Geometry and genesis of glacigenic debris flows on the North Sea Fan: TOBI imagery and deep-tow boomer evidence

Acoustic data have been used to study the upper sequence of glacigenic debris flows (GDFs) deposited during the last glacial maximum on the North Sea Fan. The presumed GDF release area at the shelf break (450 m water depth) exhibits a sequence up to 90 m thick, thinning to less than 45 m basinward at approx. 750–800 m depth, on a low slope (0.6–0.8°). A seismically featureless sequence at the shelf break changes distinctly the first kilometers basinward, showing a significant occurrence of features interpreted as infilled transport chutes. The straight or sinuous chutes feed into well-defined relief-building GDF lobes 10–40 km from the shelf break. A lack of failure scarps at the shelf break suggests that the GDFs have been sourced by a relatively continuous release of material. Flow lines concentrated along the chutes and center of the GDFs suggest a laminar movement pattern in the GDF core, supported by passive flanks. Based on the flow morphology high pore pressure and low sediment strength within a central core of the flow is assumed to have contributed to the mobility of the GDFs. Dissipation of pore pressure along the margins of the flow may have caused the margins freeze while the sediment in the core remained mobile.