Geomorphology of Buried Subglacial Valleys and Horizons Interpreted from 3-D Seismic Data, German North Sea

Lutz, Rudiger, Gerhard Best, Lutz Reinhardt, Christoph Gaedicke, Federal Institute for Geosciences and Natural Resources (BGR), Germany, Hannover, Germany

A 3D seismic survey acquired by a German oil and gas consortium was investigated down to a depth of 2 s TWT. Target of the study was the analysis of the structures of buried pre-Weichselian subglacial valleys and horizons featuring glacigenic morphology. The most prominent structures in the shallow subsurface are the widespread ramified subglacial val­leys of likely Elsterian (~400 kyr BP) and Saalian (~190 kyr BP) age. They can be up to 400 m deep and generally have a complex infill. So far, their occurrence is incompletely known for the German EEZ. In the 3D seismic data these valleys can be studied in greater detail and their morphology can be determined. Different generation phases and varying orientations of these valleys are visible.

More subtle structures are investigated in different time slices like widespread iceberg keel scouring and ice sheet scours. Observed circular structures could be pingos and/or pockmarks produced by permafrost and gas now situated at a depth of 800 ms. More elon­gated shallow depressions (“wallows”) on bedding planes could have been produced by ice masses, which stranded in shallow waters.

Also remnants of the former morphology of the glacially formed landscapes seem to be preserved represented by contrasting seismic facies of patchy, rough moraines and more uniform planes. The planes are incised by up to 4 intersecting systems of gullies, channels, and valleys that are of supposed fluvial and subglacial origin. Bright spots, gas pockets at faults and gas chimneys are indicators for the presence of shallow gas.