Evidence of bed deformation beneath the Wright Lower Glacier, South Victoria Land, Antarctica

A tunnel excavated into the margin of Wright Lower Glacier revealed a basal ice sequence dominated by the presence of frozen blocks of sand that contained well-preserved fluvial sedimentary structures. The sedimentary structures, together with the presence of ice between the frozen blocks of sand and ice wedges in the uppermost block of sand, suggest that the material is an overridden proglacial permafrost environment. Velocity and strain measurements made in the tunnel show that the permafrost is being deformed and the glacier has entrained the sediment blocks. The measurements also reveal a compound basal velocity profile that is the result of no or very low internal shear in the sand blocks whereas the relatively clean ice experiences relatively high strain rates. The pattern of strain and displacement strongly resembles the velocity structure associated with the deformation of subglacial sediment. However, the low basal ice temperature of the glacier (-16°C) is inconsistent with previous accounts of subglacial sediment deformation that attribute deformation to elevated pore water pressures. We conclude that subglacial sediment deformation beneath cold-based glaciers can occur the glacier substrate contains ice-rich sediments.