Sedimentation processes and acoustic stratigraphy in the Bellingshausen Basin

Abstract

DSDP Leg 35 drilling results and seismic reflection profiles from the Bellingshausen Basin allow definition of the sedimentary processes which have shaped much of the deepwater West Antarctic continental margin. The continental rise exhibits three distinct types of acoustic signature characteristic of specific sedimentary processes. The upper rise has a smooth to irregular seafloor above highly reflective sediments with poor acoustic lamination or no coherent reflectors; the sediment probably has been emplaced chaotically by slumping from the adjacent continental slope and has been modified by local slumping and turbidity currents. The central continental rise exhibits moderately reflective but finely laminated sediments containing large sediment waves, smaller-scale migrating sediment dunes and leveed channels. The primary constructional processes are deposition from channel-bank overflow of turbidity currents and from the nepheloid layer maintained by the westward-flowing Antarctic Rise Countercurrent. The lower rise has very reflective sediments with moderate to strong acoustic lamination which represent proximal deposition from turbidity currents and local reworking by bottom currents. Development of the continental rise has caused seaward migration of these acoustic provinces with time. In addition, a marked change in depositional style on the central continental rise probably during Late Oligocene or Early Miocene time may have been caused by the formation of the Antarctic Rise Countercurrent at the time when the Drake Passage was deeply breached to the flow of bottom water. Correlation of specific reflecting horizons with events of Antarctic glaciation generally is not possible, although one widespread and highly reflective horizon may represent coarse debris deposited during a glacial maximum in the latest Miocene or Early Pliocene. The Bellingshausen Abyssal Plain contains highly reflective sediments which generally have well-defined, closely spaced and horizontal acoustic laminae; reflector definition is best in distal parts of the plain. These sediments represent proximal to distal deposition of turbidites, the most seaward of which are ponded into the Eltanin and adjacent fracture zones. Frequently discontinuous reflectors within the abyssal plain may indicate sediment reworking by flow at the base of the Antarctic Circumpolar Current.