PATTERNED-GROUND AND SUBLIMATION TILL OVER BURIED GLACIER ICE IN SOUTHERN VICTORIA LAND, ANTARCTICA

A thin glacial diamicton, informally termed Granite drift, occupies the floor of central Beacon Valley in southern Victoria Land, Antarctica. Granite drift is < 1.0 m thick and rests with sharp planar contacts on underlying glacier ice. Progressive sublimation of this debris-rich glacier has led to the formation of Granite drift as a lag deposit (sublimation till) that caps the ice. The thickness and texture of Granite drift strongly influences sublimation rates for the remaining ice below. This ice, > 30 m thick, is reportedly of Miocene age (> 8.1 Ma) based on ⁴⁰Ar/³⁹Ar analyses of presumed in situ ashfall deposits that occur within Granite drift. If the age is correct, then the buried glacier in Beacon Valley may yield precise constraints on atmospheric conditions over Antarctica during Miocene time. Two points are at odds with this possibility. First, theoretically derived sublimation rates calculated for Beacon Valley cannot sustain buried ice for more than 104 ky. Second, the presence of well-developed patterned ground on Granite drift implies extensive cryoturbation, sediment mixing, and reworking of ashfall deposits. If the dated ashes contained within Granite drift have been reworked and re-transported, then they cannot be used to place a minimum age on the underlying ice and the reported Miocene age is unfounded. Here, we describe concentrations of cosmogenic ³He from two profiles through Granite drift that, along with detailed sedimentologic and geomorphic analyses of the drift and underlying ice, are used to construct a model for patterned ground formation that is consistent with in situ Miocene ashes, slow ice sublimation ~ 5-60 m/ Ma, and preservation of Miocene-age glacier ice.