geomorphology
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The upper Renmark Group is a widespread non-marine succession of Miocene age in the Murray Basin. Unlithified and nowhere exposed, it is a poorly understood unit that provides a lower confining horizon to important aquifers in the overlying Calivil succession, and includes deeper aquifers exploited for water for mineral sands mining. The use of sonic drilling has allowed extensive coring of the previously poorly sampled unit, documentation of its facies and collection of a diverse assemblage of spores and pollen. The upper Renmark Group was deposited on a low relief sedimentary plain dominated by anastomosing fixed channel streams, flowing southward into a complex low energy coastal plain with numerous lagoons and bays. Hydrological connection between the more (Renmark) and less saline (Calivil) aquifers is low, except where Renmark channel sands immediately underlie the Calivil. In these areas can be a distinct salinity gradient from the Calivil down into the Renmark, indicating mixing. High water tables during deposition facilitated preservation of an unusually detailed record of inland Australian vegetation from this period. Late Early to Middle Miocene climates were wet and warm with low seasonality. These conditions supported complex rainforest communities dominated by (1) Nothofagus (Brassospora) spp. and gymnosperms on floodplains inland of the palaeoshoreline, or (2) Myrtaceae (Syzygium-type) and unidentified mangrove angiosperms in areas subjected to a marine influence. Many of the latter pollen types are very small (<10 micron diameter), resulting in the 10 micron-filtered extracts being dominated by larger types such as Casuarinaceae and Nothofagus (Brassospora) spp.
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International efforts to protect the Vulnerable Marine Ecosystems (VMEs) that live on cold seeps and hydrothermal vents requires methods to predict where these features might be in advance of human activity. We suggest an approach to identifying seeps and vents in the CCAMLR region that uses existing data to highlight areas of possible seep and vent communities. These hierarchical criteria can be used to reduce the accidental disturbance of seep communities. We propose a 4 level classification of indicators: Class 1 Areas: VME confirmed by recovery of organisms or observation (video, stills). This level would qualify for VME status and high levels of protection. Class 2 Areas: Seepage/venting present but VME not confirmed. These locations would have a number of indicators of active seepage but VMEs have not been identified. Class 3 Areas: Seepage suspected from geophysical, geochemical or oceanographic observations. These areas have seismic indications of shallow gas or clathrates , structures suggesting fluid escape but where bubble flares or water column plumes have not been detected or where plume has been detected but not tied to an area of sea floor. Class 4 Areas: Area or geomorphic features associated with seepage and vents. These areas are large-scale geomorphic features such as Mid-Ocean Ridge rift valleys or volcanoes where vents are likely but not yet detected. Class 3 and Class 4 areas have been mapped from 45oE to 160oE using global bathymetry grids and seismic data from the SCAR Seismic Data Library.
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Abstract: The Collaborative East Antarctic Marine Census (CEAMARC) surveys to the Terre Adélie and George V shelf and margin highlight the requirement for a revised high resolution depth model that can be used as a spatial tool for improving physical models of the region. We have combined available shiptrack and multibeam bathymetry, coastline and land topographic data to develop a new high-resolution depth model, called GVdem. GVdem spans an area 138°E to 148°E longitude and 63°S to 69°S latitude, with a choice of three ESRI grids with cell pixel sizes: 15 arcsec, 9 arcsec and 3.6 arcsec. The revised depth model is an improvement over previously available regional-scale grids, and highlights seabed physiographic detail not previously observed for this part of East Antarctica. In particular, the extent and complexity of the inner-shelf depressions are revealed and their relationship with large shelf basins and adjacent flat-topped banks.
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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.
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Antarctic ice shelves and fast flowing ice streams are key drainage features of the Antarctic Ice Sheet and their behaviour determines the sensitivity of the ice sheet to climate change and sea level rise. Some fast flowing ice streams are thinning rapidly and could be the 'soft underbelly' of the East Antarctic Ice sheet. Processes across the grounding zone are important in understanding the retreat behaviour of ice streams but are poorly understood because of the difficulty of accessing the region. The Antarctic Shelf preserves geomorphic features and sedimentary structures left by ice retreat which can provide insights into processes in and close to the grounding zone. Sidescan sonar records from Prydz Bay image a range of features that reflect changes in processes across the Amery Ice Shelf grounding zone during retreat after the Last Glacial Maximum. The major features identified are: Mega-scale Glacial Lineations Linear ridges of sediment formed by moulding of mobile subglacial sediment parallel to ice flow. Flutes and Mega flutes - Smaller linear ridges moulded by ice flow. Inter-flute dunes - Large bedforms formed by bottom currents flowing the grounding zone in the sub-ice shelf cavity. Transverse steps - Ice flow parallel ridges that terminate in steps running oblique to normal to the ice flow direction. Sinuous ridges (Eskers) - Gently sinuous ridges that run generally parallel to obliquely across fluted surfaces. Polygonal crevasse infills - Irregular polygonal ridges on the crest of grounding zone wedges. The presence of fluted and mega-scale glacial lineations indicates that the ice moved over an unfrozen, deforming bed in the zone up stream of the grounding zone. For most of the Amery Ice Shelf, the inter-flute dunes reflect strong thermohaline circulation in the ice shelf cavity. Sand and gravel recovered in cores from beneath the Amery Ice Shelf indicate significant current speeds, possibly enhanced by tidal pumping.
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Lithostratigraphy, grain sizes and down-hole logs of Site 1166 on the continental shelf, and Site 1167 on the upper slope, are analyzed to reconstruct glacial processes in eastern Prydz Bay and the development of the Prydz trough-mouth fan. In eastern Prydz Bay upper Pliocene-lower Pleistocene glaciomarine sediments occur interbedded with open-marine muds and grade upward into waterlaid tills and subglacial tills. Lower Pleistocene sediments of the trough-mouth fan consist of coarse-grained debrites interbedded with bottom-current deposits and hemipelagic muds, indicating repeated advances and retreats of the Lambert Glacier-Amery Ice Shelf system with respect to the shelf break. Systematic fluctuations in lithofacies and down-hole logs characterize the upper Pliocene-lower Pleistocene transition at Sites 1166 and 1167 and indicate that an ice stream advanced and retreated within the Prydz Channel until the mid Pleistocene. The record from Site 1167 shows that the grounding line of the Lambert Glacier did not extend to the shelf break after 0.78 Ma. Published ice-rafted debris records in the Southern Ocean show peak abundances in the Pliocene and the early Pleistocene, suggesting a link between the nature of the glacial drainage system as recorded by the trough-mouth fans and increased delivery of ice-rafted debris to the Southern Ocean.
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During the late Neogene, the Lambert Glacier-Amery Ice Shelf drainage system flowed across Prydz Bay and showed several changes in flow pattern. In the Early Pliocene, the Lambert Glacier ice stream reached the shelf edge and built a trough mouth fan on the upper continental slope. This was associated with an increase in ice discharge from the Princess Elizabeth Land coast into Prydz Bay. The trough mouth fan consists mostly of debris flow deposits derived from the melting out of subglacial debris at the grounding line at the continental shelf edge. The composition of debris changes at around 1.1 Ma BP from material derived from erosion of the Lambert Graben and Prydz Bay Basin to mostly basement derived material. This probably results from a reduction in the depth of erosion and hence the volume of ice in the system. In the trough mouth fan, debris flow intervals are separated by thin mudstone horizons deposited when the ice had retreated from the shelf edge. Age control in an Ocean Drilling Program hole indicates that most of the trough mouth fan was deposited prior to the Brunhes Matuyama Boundary (780 ka BP). This stratigraphy indicates that extreme ice advances in Prydz Bay were rare after the mid Pleistocene, and that ice discharge from Princess Elizabeth Land became more dominant than the Lambert Glacier ice in shelf grounding episodes, since the mid Pleistocene. Mechanisms that might have produced this change are extreme inner shelf erosion and/or decreasing ice accumulation in the interior of East Antarctica. We interpret this pattern as reflecting the increasing elevation of coastal ice through time and the increasing continentality of the interior of the East Antarctic Ice Sheet. The mid Pleistocene change to 100 ka climatic and sea level cycles may also have affected the critical relationship between ice dynamics and the symmetry or asymmetry of the interglacial/glacial climate cycle duration.
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This is a compilation of all the processed multibeam bathymetry data that are publicly available in Geoscience Australia's data holding for the Macquarie Ridge.
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High-resolution marine sonar swath mapping, covering an area of ca. 33 km2 in the vicinity of the Windmill Islands (67° S, 110° E), Wilkes Land, east Antarctica, permits visualisation and description of the near-shore geomorphology of the seafloor environment in unprecedented detail and provides invaluable insight into the ice-sheet history of the region. Mesoproterozoic metamorphic basement exhibits prominent sets of parallel northwest-trending linear fault sets that probably formed during fragmentation of eastern Gondwana during the Mesozoic. The fault systems appear to control regional coastal physiographic features and have, in places, been preferentially eroded and exploited by subsequent glacial activity. Possibly the earliest formed glacially-derived geomorphological elements are networks of sub-glacial meltwater channels which are preserved on bedrock platforms and ridges. Subtle glacial lineations and streamlined landforms record evidence of the westward expansion of the grounded, Law Dome ice sheet margin, probably during the late Pleistocene Last Glacial Maximum, the direction of which coincides with glacial striae on onshore crystalline bedrock outcrops. The most striking glacial geomorphological features are sets of arcuate ridges confined mostly within glacially excavated `U-shaped valleys, exploiting and developed along bedrock fault sets. These ridge sets are interpreted as `push moraines or grounding zone features, formed during episodic retreat of highly channelised, topographically controlled ice-streams following ice surging, possibly in response to local environmental forcing during the mid-late Holocene. Minor post-glacial marine sedimentation is preserved in several small (1 km2) `isolated marine basins with shallow seaward sills.
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Faults of the Lapstone Structural Complex (LSC) underlie 100 km, and perhaps as much as 160 km, of the eastern range front of the Blue Mountains, west of Sydney. More than a dozen major faults and monoclinal flexures have been mapped along its extent. The Lapstone Monocline is the most prominent of the flexures, and accounts for more than three quarters of the deformation across the complex at its northern end. Opinion varies as to whether recent tectonism, or erosional exhumation of a pre-existing structure, better accounts for the deeply dissected Blue Mountains plateau that we see today. Geomorphic features such as the abandoned meanders at Thirlmere Lakes illustrate the antiquity of the landscape and favour an erosional exhumation model. According to this model, over-steepened reaches developed in easterly flowing streams at the Lapstone Monocline when down-cutting through shale reached more resistant sandstone on the western side of the LSC. These over-steepened reaches drove headward (westerly) knick point retreat, ultimately dissecting the plateau. However, a series of swamps and lakes occurring where small easterly flowing streams cross the westernmost faults of the LSC, coupled with over-steepened reaches 'pinned' to the fault zones in nearby larger streams, imply that tectonism plays a continuing role in the development of this landscape. We present preliminary results from an ongoing investigation of Mountain Lagoon, a small fault-bound basin bordering the Kurrajong Fault in the northern part of the LSC.