P.E. O'Brien

Bottom currents, sedimentation and ice-sheet retreat facies successions on the Mac Robertson shelf, East Antarctica

Abstract Measurements of water turbidity, currents, seafloor sediment samples and geophysical data document the sedimentary processes and the Late Quaternary sedimentary history of a continental shelf valley system on the East Antarctic continental margin. The valley is up to 1200 m in depth and strikes across the shelf; it is interpreted as having formed by glacial erosion processes. On the outer-shelf sill of the valley, northwestward (offshore) currents with speeds of up to 0.47 m/s referenced to 100 cm above the seabed were recorded over a 10-month period. Such currents are competent to initiate bedload transport of medium sand and formation of small (ripple-sized) bedforms which explains the occurrence of ripple cross-bedding observed in some X-radiographs of cores. A nepheloid layer with concentrations of up to 3.5 mg/l was noted at two stations and the available evidence suggests that density flows resulting from the sea-ice brine-rejection mechanism are competent to entrain shelf bottom sediments episodically. Sediment cores taken from this environment document the succession of facies resulting from retreat of glacial ice from the shelf during the Holocene transgression. Under low sea-level, glacial conditions, the ice sheet is interpreted to have been grounded on the outer shelf, where it deposited a grounding line moraine. Rising sea level and global warming caused the ice shelf to retreat leaving a sub-ice shelf, glacial-marine mud. This is overlain by a laminated to massively bedded ice-rafted debris-rich facies, interpreted as being deposited at the glacial calving front. Above this is a siliceous mud and ooze facies having a basal age of about 10,000 years BP, characterising modern, open-marine conditions. Two types of iceberg keel marks on the outer part of the shelf valley are recognised from sidescan sonographs: one relict, arcuate type formed during the retreat of the ice shelf prior to 10,000 years BP and a second modern, elongate type that is presently being formed near the shelf break by iceberg grounding processes. This facies succession has been described from other Antarctic shelf environments and is probably representative of facies deposited during Holocene glacial retreat from the Antarctic shelf.

A Small, Alluvial Crevasse Splay

ABSTRACT A crevasse splay deposited on the floodplain of the Clarence River in northeastern New South Wales, Australia, by a small, ephemeral tributary provides a guide to the geometry and sedimentary structures of crevasse splays in general and to the processes affecting their final form. The splay consists of a main lobe of sand with smaller sand fingers that end in slipfaces around its edges. Water seeping out of the channel reworks part of the splay into a gently thinning sheet. Small-scale facies models derived from different parts of this splay explain the origins of solitary planar cross sets and gently inclined bedding in some ancient crevasse-splay sandstones.

Physical controls on deep water coral communities on the George V Land slope, East Antarctica

Abstract Dense coral-sponge communities on the upper continental slope at 570–950 m off George V Land, East Antarctica have been identified as Vulnerable Marine Ecosystems. The challenge is now to understand their probable distribution on other parts of the Antarctic margin. We propose three main factors governing their distribution on the George V margin: 1) their depth in relation to iceberg scouring, 2) the flow of organic-rich bottom waters, and 3) their location at the head of shelf cutting canyons. Icebergs scour to 500 m in this region and the lack of such disturbance is a probable factor allowing the growth of rich benthic ecosystems. In addition, the richest communities are found in the heads of canyons which receive descending plumes of Antarctic Bottom Water formed on the George V shelf, which could entrain abundant food for the benthos. The canyons harbouring rich benthos are also those that cut the shelf break. Such canyons are known sites of high productivity in other areas due to strong current flow and increased mixing with shelf waters, and the abrupt, complex topography. These proposed mechanisms provide a framework for the identification of areas where there is a higher likelihood of encountering these Vulnerable Marine Ecosystems.

Topographic controls on post-Oligocene changes in ice-sheet dynamics, Prydz Bay region, East Antarctica

Within the general trend of post-Eocene cooling, the largest and oldest outlet of the East Antarctic Ice Sheet underwent a change from ice-cliff to ice-stream and/or ice-shelf dynamics, with an associated switch from line-source to fan sedimentation. Available geological data reveal little about the causes of these changes in ice dynamics during the Miocene Epoch, or the subsequent effects on Pliocene‐Pleistocene ice-sheet history. Icesheet numerical modeling reveals that bed morphology was probably responsible for driving changes in both ice-sheet extent and dynamics in the Lambert-Amery system at Prydz Bay. The modeling shows how the topography and bathymetry of the Lambert graben and Prydz Bay control ice-sheet extent and flow. The changes in bathymetric volume required for shelf-edge glaciation correlate well with the Prydz Channel fan sedimentation history. This suggests a negative feedback between erosion and glaciation, whereby the current graben is overdeepened to such an extent that shelf-edge glaciation is now not possible, even if a Last Glacial Maximum environment recurs. We conclude that the erosional history of the Lambert graben and Prydz Bay in combination with the uplift histories of the surrounding mountains are responsible for the evolution of this section of the East Antarctic Ice Sheet, once the necessary initial climatic conditions for glaciation were achieved at the start of the Oligocene Epoch.

Mesozoic palynofloras from the Mac. Robertson shelf, East Antarctica: geological and phytogeographic implications

Gravity cores taken from five sites in the Nielsen basin, a sinuous trough on the Mac. Robertson shelf of the East Antarctic continental margin, have yielded non-marine palynofloras of late Early Jurassic to Early Cretaceous age. The palynomorphs occur in glacimarine muds and are thus not in situ, but the composition of the samples indicates that no mixing of individual assemblages has occurred, and that the sediments were deposited very close to outcrop of the source of the palynomorphs. The sequence is oldest at the seaward end of the Nielsen basin, and youngest near the fault-bounded landward end. It is interpreted as reflecting pre-breakup deposition on a passive, rifted continental margin. The age of the assemblages has been established by comparison with the zonal scheme developed in the Perth Basin of Western Australia. Similarity of the Antarctic palynofloras with those known from basins on the east coast of India confirms continental reconstructions that show this part of East Antarctica to have been contiguous with eastern India prior to break-up. The non-marine character of the palynomorphs suggests that marine conditions in this region of the Antarctic margin did not develop until at least late Early Cretaceous time.

Sedimentological signatures of the sub-Amery Ice Shelf circulation

Abstract Two sediment cores collected from beneath the Amery Ice Shelf, East Antarctica describe the physical sedimentation patterns beneath an existing major embayed ice shelf. Core AM01b was collected from a site of basal freezing, contrasting with core AM02, collected from a site of basal melting. Both cores comprise Holocene siliceous muddy ooze (SMO), however, AM01b also recovered interbedded siliciclastic mud, sand and gravel with inclined bedding in its lower 27 cm. This interval indicates an episode of variable but strong current activity before SMO sedimentation became dominant. 14C ages corrected for old surface ages are consistent with previous dating of marine sediments in Prydz Bay. However, the basal age of AM01b of 28250 ± 230 14C yr bp probably results from greater contamination by recycled organic matter. Lithology, 14C surface ages, absolute diatom abundance, and the diatom assemblage are used as indicators of sediment transport pathways beneath the ice shelf. The transport pathways suggested from these indicators do not correspond to previous models of the basal melt/freeze pattern. This indicates that the overturning baroclinic circulation beneath the Amery Ice Shelf (near-bed inflow–surface outflow) is a more important influence on basal melt/freeze and sediment distributions than the barotropic circulation that produces inflow in the east and outflow in the west of the ice front. Localized topographic (ice draft and bed elevation) variations are likely to play a dominant role in the resulting sub-ice shelf melt and sediment distribution.

LITHOFACIES DISTRIBUTION IN RELATION TO THE GEOMORPHIC PROVINCES OF PRYDZ BAY, EAST ANTARCTICA

Over the past 15 years, Japanese, Australian and Russian expeditions to Prydz Bay have collected about 30 000 km of bathymetric data, 6000 km of sidescan sonar data and more than 250 sediment grab and core samples. These data were used in the present study to compile surficial sediment, bathymetric, and geomorphological maps of the Prydz Bay region. Lithofacies distribution was determined by surficial sediment data analysis using sample matrix (Q-mode) and cluster analysis techniques based on data from 206 sites. Data included percentage biogenic silica (opal), calcium carbonate, gravel, mud, and relative abundance of two diatom species (Fragilariopsis curfa and F. kerguelensis). Five lithofacies are identified from the available data: (1) slightly gravelly sandy mud (g)sM lithofacies, (2) siliceous mud and diatom ooze (SMO) lithofacies, (3) F. kerguefensis pelagic ooze lithofacies, (4) F. cum gravelly muddy sand gmS lithofacies and (5) calcareous gravel lithofacies. In many areas the lithofacies correlate to geomorphological provinces as defined by previous investigators using 3.5 kHz and sidescan sonar data. In some cases, Holocene SMO sediments are seen to drape over iceberg plough marks, implying that these are relict features. These five lithofacies are likely to dominate most of the East Antarctic shelf region and may be helphl in defining sedimentary successions resulting from ice-sheet advance and retreat over glacial-interglacial cycles. Received 16 December 1997, accepted 29 May 1998

Development of Mesozoic transtensional basins in easternmost Australia

Seismic reflection profiles across Mesozoic basins in easternmost Australia show that the Esk Trough, Ipswich Basin and Clarence-Moreton Basin were initiated by transtensional events in the Late Permian or Early Triassic. The initial sediments in the basins are unexposed sequences below the Esk Trough and Ipswich Basin. In the Scythian, strike-slip faulting moved eastwards to the site of the present Ipswich Basin, and the Esk Trough subsided due to thermal relaxation. After oblique extension ceased in the Carnian, thermal relaxation subsidence led to deposition of sediments firstly in the Ipswich Basin and then in the more extensive Clarence-Moreton Basin. The structures and sediments of the extension and thermal relaxation phases show a pronounced asymmetry in cross-section, as expected during pure extension, but no low-angle detachment faults have been detected indicating strike-slip movement on the bounding fault of the Esk Trough (the West Ipswich Fault) during extension. Thus the basins developed dur...

Geomorphology and sedimentology of the continental shelf adjacent to Mac. Robertson Land, East Antarctica: A scalped shelf

During the Quaternary, the Mac. Robertson shelf of East Antarctica was deeply eroded by glaciers and currents exposing the underlying basement, resulting in a scalped shelf. Major geomorphic zones are: (1) high-relief, ridge and valley topography (200–1400 m); (2) smooth sea floors associated with low-energy, depositional shelf valleys and basins (400–800 m); (3) low-relief, planated banktops (100–200 m); and (4) iceberg gouged and current reworked seaward-bank margins and upper slope (200 to < 630 m). About 90% of the shelfs surface has net erosional conditions and about 10% is net depositional. The sedimentary processes and deposits may be common to large areas of the East Antarctic margin.

Sequence stratigraphy of a sandstone‐rich Permian glacial succession, Fitzroy Trough, Canning Basin, Western Australia

The Canning Basin in northwest Australia contains hydrocarbon‐bearing Permo‐Carboniferous glacial successions, one of a number of such units that occur widely distributed across Gondwana. Up to 2.5 km of clastic sediment eroded by Permian ice sheets from the adjacent Precambrian craton was trapped in the Fitzroy Trough in the northeast part of the Canning Basin. These sedimentary rocks are between 60 and 80% fine to medium sandstone. During relative sea‐level lowstands, regionally extensive erosion surfaces with a relief of several hundred metres developed. Lowstand and early transgressive deposits consisting of sandstone and finer grained elastics that infill topographic lows were deposited on these erosion surfaces. Later transgressive deposits consist of glaciomarine mudstones and rain‐out diamictites, subaqueous outwash fans and deltaic deposits, composed mostly of sandstone. Alternatively, in some situations, the sequences consist entirely of aggrading sandy fluvial braidplain deposits. Some relative...