HR: 11:35h
AN: C52A-06    [Abstracts]
TI: Subglacial Landforms and Processes: new Information From 3D Seismic Technology
AU: * Andreassen, K
EM: karin.andreassen@ig.uit.no
AF: University of Tromso, Dramsvn 201, Tromso, 9037, Norway
AB: Three-dimensional (3D) seismic interpretation and imaging techniques provide a unique means of investigating submarine geomorphic features produced by former ice sheets. An extensive two-dimensional (2D) and 3D seismic data base is here used to image the imprints left behind by glaciers that flowed out a major cross-shelf trough (Bjornoyrenna) of the north-Norwegian continental shelf during repeated glacial episodes. Mega-scale glacial lineations characterize the seafloor geomorphology of Bjornoyrenna and smaller, contributing cross-shelf troughs, where they are inferred to represent flow-lines of former ice streams that where active during the most recent (Weichselian) glacial period. Similar features are commonly observed on buried horizons. Large- scale seafloor imprints from an early readvance after the last glacial maximum are especially well preserved. Streamlined landforms and associated lobe-shaped ridges indicate that this major cross-shelf trough hosted six separate ice stream lobes that diverged fan-like at their margins, but were not all active simultaneously. A 300 km wide grounding-zone wedge results from high sediment flux within sub-ice stream deformable beds. A 2 to 3 km thick Pleistocene record is preserved at the mouth of Bjornoyrenna, in the Bjornoya Trough Mouth Fan. The preservation of up to several hundred meters of glacigenic sediments between the buried, glacially eroded surfaces, provides here the opportunity to study the internal structure of till units. 3D seismic attribute maps reveal that megablocks and rafts commonly occur within the till units. The sediments blocks are often aligned in chains that may be up to 2 km wide and over 50 km long. The largest individual megablocks have an areal extent of over 2 km2. The sediment chains are interpreted to have been eroded, transported and deposited by grounded ice, most probably fast-flowing ice streams. This is based on the relationship between the sediment chains and the horizons revealing mega-scale glacial lineations. A borehole documents that one of the sediment blocks consists of a 15 to 25 m thick sequence of consolidated mid Cretaceous sedimentary rock, buried in a muddy till. Both mega-scale glacial lineations and chains of sediment blocks and rafts are inferred to be products of fast- flowing ice streams, and hypothesized to represent different modes of ice-stream sediment deformation. The mega-scale glacial lineations indicate deformation of subglacial sediments close to the ice-sediment interface. The megablocks and rafts suggest deformation deeper in the sediments, probably related to freezing-on to the base of the ice.
DE: 3022 Marine sediments: processes and transport
DE: 3025 Marine seismics (0935, 7294)
DE: 3045 Seafloor morphology, geology, and geophysics
SC: Cryosphere [C]
MN: 2007 Fall Meeting