FORM AND FLOW OF A PALEO-ICE STREAM ON THE ANTARCTIC PENINSULA CONTINENTAL MARGIN

Geophysical and geological data from the continental margin west of the Antarctic Peninsula reveal the former path of a large palaeo-ice stream on the continental shelf. This ice stream drained a 60,000 km2 ice-sheet basin through Marguerite Bay to the shelf edge via a bathymetric trough during the last glacial maximum. Swath bathymetric data from the floor of the trough show streamlined subglacial bedforms that exhibit progressive elongation with distance along the trough. Downflow evolution of the bedforms relates to a switch from crystalline bedrock (hard bed) on the inner shelf to a sedimentary substrate (soft bed) on the outer shelf. Streaming flow operated across both substrates but the longest bedforms occur on the area of the soft bed and indicate that the highest flow velocities occurred in this region. The ice stream was about 75-80 km wide and about 370 km long. Well-developed mega-scale glacial lineations are present in the outer-shelf trough and sub-bottom profiler data indicate that the lineations are formed in a thick sediment layer. The layer consists of massive, matrix-supported, diamict facies that is relatively soft and water-rich. This soft diamict overlies a much stiffer massive, matrix-supported diamict facies. The soft diamict facies is interpreted as a deformation till, formed, at least in part, by subglacial reworking of the underlying stiff diamict facies. Using acoustic stratigraphic and core data, the extent and thickness of the deformation till layer has been mapped throughout Marguerite Trough. The till layer is <15 m thick and is confined to the outer-shelf trough. Based on the thickness of the layer and its association with mega-scale glacial lineations, it appears that the zone of fastest flow within the ice stream occurred over the area of the trough floored by deformation till. Rapid deglaciation of the ice stream along the length of the trough is supported by an absence of coarse-grained ice-proximal glacimarine lithofacies in the cores, the well-preserved nature of the subglacial bedforms, and absence of recessional moraines or zones of cross-cutting bedforms. This rapid retreat contrasts with other locations on the Antarctic margin, such as the northern Antarctic Peninsula and the Ross Sea where ice-stream retreat was slower.