Bedrock Plateaus within the Ross Embayment and beneath the West Antarctic Ice Sheet, Formed by Marine Erosion in Late Tertiary Time

Abstract

Ice penetrating radar (mostly airborne) and marine seismic surveys have revealed plateaus and terraces about 100–350 m below sea level beneath parts of the Ross Embayment including the West Antarctic ice sheet, the Ross Ice Shelf, and the eastern Ross Sea. These surfaces cover many thousands of square kilometers and are separated by bedrock troughs occupied by the West Antarctic ice streams. We interpret these surfaces as remnants of a level surface formed by wave erosion when the coastal regions of Antarctica were relatively free of ice. The flat and level nature of the surfaces that are near the same depth over large distances supports an origin by marine rather than glacial erosion. Marine seismic reflection profiles over one of the plateau remnants show thin, flat-lying glacial marine sediments draped with angular unconformity over gently dipping sediments of early Miocene age. Ice sheet and global sea level histories suggest that the shallower plateaus were last eroded in the middle Miocene, possibly within a warm interval from about 17 to 14 Ma, prior to formation of the modern West Antarctic ice sheet. The plateau surfaces cannot be directly correlated to Ross Sea unconformities but they may be extensions of ∼14-Ma unconformity RSU4. The plateaus along the Siple Coast, with depths around 350 m, do not rebound to close to sea level for models of removing past and present ice load. Another possible factor is that western Marie Byrd Land lithosphere was heated in Oligocene time due to substantial extension or intensified mantle plume activity. Subsequent cooling has caused a moderate amount of crustal subsidence since then. These plateaus are similar to the emergent wave cut platforms interrupted by deep fjords, termed “strand” flats that rim the coasts of Norway.