Glacigenic debris flows on the North Sea Trough Mouth Fan during ice stream maxima

Numerous short sediment cores which penetrate through the glacimarine section and into glacigenic debris flow debrites (GDFs) on the North Sea Fan are investigated. The sampled GDFs represent the uppermost of several sequences of these stacked bodies, many hundreds of metres thick. The GDFs occurred over slopes from 0.3 to 0.7° and examples are found from the shelf break at the mouth of the Norwegian Channel to the Norway Basin floor, 300 km distant. The GDF diamicts are structureless and homogeneous in terms of grain size, magnetic susceptibility, NRM, bulk density, water, carbonate, foraminiferal and rock fragment content. This applies both from GDF to GDF across the fan and in a proximal/distal sense. Comparison of the investigated lithologic parameters with Weichselian till from a borehole in the Norwegian Channel (200 km up-ice) shows striking similarity to the GDF debrites. AMS 14C dates at the GDF/glacimarine contact and stratigraphic relationships indicate voluminous deposition before 18 ka, continuing, possibly uninterrupted, until ca. 16 ka, and a final phase halting just before 15 ka when the Norwegian Channel became ice free. This chronology, the comparable lithologic character between till and GDF debrites, and a seismic continuity confirms inferences that the GDFs are directly associated with glacial ice at the shelf break. The glacial efflux has not been subject to subsequent sorting and the homogeneity of the GDF is thought to reflect thorough mixing in a deforming till layer beneath a fast-flowing ice stream. The glacimarine section subsequent to GDFs shows no indications of turbidity flow. Observations are compatible with a slow, non-disintegrating (cohesive) `plug' flow transport mechanism with little sediment/water interaction on the upper surface. Minor distal differentiation might reflect a change in flow process. Deposition on the fan margins from water-borne glacimarine processes simultaneous with GDF deposition is limited.