Bottom currents, sedimentation and ice-sheet retreat facies successions on the Mac Robertson shelf, East Antarctica

Abstract

Measurements of water turbidity, currents, seafloor sediment samples and geophysical data document the sedimentary processes and the Late Quaternary sedimentary history of a continental shelf valley system on the East Antarctic continental margin. The valley is up to 1200 m in depth and strikes across the shelf; it is interpreted as having formed by glacial erosion processes. On the outer-shelf sill of the valley, northwestward (offshore) currents with speeds of up to 0.47 m/s referenced to 100 cm above the seabed were recorded over a 10-month period. Such currents are competent to initiate bedload transport of medium sand and formation of small (ripple-sized) bedforms which explains the occurrence of ripple cross-bedding observed in some X-radiographs of cores. A nepheloid layer with concentrations of up to 3.5 mg/l was noted at two stations and the available evidence suggests that density flows resulting from the sea-ice brine-rejection mechanism are competent to entrain shelf bottom sediments episodically. Sediment cores taken from this environment document the succession of facies resulting from retreat of glacial ice from the shelf during the Holocene transgression. Under low sea-level, glacial conditions, the ice sheet is interpreted to have been grounded on the outer shelf, where it deposited a grounding line moraine. Rising sea level and global warming caused the ice shelf to retreat leaving a sub-ice shelf, glacial-marine mud. This is overlain by a laminated to massively bedded ice-rafted debris-rich facies, interpreted as being deposited at the glacial calving front. Above this is a siliceous mud and ooze facies having a basal age of about 10,000 years BP, characterising modern, open-marine conditions. Two types of iceberg keel marks on the outer part of the shelf valley are recognised from sidescan sonographs: one relict, arcuate type formed during the retreat of the ice shelf prior to 10,000 years BP and a second modern, elongate type that is presently being formed near the shelf break by iceberg grounding processes. This facies succession has been described from other Antarctic shelf environments and is probably representative of facies deposited during Holocene glacial retreat from the Antarctic shelf.