Late glacial fans in the eastern Skagerrak; depositional environment interpreted from swath bathymetry and seismostratigraphy

The origin of acoustically transparent fan deposits overlying glacial till and ice-proximal sediments on the southern margin of the Norwegian Channel has been studied using high-resolution seismic-reflection profiles and multibeam bathymetry. The first deposits overlying glacigenic sediments are a series of stacked, acoustically transparent submarine fans. The lack of glaciomarine sediments below and between individual fans indicates that deposition was rapid and immediately followed the break up of the Late Weichselian ice cover. The fans are overlain by stratified glaciomarine sediments and Holocene mud. Because of the uniformity of this drape, the upper surface of the fan deposits is mimicked at the present seafloor, and the bathymetric images clearly show the spatial relationship of the fans to bedrock ridges and the presence of braided channel-levee systems on the surface of the youngest fans. The acoustically transparent character of the fan deposits indicates that they comprise silt and clay, and their lobate form and lack of internal stratification indicates that they were deposited by debris flows. The channel-levee morphology indicates deposition from more watery hyperconcentrated fluid flows. The fan sediments were either derived from 1) erosion of Mid Weichselian lake deposits in southern Skagerrak or 2) from Late glacial ice-margin lake deposits, ponded against the Norwegian Channel ice stream, which collapsed catastrophically when the lateral support was removed as the ice disintegrated. Fans composed almost exclusively of fine-grained sediment need not, therefore, rule out an origin in a deglacial setting relatively close to the former margins of glaciers and ice sheets.