UNDERSTANDING GLACIER SLIDING FROM SUBGLACIALLY DEPOSITED SILT SKINS

In the past two decades, the retreating Mendenhall Glacier in Juneau, Alaska has exposed a bedrock ridge spotted with patchy coatings of calcite-cemented silt to sand-sized lithic grains that can aid our understanding of subglacial processes. These coatings, referred to here as 'silt skins', range from 0.5 to 20 mm in thickness and display two distinct morphologies: striated and corrugated. Striated silt skins are thin, located mainly on stoss slopes, and preserve local striation direction. Thicker, corrugated skins form on lee slopes and consist of parallel micro-ridges elongated in the local down-dip direction. Micro-ridges are constructional features enhanced by erosional processes; wavelengths generally range from 1 to 10 mm.

Thin sections, SEM images, and hand specimens demonstrate that matrix/cement ratios, average grain size, and micro-ridge height and wavelength vary with position on the rock ridge. Highest, thickest skin populations are concentrated on lee slopes in topographic depressions on the rock ridge. Dirty basal ice samples have a relatively high concentration of dissolved calcium to magnesium despite the low concentration of carbonate in the garnet-staurolite grade meta-sediments of the area.

Features morphologically similar to silt skins have been described by Hallet (1976) at temperate glaciers in Alberta and the northern United States, and attributed to subglacial precipitation of CaCO₃ on limestone bedrock. Silt skins at the Mendenhall Glacier formed in the subglacial cavity system from processes likely related to regelation flow and cyclical fluctuations in subglacial hydrology. While they do not persist in subaerial exposures for longer than ~ 20 years near the Mendenhall Glacier, silt skins may provide a useful tool for understanding the dynamics of glacier flow over bedrock.