Basal till fabric and deposition at Burroughs Glacier, Glacier Bay, Alaska

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Abstract
Basal till and debris-rich ice in the basal transport zone of Burroughs Glacier, southeast Alaska, were studied to develop a model for till deposition and to acquire new pebble fabric measurements from till at a modern glacier. Studies of till deposition and fabric measurements from modern glacial deposits are important for studying the genesis of Pleistocene and pre-Pleistocene glacial sediment. Basal till at Burroughs Glacier is deposited at the ice margin by the melting of basal debris-rich ice that stagnated progressively, from the bottom upward, beneath moving ice during deglaciation. The debris-rich ice is typically ∼1-3 m thick and has very high sediment content ranging from 60% to 70% by volume. The long axes of pebbles in the debris-rich ice, and in other parts of the basal transport zone of the glacier, are typically well oriented parallel to ice flow. The average S1 eigenvalue for nine fabrics measured in the basal transport zone is 0.8092 ± 0.067σ. The basal till is massive, matrix-supported, silty-sandy diamicton. Striated, lodged boulders, and crag and tails are common on the till surface. S1 eigenvalues calculated from 27 pebble fabrics measured in the till range from 0.4930 (no fabric) to 0.8261; the average value is 0.6757 ± 0.092σ. The range of values is nearly identical to that reported for fabrics from "undeformed lodgment till" by Dowdeswell and Sharp (1986). Although the final process of till deposition at Burroughs Glacier is passive melt out from stagnant, debris-rich ice, the nature and pebble fabric of the till are inherited from active ice processes, specifically the progressive stagnation of debris-rich ice beneath active ice. Evidence of differential movement (shear) within the debris-rich ice during deposition is reflected in a change in fabric azimuth with depth in the till, beveled bullet boulders, and shear surfaces. These features are typically thought to be characteristic of lodgment till or brittle deformation of subglacial sediment. The melt-out process is passive because of the very high debris content of the debris-rich ice. If the same sequence of till deposition occurred in older glacial deposits and the debris content of the basal ice was high, the final process of melt-out would not be reflected in properties of the sediment.

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