A major trough-mouth fan on the continental margin of the Bellingshausen Sea, West Antarctica: The Belgica Fan

A 330-km length of the little known continental shelf edge and slope of the Bellingshausen Sea, West Antarctica, is investigated using multibeam swath-bathymetric and sub-bottom profiler evidence. The shelf break is at 650–700 m across the 150-km wide Belgica Trough, and to either side is about 500 m. When full-glacial ice advanced across the shelf to reach the shelf break, it was partitioned into fast- and slow-flowing elements, with an ice stream filling the trough. This had important consequences for the nature and rate of sediment delivery to the adjacent continental slope. Off Belgica Trough, the upper continental slope has convex-outward contours indicating a major sedimentary depocentre of gradient 1–2°. Acoustic profiles and cores from the depocentre show a series of diamictic glacigenic debris flows. The depocentre is interpreted as a trough-mouth fan, built largely by debris delivered from the ice stream. The slope is steeper beyond the trough margins at up to 6°. The main morphological features on the Bellingshausen Sea slope are gully systems and channels. Major canyons and Late Quaternary slides are absent. Most gullies and channels are found on the fan. Gullies are about 15–25 m deep, a few hundred metres wide and some are > 25 km long. The largest channel is over 60 km long, about a kilometre wide and 10 to 15 m deep. The channels provide pathways for sediment by-passing of the upper slope and transfer to the continental rise and beyond by turbidity currents. Gullies on the Bellingshausen Sea margin cut through debris flows on the slope. Assuming the debris flows are linked mainly to downslope transport of diamictic debris when ice was at the shelf edge under full-glacial conditions, then those gullies cut into them formed during deglaciation. Belgica Fan is > 22,000 km2 in area and about 60,000 km3 in volume. It is the largest depocentre identified to date on the continental margin of the West Antarctic Ice Sheet, fed by an interior ice-sheet basin of approximately 200,000 km2.