Till characteristics, genesis and transport beneath Antarctic paleo-ice streams

Abstract

Marine geophysical data from the Antarctic continental shelf indicate the former presence of ice streams that drained through bathymetric troughs during the last glacial cycle. Streaming flow is recorded by elongate subglacial bedforms, formed in the upper part of an acoustically transparent sediment layer. Cores from the layer demonstrate that it is a weak and porous subglacial till (“soft till”). Shear within the soft till was concentrated in zones that were 0.1–0.9 m thick. The soft till is a “hybrid” till, formed by a combination of subglacial sediment deformation and lodgment. The base of the soft till is marked by a strong reflector which represents the top of a denser and stronger “stiff till.” The form of this reflector ranges from flat and continuous to deeply grooved and irregular, implying the operation of different mechanisms of sediment mobilization and incorporation into the soft till layer. The irregular form of the basal reflector is consistent with grooving, whereas the flat and continuous form is more consistent with sediment mobilization as a subglacial traction carpet related to changes in basal effective stress. Geophysical evidence for large-scale advection of the soft till, combined with evidence for deformation partitioning into relatively thin layers up to 0.9 m thick, indicates that localized shear zones integrated to transport significant volumes of sediment beneath Antarctic paleo-ice streams.