The importance of sediment gravity flow to sediment transport and sorting in a glacial marine environment: Eastern Weddell Sea, Antarctica

Abstract

Offshore sediment transport on the eastern Weddell Sea continental margin cannot be modeled after examples from nonglaciated margins. Transport of terrigenous material to the marine environment is accomplished by floating or grounded ice instead of fluvial processes. There is no coastal zone where sediments are washed as they enter the marine environment, and, because the average shelf depth is 400 m, wind- or wave-generated currents have little, if any, influence on sediments. Texturally immature glacial sediment on the continental shelf and slope remains unsorted, except by processes of sediment gravity flow. Piston core coverage demonstrates that downslope transitions between slumps, debris flows, and turbidity currents are active on the unstable intercanyon slope areas and on the glacially scoured continental shelf. Sediment textural analyses indicate that size sorting and mineralogic maturity are achieved within sediment gravity flow transitions and that lithic (>40% lithics) glacial sediment is transformed into sorted, arkosic sand (< 3% lithics) over distances of less than 10 km on the upper continental slope and shelf. Arkosic or quartz sand thus generated is often interbedded with diamictons, which are either glacial or nonglacial (slumps and debris flows) in origin. Therefore, such associations in ancient sequences do not rule out a glacial origin for diamictites (or mixtites), nor do they mandate deep-water sedimentation. Canyon systems are at present isolated from major sediment input; however, an extensive (∼ 662,000 km2) abyssal submarine fan complex containing glaciogenic sands implies canyon activity since initial glaciation of the region. Sediment may have been supplied to canyons in the past by subglacial meltwater discharge at times of maximum ice grounding. Sand may also have been generated directly at canyon heads by strong contour currents impinging on the shelf break, as typically occurs elsewhere around the Antarctic margin today. This focus of current energy at the shelf break does not exist in the eastern Weddell Sea today. Thus, given the great depth of the continental shelf, eustasy is far less important than glacial and oceanographic conditions (that is, the position of the ice sheet grounding line relative to the shelf break and the level and intensity of contour currents) in regulating sediment supply to submarine canyons. In the absence of major canyon activity, intercanyon sediment transport appears to be the most significant source of slope and abyssal sands.