MICROMORPHOLOGIC ANALYSIS OF TILLS IN THE ROSS SEA, ANTARCTICA: DEMONSTRATING DEFORMATION AND CORRELATION TO ICE BEHAVIOR

During the last glacial maximum, the West Antarctic Ice Sheet expanded, covering most of the continental shelf in the Ross Sea. Sedimentological and geophysical data have been used to infer that the expanded ice sheet included ice streams whose fast flow was facilitated by a deforming bed at the base of the ice. Micromorphologic analysis of sediment thin section samples gives weight to this hypothesis. Thin section samples were taken from the sediment along the axis of a paleo-ice stream in five of seven geomorphic zones as well as off the trough axis. The microfabric in these samples includes lineations, rotation structures, necking structures, and Riedel shears among others. Till pellets are found in many samples, including those from depth within the till. In several locations, tills can be observed that exhibit strong deformation indicative of high strain rates (cf. Thomason & Iverson, 2006) overlying undeformed layers, possibly illustrating the insertion of rafted older sediment into the glacial depositional system or alternately sudden changes in stress level transmission within the till package. This evidence from the Ross Sea indicates that a complex package of sediments has been deformed, transported, emplaced, and in some cases overridden and re-deformed as the ice sheet advanced over its own glacial-marine deposits in a series of small steps backwards and forwards. While micromorphology cannot delineate diamicton facies as previously identified, i.e., soft versus stiff tills, the fabric analysis does not contradict the previous facies delineation and in some cases is able to clarify the classification of certain samples. As has been observed elsewhere in Antarctica (O'Cofaigh et al., 2005), micromorphology indicates that the stiff tills previously interpreted as lodgement till record significant deformation and are members of a spectrum of tectomicts. Microfabric analyses allows separation of glacial diamictons from marine diamictons and thus provides an additional tool for determining the position of the paleo-grounding line of the ice sheet, which to date has remained in question.