Ice-rafting at a temperate tidewater glacier : McBride Glacier, Glacier Bay, Alaska
Gottler, Paul F.
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Rafting of debris by glacier ice is an integral part of glacimarine sedimentation. Changes in the debris characteristics and proportions of iceberg-rafted debris (IBRD) versus hemipelagic sedimentation were analyzed in a modern glacial setting to assist in determining the positions and types of paleoglacial termini from successions in the stratigraphic record. Characteristics of glacially transported debris are dependent on the mode of glacial transport. Basal debris is generally well rounded and contributes approximately 82% of all IBRD deposited in McBride Inlet. Supraglacial debris is angular and the generally singular rock type as its sole source is the medial moraine which contributes approximately 8% of all IBRD. Horizons in bottom sediments where the oligomictic clasts occur are correlatable with calving events at the medial moraine. The remaining 12% of IBRD in the inlet is from englacial sources that are derived from both supraglacial and basal sources. Rafted debris ranges from clay to boulder sizes and is generally released from bergs when melting causes rolling, break-up, or debris flows off the ice. Particle-by-particle release also occurs in less debris-laden bergs and contributes significant quantities in the distal areas where sedimentation from suspension settling is greatly decreased relative to ice proximal sedimentation rates. Overall, samples of bottom sediment are quite variable in their proportions of IBRD, ranging from less than 1% to greater than 50%, which is attributed to episodic release of debris from bergs. Dispersal of most IBRD within McBride Inlet is controlled by the travel paths of larger debris-laden bergs. Most larger bergs are derived from the debris-rich basal zone of McBride Glacier; consequently, deposits in the gyre receive higher proportions of IBRD than elsewhere in the inlet. The tidal prism ranges to 7 m and during falling tides bergs ground on bathymetric highs. Once grounded they often become unstable and dump their debris, creating an area outside the gyre where proportions of IBRD are elevated with respect to other areas within the inlet. Ice proximal IBRD contains debris originating from all levels of glacial transport, allowing differentiation of tidewater fronts from ice shelf conditions.