SEDIMENTARY PROCESSES AT THE BASE OF A WEST ANTARCTIC ICE STREAM - CONSTRAINTS FROM TEXTURAL AND COMPOSITIONAL PROPERTIES OF SUBGLACIAL DEBRIS

Samples of sediments from beneath Ice Stream B (at camp UpB), West Ant arctica, provide the first opportunity to study the relationship betwe en sediment properties and physical conditions in a sub-ice-stream env ironment. Piston coring in holes bored by hot-water drilling yielded f ive 1-3 m long, undisturbed subglacial sediment cores. We analyzed gra nulometry, composition, and particle morphology in these cores. The Up B cores are composed of a clay rich, unsorted diamicton containing rar e marine diatoms, Sedimentary particles in these cores bear no evidenc e of the recent crushing or abrasion that is common in other subglacia l sedimentary environments. The presence of reworked diatoms and their state of preservation, as well as the relative spatial homogeneity of this diamicton, suggest that the UpB cores sampled a several-meter-th ick till layer and not in situ glacimarine sediments. The till does in corporate material recycled from the subjacent poorly indurated Tertia ry glacimarine sediments of the Boss Sea sedimentary basin, which exte nds beneath this part of the West Antarctic Ice Sheet. We propose that the lack of significant comminution in the UpB till is ultimately due to its setting over these easily erodible, clay-rich source sediments . The resulting fine-grained till matrix inhibits glacial comminution, because it facilitates buildup of high pore-water pressures and hinde rs interparticle stress concentrations, Our observations are consisten t with the conjecture that subglacial deformation of weak, fine-graine d tills does not produce significant comminution of till debris (Elson 1988), Based on our findings, we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting, poorly indurated, fine grained sediments, Since such weak till layers create a permissive condition far ice streaming, subglacial geology m ay have an indirect but strong control over the location, extent, and basal mechanics of ice streams.