonomic Units, OTUs) were recorded within the higher taxa Demospongiae, Decapoda, Gastropoda and Echinodermata. Multispecies analyses showed clear changes in the assemblage structure with depth; sponges and echinoderms dominated the overall biomass and density, with the former being more prominent in shallower depths. The assemblage structure is consistent with the GAB being a single provincial-scale bioregion, with no longitudinal pattern in assemblage, biomass or density distribution. Approximately 70% of species that could be assigned biogeographic data were previously recorded from Australia, with less than half (146 species, 39%) previously known from the GAB. Only two described species, the crab Choniognathus granulosus and barnacle Arcoscalpellum inum, appear restricted to the GAB; it would be premature to assign any undescribed species as having endemic status. The clear eastwards biogeographic affinity of the GAB fauna is influenced by the relatively high deep-sea sampling effort to the east off southeastern Australia and New Zealand. Our survey of invertebrate megafauna at baseline (unperturbed) sites provides the basis to evaluate indicators and metrics using a reference-site monitoring approach. A robust (consistent species-level) taxonomic foundation will enable a variety of assemblage-level (composite) metrics (e.g. richness, diversity, distinctness) to be derived, and this is possible across several major taxa including Porifera, Cnidaria, Mollusca, Echinodermata and Crustacea. Species-level data also permit structural and functional changes (including recovery) to be assessed in response to disturbance. Where reference sites should be established can only be determined once the exploration phase of industry development is further advanced because the spatial scales of potential impact are highly activity-specific. However, our data show the high importance of depth to selecting monitoring sites because invertebrate megafaunal assemblage composition (turnover), diversity, and abundance are all highly correlated with depth. Conversely, latitude is not important because the central GAB is a single biogeographic province.

Composition, diversity and biogeographic affinities of the deep-sea (200–3000 m) fish assemblage in the Great Australian Bight, Australia

The deepest systematic collection of benthic fishes in Australian waters (108 species from 48 families in 200–3000 m depths) was taken by beam trawl during two surveys in the central Great Australian Bight (GAB) in 2015. All samples were on sediment habitats, but some were in close proximity to volcanic seamounts and outcropping rocky seabed in submarine canyons. The great majority of species were previously recorded from Australian waters (90%), and the GAB (75%). A markedly lower proportion of recorded species occurred at greater depths (1700–3000 m) where there had been virtually no previous sampling: 74% in Australian waters, and 30% in GAB waters. The fauna is dominated by families that typify many areas of the global deep sea: rattails (Macrouridae), cut-throat eels (Synaphobranchidae), morid cods (Moridae), oreo dories (Oreosomatidae), slickheads (Alepocephalidae), cusk-eels (Ophidiidae) and halosaurs (Halosauridae). The Macrouridae was most diverse, occurred most frequently, was most abundant (density) in 400 m and all deeper strata, and top ranked by biomass and density in standardised data. The predominant patterns of biogeographical affinities were strongly influenced by depth. Endemic species were most prevalent in shelf break and upper to mid-slope depths (both 52%), and declined with increasing depth (upper to mid-slope species 25%; lower slope/rise 4%), consistent with a general pattern in the Australian ichthyofauna. Fish biomass (gm−2) was significantly related to depth, being relatively very low (~0.5 gm−2) at 200 m depth, highest at 400 m (~3.4 gm−2), then steadily declining with increasing depth to about 0.4 gm−2 at 3000 m depth. Depth was the main factor explaining assemblage structure. There was no clear distinction of assemblages at sites close to structured habitats, and no longitudinal pattern across a gradient of productivity (higher in the east due to summertime upwelling). The GAB is presently being considered for oil and gas exploration in areas extending from the shelf break (200 m) to beyond 4000 m depths. The current data set has the potential to inform biodiversity management in the GAB, including the design of an ecological monitoring program prior to oil and gas exploration, because it includes samples from within and immediately adjacent to the Great Australian Bight Australian Marine Park (GAB AMP) and six active lease blocks. The data set outlined provides a quality assured inventory of fishes, mapping species distributions and associations with habitats in the area of interest, and identifies potential sites of impact and suitable control sites inside and outside marine parks.

Cross-shelf seawater exchange controls the distribution of temperature, salinity, and neritic carbonate sediments in the Great Australian Bight

The seasonally averaged wind stress of the Great Australian Bight (GAB) during the austral winter is directed to the east along the shelf and results in downwelling that extends to depths of 250 m. This downwelling is enhanced in the eastern GAB through the outflow of cold saline water formed in the broad shallow regions of the GAB and gulfs. During the austral summer, the averaged wind stress field of the GAB is anticyclonic with upwelling favorable winds along much of the coastline. In general however, significant slope upwelling is only observed in the eastern and western GAB where the shelf is narrow. Upwelling there provides nutrients to support the planktic and pelagic communities and the observed distribution of benthic invertebrate communities that lead to the formation of neritic carbonate sediments. At the shelf break of the central GAB where the shelf is very wide, the observed cross-shelf distributions of temperature, salinity, and sediments indicate that downwelling occurs year round. The implied lack of nutrients is argued to explain the smaller communities of invertebrates found at the shelf edge (100–250 m) in these areas. A previous numerical study is cited to show that summer downwelling very likely results from a convergence of the deep ocean and poleward Sverdrup transports. The unique aspect of this integrated oceanographic-sedimentological investigation is to tie outcomes of that numerical study with extant observations and benthic habitat to provide a consistent picture of how cross-shelf exchange regulates water properties and benthos in the GAB.

Variations in rift symmetry: cautionary examples from the Southern Rift System (Australia–Antarctica)

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.

Analysis of hydrographic and stable isotope data to determine water masses, circulation, and mixing in the eastern Great Australian Bight

Hydrographic and stable isotope data from waters in the eastern Great Australian Bight (GAB) sampled during March–April 1998 indicate that both mixing and evaporative processes are important on the shelf. Five water masses are defined on the basis of their temperature, salinity, δ2H and δ18O values. Two of these are end‐members, the Flinders Current (FC) and the Great Australian Bight Plume (GABP), whereas the other three are a result of mixing between these two end‐members. Water mass distribution reflects an anticyclonic gyre in the eastern GAB. Cool and fresh water present at depth along the Eyre Peninsula is sourced from upwelling of Flinders Current water directly from the shelf break. This water is progressively heated, evaporated, and mixed with warmer and more saline shelf waters as it flows around the gyre. High temperatures, salinities, and δ2H values in surface waters in the central GAB suggest that the Great Australian Bight Plume has a greater spatial extent than previously recorded, also occurring along the shelf edge between 130°E and 133°E. A high temperature, high salinity, low δ2H water mass that is isotopically similar to the Flinders Current occurs in the west of the study area, indicating intrusion of Flinders Current water into the central GAB. Differences in isotopic compositions of off‐shelf water suggest that the Great Australian Bight Plume is flowing off the shelf and mixing with Flinders Current water at ∼132°E; however, this outflow does not generate an eastward flowing current during the period when samples were collected.

Cyclicity in Pleistocene upper-slope cool-water carbonates: Unravelling sedimentary dynamics in deep-water sediments, Great Australian Bight, Odp Leg 182, Site 1131A

Ocean Drilling Program Site 1131, located in the central Great Australian Bight, recovered an expanded series of Pleistocene cool-water carbonates. Three distinct facies are arranged in repeating sedimentary cycles. Omission intervals form the base of the cycles and are overlain by laminated deposits. The upper part of the cycles consists of bioturbated sediments. Facies cyclicity parallels glacial–interglacial sea-level changes. Firmground/hardground surfaces formed during latest stages of sea-level falls. Laminated sediments were deposited during later sea-level fall and/or early phases of the sea-level lowstand, and the bioturbated intervals are related to interglacial sea-level highstands. Bottom current activity is interpreted to play an important control on facies and cycle development. During the late stages of sea-level fall, upwelling currents winnowed the slope sediments, leading to hardground formation. During latest sea-level falls and early sea-level lowstands, a relative increase in the ramp sediment supply to the slope leading to the formation of the laminated facies was also a response to the continued action of the near-bottom upwelling currents that reworked previously deposited sediments along the slope. During the early sea-level rises, laminated, upwelling-influenced facies were progressively covered by the bioturbated facies. The adopted approach to decipher sedimentary cyclicity, involving the integrated use of wireline logs, geochemical data, FMS images and sedimentological analysis of the lithofacies has allowed to recognise subtle facies changes in upper-slope settings of a distally-steepened carbonate ramp and their relationship with the sea level, as well as to unravel the importance of the oceanic-current regime in the deposition style on the ramp that otherwise would remain unsolved. This integrated method could be applied to study more precisely apparently homogeneous fine-grained calcareous sediment successions of distal-ramp outcrops.

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

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.

Pelagic ecology of a northern boundary current system: effects of upwelling on the production and distribution of sardine (Sardinops sagax), anchovy (Engraulis australis) and southern bluefin tuna (Thunnus maccoyii) in the Great Australian Bight

AbstractShelf waters of southern Australia support the world's only northern boundary current ecosystem. Although there are some indications of intense nitrate enrichment in the eastern Great Australian Bight (GAB) arising from upwelling of the Flinders Current, the biological consequences of these processes are poorly understood. We show that productivity in the eastern GAB is low during winter, but that coastal upwelling at several locations during the austral summer–autumn results in localized increases in surface chlorophyll a concentrations and downstream enhancement of zooplankton biomass. Sardine (Sardinops sagax) and anchovy (Engraulis australis) eggs and larvae are abundant and widely distributed in shelf waters of the eastern and central GAB during summer–autumn, with high densities of sardine eggs and larvae occurring in areas with high zooplankton biomass. Egg densities and distributions support previous evidence suggesting that the spawning biomass of sardine in the waters off South Australia is an order of magnitude higher than elsewhere in southern Australia. Sardine comprised >50% of the identified prey species of juvenile southern bluefin tuna (SBT, Thunnus maccoyii) collected during this study. Other studies have shown that the lipid content of sardine from the GAB is relatively high during summer and autumn. We suggest that juvenile SBT migrate into the eastern and central GAB during each summer–autumn to access the high densities of lipid‐rich sardines that are available in the region during the upwelling period. Levels of primary, secondary and fish production in the eastern GAB during summer–autumn are higher than those recorded in other parts of Australia, and within the lower portion of ranges observed during upwelling events in the productive eastern boundary current systems off California, Peru and southern Africa.

Origin of Late Pleistocene Bryozoan Reef Mounds; Great Australian Bight

Bryozoan-rich biogenic mounds grew periodically on the prograding carbonate slope of the central Great Australian Bight throughout Pliocene-Pleistocene time. Cores from three ODP Leg 182 drill sites provide a record of mound growth during the last 300,000 years over a stratigraphic thickness of similar to 150 m. These mounds, the first such structures described from the modern ocean, grew between paleodepths of 100 and 240 m; we infer that the upper limit of growth was established by swell wave base, and the lower boundary was fixed by an oligotrophic water mass. Detailed chronostratigraphy, based on radiometric and U-series dating, benthic foraminifer stable-isotope stratigraphy, and planktonic foraminifer abundance ratios, confirms that buildups flourished during glacial lowstands (even-numbered marine isotope stages) but were largely moribund during interglacial highstands and are not extant today. Mound floatstones are compositionally a mixture of in situ bryozoans comprising 96 genera and characterized by fenestrate, flat robust branching, encrusting, nodular-arborescent, and delicate branching growth forms. The packstone matrix comprises autochthonous and allochthonous sand-size bryozoans, benthic and planktonic foraminifers, serpulids, coralline algae, sponge spicules, peloids, and variable glauconite and quartz grains, together with mud-size ostracods, tunicate spicules, bioeroded sponge chips, and coccoliths. Intermound, allochthonous packstone and local grainstone contain similar particles, but they are conspicuously worn, abraded, blackened, and bioeroded. An integrated model of mound accretion during sea-level lowstands begins with delicate branching bryozoan floatstone that increases in bryozoan abundance and diversity upward over a thickness of 5-10 m, culminating in thin intervals of grainstone characterized by reduced diversity and locally abraded fossils. Mound accumulation was relatively rapid (30-67 cm/ky) and locally punctuated by rudstones and firmgrounds. Intermound highstand deposition was comparatively slow (17-25 cm/ky) and typified by meter-scale, fining-upward packages of packstone and grainstone or burrowed packstone, with local firmgrounds overlain by characteristically abraded particles. Mound growth during glacial periods is interpreted to have resulted from increased nutrient supply and enhanced primary productivity. Such elevated trophic resources were both regional and local, and thought to be focused in this area by cessation of Leeuwin Current flow, together with northward movement of the subtropical convergence and related dynamic mixing.

Surficial Sediments of the Great Australian Bight: Facies Dynamics and Oceanography on a Vast Cool-Water Carbonate Shelf

ABSTRACT The Great Australian Bight (GAB), the largest sector of the southern Australia continental margin, is a site of cool-water carbonate sedimentation throughout, ranging from locally warm-temperate inboard to cool-temperate outboard. Surficial sediments are a mixture of calcareous Pleistocene skeletal and lithic intraclasts (relict grains), and Holocene biofragments, with minor amounts of quartz inboard. The inner is an area of abundant macrophytes and seagrasses, active carbonate sediment production and accumulation, and little relict sediment. The huge middle portion is a shaved shelf with active sediment winnowing and mostly relict sediment. The outer and upper slope is a variably productive sediment factory characterized by prolific calcareous epibenthic growth on hard substrate subaqueous islands shedding particles into surrounding sands and muds. Patterns of Holocene sedimentation are linked to modern oceanographic parameters in this high-energy setting characterized by overall downwelling. Prolific rhodoliths occur on the NW inner shelf, where shallow summer waters are the warmest in the GAB. These warm, saline, nutrient-depleted waters then drift eastward across the shelf, suppressing heterozoan carbonate production on the central and eastern mid-shelf. This arrested production in the eastern GAB is countered locally by summer coastal upwelling along western Eyre Peninsula, with bryozoan-rich sediment extending well inboard onto the mid-shelf. The outer and upper slope is an area of prolific bryozoan growth, likely linked to upwelling, except in the central GAB, a region of year-round downwelling, where the area is one of off-shelf fine sediment transport and carbonate mud deposition. These patterns, in the central GAB at least, are present in the underlying Holocene and Pleistocene, suggesting that the general modern oceanographic dynamics and resultant carbonate sedimentation have persisted throughout the Quaternary.

Nature of the continent-ocean transition on the non-volcanic rifted margin of the central Great Australian Bight

Abstract A region of 50–120 km width defines the continent-ocean transition (COT) in the central Great Australian Bight. It is characterized by a thin apron of post-break-up sediments overlying complexly deformed sediments and intruded crust bounded landward by a basement ridge complex and oceanward by rough oceanic basement. Recently acquired deep reflection and refraction seismic data have significantly enhanced understanding of the COT and basement ridge. Modelled gravity and magnetic data, and features interpreted from seismic data, are consistent with aspects of extensional and break-up models proposed for the West Iberia margin. Many of the features and relationships observed beneath the outer margin of the central Great Australian Bight can be explained by extension within a lithosphere-scale ‘pure-shear’ environment involving four layers: brittle upper crust and upper mantle, and ductile lower crust and lower lithospheric mantle. The COT is interpreted to be underlain by extended continental lithosphere. Thus, the continent-ocean boundary is unequivocally defined between oceanic crust and the COT and appears to be associated with sea-floor spreading magnetic anomaly 33, indicating that break-up and sea-floor spreading did not commence until c. 83 Ma (early Campanian time), later than the currently accepted 95 Ma age. The major part of the basement ridge complex is probably a combination of serpentinized peridotites and mafic intrusions or extrusions derived by mantle upwelling and limited partial melting. The magmatic products of this process probably cooled during chron 34 producing a distinctive magnetic anomaly, but one that does not relate to break-up and sea-floor spreading.

A case for Australia-Antarctica separation in the Neocomian (ca. 125 Ma)

The age of separation of Australia and Antarctica (that is, the onset of seafloor spreading) has usually been determined by the identification of seafloor-spreading magnetic anomalies adjacent to the margin, or by extrapolation of the spreading rate/time span between the oldest identified anomaly and the continent-ocean boundary (COB) as interpreted from magnetic and single-channel seismic data. The most recent estimate of the age of breakup using these methods is 95 ± 5 Ma, in the Cenomanian. However, identification of the oldest magnetic anomalies, formed during the early phase of slow drift between Australia and Antarctica, is tenuous, particularly between southwest Australia and the central Great Australian Bight (GAB). An alternative approach for establishing the age of breakup is to determine the relationship of the continental margin sequences and the oceanic crust, using seismic data. Application of this technique to the western Great Australian Bight demonstrates that it is likely that the oldest interpreted oceanic crust is overlain by several hundred metres of sediment that is no younger than Valanginian (ca. 125 Ma); that is, separation of Australia and Antarctica must have occurred prior to this time, perhaps contemporaneously with the separation of Australia and “Greater India” off Western Australia. The similarity of structural and magnetic anomaly trends between the southern and western margins of Australia further suggests that breakup and spreading on those two margins are closely linked. However, there may be further complicating factors in the Australian-Antarctic extension and breakup history. High-quality migrated seismic data from the central Great Australian Bight, across what has been previously interpreted as oceanic crust, show the presence of listric faulting and apparent syn-rift sedimentation within “basement”. The gross structure is similar in appearance to published models of a metamorphic core complex. Other possibilities are that the observed structuring is the product of the re-rifting of oceanic crust of undetermined (pre-Valanginian) age, or that the zone oceanwards of the “COB” consists of highly thinned lower plate, probably injected by ribbons of oceanic basalt towards its southern edge. If such structures can also be identified in the western GAB, then it is clear that the age of the emplacement of the first oceanic crust between Australia and Antarctica is still undetermined.

Australia's southern margin: a product of oblique extension

Recently developed detachment models of continental margin formation interpret the southern margin of Australia to have formed when the lower-plate Australian margin was pulled out from beneath the upper-plate Antarctic margin. Data now available and summarised in this paper, point very strongly to a generally NW-SE direction of initial continental extension for the southern margin, in contrast to the widely held picture of simple N-S rifting. The evidence for this extension direction comes from the analysis of deep Seismic data acquired by the Bureau of Mineral Resources in 1986 in the central Great Australian Bight (GAB), the gravity field of the GAB, Seismic and magnetic basement structures in the Eyre Sub-basin, Polda Trough, Ceduna Depocentre and Duntroon Basin and from the analysis of the magnetic seafloor spreading anomalies produced during the slow first phase of drifting between Australia and Antarctica.Further, it is now believed that the formation of the southern margin of Australia can be described in terms of three phases of continental extension (El to E3) and two phases of drifting (D1 and D2). In summary, these phases were as follows. 1.E1: approximately 300 km of Late Jurassic (?or older) to Early Cretaceous NW-SE-oriented extension in the GAB, with strike-slip motion in the nascent Otway Basin and along the Tasmanian margin.2.E2: 120 km of Early Cretaceous NNE-SSW-oriented extension which formed the basins of southeastern Australia (Otway, Bass, Gippsland) and which probably produced a structural overprinting in the GAB Basin.3.E3/D1: minor continental extension and the first 500 km of slow drift between Australia and Antarctica on an azimuth of 165°; wrenching on the Tasmanian margin.4.D2: 2600 km of fast drifting between Australia and Antarctica on a N-S azimuth.