DISTRIBUTION OF THE GRANITE-RICH DRIFT ASSOCIATED WITH OLD ICE IN BEACON VALLEY, ANTARCTICA

In 1903 H. T. Ferrar observed granite boulders “resting on the side of Beacon Height West on a surface of sandstone.” Those boulders are glacial erratics associated with a granite-rich drift in the northern half of Beacon Valley (BV). That drift overlies stagnant glacier ice on the central valley floor reportedly >8.1 Ma (Sugden et al., 1995; Marchant et al., 2002). The granite drift is inferred to be >11.3 Ma in adjacent Arena Valley (Marchant et al., 1993). The outer and upper limit of the granite drift is easily mapped because there is no granite bedrock in BV. Except near its upper limit, where there are only scattered granite erratics, granite drift is mud-rich and contains abundant striated clasts. Granite drift in northern BV is distinct from colluvium and alpine glacial deposits at the head of BV and tributaries. These deposits, some of which also overlie buried ice lack granite erratics, are mud-poor, and derived entirely from local bedrock.

The outcrop pattern of granite drift in BV can be used to constrain the shape and thickness of the expanded Taylor Glacier that extended into BV from the north. Granite drift exists up to the elevations given at the following locations from N to S: a) 1750 m in the mouth of the pass between West and East Beacon, b) 1700 m both west and east of Aztec and Maya Mts, c) 1675 m in the first valley S of West Beacon, d) 1700 m in the mouth of University Valley, and e) 1600 m at the north side of the mouth of Farnell Valley. This implies that the lobe of Taylor Glacier that left the granite drift in BV was at least 650 m thicker than today where present Taylor Glacier ends in BV, and 350-450 m thicker than today where the stagnant old ice is preserved beneath granite drift on central BV floor. Sublimation of ~400 m of ice in ~8 Ma to ~11.3 Ma, or ~50 m/Ma to ~35 m/Ma, is consistent with estimates of sublimation rates derived from cosmogenic iotopes in debris over the old ice of 5 m/Ma (Schafer et al., 2000) to 50 m/Ma (Stone et al., 2000; Marchant et al., 2002).