Southeast Australian Margin Research Articles

Preferential Riverine Export of Fine Volcanogenic Particles to the Southeast Australian Margin

We report on rare earth element and neodymium isotopic compositions in a series of grain-size fractions separated from river suspended matter in the Murray-Darling Basin (MDB) and a nearby marine sediment core (MD03-2607) offshore south-eastern Australia. This source-to-sink approach was used to further investigate the extent to which sediment sorting may control the geochemistry of suspended loads in rivers, and to provide constraints on the source of the detrital sediment material exported to the ocean. Our results provide further compelling evidence that significant size-dependent geochemical decoupling can occur in river systems, accounting here for Nd isotopic (Nd) differences of up to 8 epsilon-units between silt (>25µm) and colloidal (0.2-0.006 µm; 0.006-0.003 µm) fractions. All suspended particulate samples from the River Murray watershed display a trend towards more radiogenic (higher Nd) Nd signatures with decreasing grain-size, in addition to differing REE signatures, which collectively point towards a preferential volcanogenic origin for the fine-grained inorganic particles transported by MDB rivers. Furthermore, we show that the same river-borne volcanogenic material dominates in the fine-grained detrital fractions extracted from core MD03-2607 at the south-eastern Australian margin; a finding corroborated by REE signatures in a series of copepod faecal pellet separates from the same core. Collectively, our results suggest that river sediment discharge is accompanied by preferential export of fine-grained volcanogenic particles to the ocean. This potential source of bioavailable trace metals and nutrients in ocean surface waters could impact marine productivity.

Phosphate‐rich deposits associated with the Mio‐Pliocene unconformity in south‐east Australia

AbstractPhosphates are present on the surface of the Mio‐Pliocene unconformity in the Otway, Port Phillip and Gippsland basins of south‐east Australia. The phosphates occur as lenticular lag deposits and include reworked phosphatic intraclasts, vertebrate bone and teeth. In situ phosphatized burrows are also found in sediments of Late Miocene and Early Pliocene age. The phosphatic intraclasts on the unconformity are interpreted as reworked phosphatized burrows derived from latest Miocene sediments (6 to 5 Ma). The phosphatization of these intraclasts is temporally related to the unconformity. The timing of phosphogenesis coincides with a period of transgression across the south‐east Australian margin following Late Miocene uplift. This transgression is responsible for initial marine erosion of the underlying Miocene sequence, creation of a period of very slow sedimentation that was favourable to phosphate formation and subsequent deposition of the latest Miocene through to Pliocene sediments. The continental weathering of the uplifted highlands adjacent to the sedimentary basins, global phosphorus enrichment in the Late Miocene oceans and localized upwelling may all have contributed to phosphatization in south‐eastern Australia.

Geomorphology and surficial sediments of the southeast Australian continental margin

The southeast Australian continental margin stretches 1500 km north from Bass Strait to the Great Barrier Reef. This part of the Australian margin represents a distinct oceanographic, climatic and geological province. Recent initiatives by the Australian Hydrographic Office, Geoscience Australia and the Environmental Geoscience Group at the University of Newcastle have provided a new digital database of bathymetric soundings and surface‐sediment distribution for the southeast Australian margin. This database is used here to provide the first detailed digital 3–D representations of the southeast Australian margin, leading to new morphological classifications and integrated sedimentation models. As a result of its tectonic history of asymmetric passive margin rifting, the continental margin as a whole is narrow, steep and sediment deficient. The shelf can be divided into an inner zone with bedrock cropping out or close to the surface, and an outer zone that represents the top of a Cenozoic sediment wedge. The continental shelf and slope can be classified into a variety of morphological units based on detailed analysis of geometry and slope. These units include a succession of shore‐normal zones (shoreface, inner plain, inner and mid‐slope, outer plain and continental slope) and a number of local regions (mounds, lobes, tongues, depressions, deltas and bedrock). Sediments are generally siliciclastic in composition on the inner margin and carbonate‐rich on the outer margin, reflecting carbonate production at low sea‐levels on the outer shelf/continental slope and terrestrial sediment input to the inner shelf. Fine‐grained sediments are rare on the shelf north from Port Stephens, and occur draped across the inner and mid‐slope further south. Outer shelf carbonate sediments are coarse grained with a patchy distribution related to winnowing by the East Australian Current. The East Australian Current also produces erosional depressions and southward prograding sediment tongues and lobes where it separates from the shelf off Port Stephens. The margin character varies markedly from south to north in terms of fine‐sediment distribution, depth to the shelf break and width of the outer shelf. These outer shelf features reflect an inherited palaeotopography from underlying Cenozoic sediment wedges that are better developed on the southern margin.

Apatite (U–Th)/He age constraints on the development of the Great Escarpment on the southeastern Australian passive margin

The southeast Australian margin, like other high elevation passive margins, is characterised by a steep escarpment that separates a dissected coastal plain from a low relief inland plateau. Quantitative constraints on the generation of escarpments can be provided by apatite (U–Th)/He ages. Here we use a coast-perpendicular traverse across the coastal lowlands, escarpment and plateau to test the three prevailing models of SE Australian escarpment formation, namely retreat into a downwarped rift shoulder, escarpment retreat and down-wearing on high elevation rift shoulder with flexural rebound. Apatites from the coastal plain have He ages of between 87 and 112 Ma, suggesting that the coastal lowlands developed very rapidly after rifting and continental break-up at 85–100 Ma. The He age data are inconsistent with the erosion of a downwarped rift margin, and cannot be explained by a constant post-break-up rate of lateral escarpment retreat across the coastal plain or by constant down-wearing. The data require either rapid escarpment retreat or rapid in-place excavation of the escarpment soon after break-up, in response to rifting and the lowering of base levels on the margin of the new continent at break-up, followed by a period of landscape stability and low erosion. Combined with the existing apatite fission track record, the He data are consistent with erosion of 3–4 km within a maximum of 28 Myr of break-up, at a minimum vertical erosion rate of 130 m Myr −1 along the coast. The rapid denudation period across the coastal plain in this region took less than 48 Myr (from the coast to the escarpment base), which corresponds to an average vertical erosion rate of 45 m Myr −1. This is equivalent to a mean escarpment retreat rate of 5–10 km Myr −1. Apatite He ages from the plateau (183–247 Ma) indicate that the highlands remained stable throughout continental break-up, experiencing average erosion rates of less than 10 m Myr −1 since the late Palaeozoic/early Mesozoic.

Otway Continental Margin Transect: Crustal architecture from wide‐angle seismic profiling across Australia's southern margin

The Otway Basin in southeastern Australia formed on a triangular‐shaped area of extended continental lithosphere during two extensional episodes in Cretaceous to Miocene times. The extent of the offshore continental margin is highlighted by Seasat/Geosat satellite altimeter data. The crustal architecture and structural features across this southeast Australian margin have been interpreted from offshore‐onshore wide‐angle seismic profiling data along the Otway Continental Margin Transect extending from the onshore Lake Condah High, through the town of Portland, to the deep Southern Ocean. Along the Otway Continental Margin Transect, the onshore half‐graben geometry of Early Cretaceous deposition gives way offshore to a 5 km‐thick slope basin (P‐wave velocity 2.2–4.6 km/s) to at least 60 km from the shoreline. At 120 km from the nearest shore in a water depth of 4220 m, sonobuoy data indicate a 4–5 km sedimentary sequence overlying a 7 km thick basement above the Moho at 15 km depth. Major fault zones affect the thickness of basin sequences in the onshore area (Tartwaup Fault Zone and its southeast continuation) and at the seaward edge of the Mussel Platform (Mussel Fault). Upper crustal basement is interpreted to be attenuated and thinned Palaeozoic rocks of the Delamerian and Lachlan Orogens (intruded with Jurassic volcanics) that thin from 16 km onshore to about 3.5 km at 120 km from the nearest shore. Basement rocks comprise a 3 km section with velocity 5.5–5.7 km/s overlying a deeper basement unit with velocity 6.15–6.35 km/s. The Moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore, and then to about 12 km in the deep ocean at the limits of the transect (water depth 5200 m). The continent‐ocean boundary is interpreted to be at a prominent topographic inflection point 170 km from shore at the bottom of the continental slope in 4800 m of water. P‐wave velocities in the lower crust are 6.4–6.8 km/s, overlying a thin transition zone to an upper mantle velocity of 8.05 km/s beneath the Moho. Outstandingly clear Moho reflections seen in deep‐marine profiling data at about 10.3 s two‐way time under the slope basin and continent‐ocean boundary place further strong controls on crustal thickness. There is no evidence of massive high velocity (>7 km/s) intrusives/underplate material in the lower crust nor any synrift or early post‐rift subaerial volcanics, indicating that the Otway continental margin can be considered a non‐volcanic margin, similar in many respects to some parts of the Atlantic Ocean margins e.g. the Nova Scotia ‐ Newfoundland margin off Canada and the Galicia Bank off the Iberian Peninsula. Using this analogue, the prominent gravity feature trending northwest‐southeast at the continent‐ocean boundary may indicate the presence of highly serpentinised mantle material beneath a thin crust, but this has yet to be tested by detailed work.

Part 1. East Autralian margin and the western marginal basins: The relationship between structures on the southeast Australian margin and in the Tasman Sea

A study of the magnetic anomaly pattern in the Tasman Sea has shown that it opened by a process of sea floor spreading between about 80 and 60 my.b.p. (Hayes and Ringis, 1973). The spreading pattern appears to have been quite complex. The basin is disected by many fracture zones (transform faults) and there appears to have been considerable variation in spreading rates within blocks bounded by these fracture zones. This is clearly shown by the varying widths of crustal blocks generated in periods of 5 m.y., as shown in Figure 1. Further, the trend of the magnetic pattern, and hence crustal isochrons, intersects the trend of the southeast Australian margin at an angle of about 45° to 50° . As a result, progressively older oceanic crust is absent north of Bass Strait.

The structure and evolution of the Tasman Sea and the Southeast Australian margin

The structure and evolution of the Tasman Sea and the Southeast Australian margin