3D SEISMIC EXPLORATON DATA OFFSHORE WESTERN NORWAY - EVIDENCE OF PLEISTOCENE ICE STREAMS, SUBGLACIAL DRAINAGE AND DEGLACIATION PATTERNS
NYGAARD, ATLE . Department of Geology, University of Bergen.
Sejrup, Hans Petter . Department of Geology, University of Bergen.
Haflidason, Haflidi . Department of Geology, University of Bergen.
Cecchi, Marcello . Enterprise Oil Norway.
Offshore western Norway, in the Northern North Sea / the southernmost Møre shelf, the Måløy Plateau is a large bank area flanking the outlet of the Norwegian Channel. A 3D survey covering 950 km2 (app. 1/4 of the Plateau) has been investigated. The strength of the 3D seismic method is its horizontal (spatial) resolution, while its weakness is the vertical (temporal) resolution, limited by the low frequency range designed to image deeper reservoirs.
The Måløy Plateau consists of a sequence of glacial, interglacial and deltaic sediments up to 500 m thick above the glacial unconformity. The stratigraphy of the Måløy Plateau is correlated with the stratigraphy of the Norwegian Channel/North Sea Fan provided by the Troll core and new geological borings on the outer Møre shelf/upper slope. In the early and Middle Pleistocene, the area was strongly influenced by northward moving ice streams, indicated by >30 km long and straight glacial lineations on top of several acoustically massive units, interpreted as tills. Superimposed on these units an up to 150 m thick deltaic sequence has been deposited, prograding from the mainland. Channels indicating subaerial exposure incise the sub-horizontal upper surface of the delta, about 250 m below the present day sea level. From its stratigraphic position a late Middle Pleistocene/Late Pleistocene age is suggested. Stratigraphically above the delta sequence, seismically massive, sheet-like deposits interpreted as subglacially deposited tills do not show the strong lineation characteristic of the lower stratigraphic levels. However, anastomosing and dendrittic channels patterns interpreted as subglacial drainage patterns are identified in the sheet-like deposits. They occur independent of local dip directions more than 300 m below the present day sea level, and suggest upslope flow directions. The uppermost sheet-like till sequence deposited by the ice which reached the shelf edge during the last glacial maximum shows weak NNW lineation. This unit forms the base of an up to 50 thick composite sequence of ridges deposited towards SW, indicated by a strong directional imprint on the seabed. The thick sequence is deposited after the collapse of the ice in the Norwegian Channel (15 ka), and a partial deglaciation of the Måløy Plateau. The lack of supporting ice in the Norwegian Channel probably created a significant imbalance in the surface profile of the ice on the Møre shelf, creating a SW directed advance before the final deglaciation of the Plateau at 12.7 ka.
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