USING TILL TEXTURES TO DETERMINE THERMAL ZONES AT THE MARGIN OF EAST ANTARCTIC OUTLET GLACIERS

The textures of tills in the western Dry Valleys region of southern Victoria Land, Antarctica, reflect the concentration of water at the ice-sediment interface. Coarse-grained, sandy tills that lack glacially molded and striated clasts are commonly associated with cold-based ice. Fine-grained, silt-rich tills with abundant glacially eroded clasts are indicative of wet-based ice. Here, we describe textural changes of Asgard till near its upper-elevation limit in Sessrumnir Valley that are best explained by deposition from polythermal glacier ice, changing from a wet-based inner zone to a frozen-based marginal zone. The Asgard till reflects deposition from lobes of an expanded outlet glacier that filled upper Wright Valley and spilled southward into north-facing valleys of the western Asgard Range. Our observed textural changes, based on analyses of 5,500 clasts from Asgard till in Sessrumnir Valley, along with the mapped distribution of undisturbed ventifacts that cap colluvium beneath Asgard till, show that the thermal transition occurred over a lateral zone about 200 m in width. This zone, which is apparent in high-elevation air photographs of Sessrumnir Valley, is concentric about the drift margin and reflects the degree to which underlying colluvial deposits were incorporated into Asgard till.