MELT-OUT TILL FORMATION AND SEDIMENTOLOGICAL PROPERTIES, MATANUSKA GLACIER, ALASKA

Tills, primary deposits of glacier activity, are deposited either subglacially or in ice marginal environments. The fact that tills may form by in situ melting of basal ice has been documented, but the sedimentological properties and how they develop during the melt-out process are not well documented. Their source, the basal ice, is a potential repository of information on the glacier itself, and the process of melt-out may permit interpretation of this past activity from rigorous sedimentological analysis of the melt-out till. However there is still controversy over whether the melt-out process preserves properties of the debris and ice within the resulting deposit, or disrupts and potentially destroys those properties.

We present an analysis of a melt-out till in the process of formation and the relationship of its sedimentology to that of its basal ice and debris source at Matanuska Glacier. The sequence was exposed in February 1997 by stream erosion of a morainal hummock near the present margin of the glacier. During the winter, cold temperatures cause ice to sublimate and sediments to dry, and thus properties of both the ice and deposit were exceptionally well exposed. The ice source was the debris-rich stratified facies of the glacier's basal zone. It was buried beneath about 0.5 meter of stratified diamicton derived from it, which in turn was overlain by about 2 meters of stratified sediments deposited by sediment gravity flows and thin flows of meltwater originating at the active ice margin. Debris strata in the ice extended uninterrupted into the diamicton. Properties of these strata, including their texture and the delicate features of aggregates and grains, were unaltered in the diamicton, except for the strata being thinner due to loss of the ice on melting (~55 cm of basal ice reduced to ~20 cm). Pebble orientations in the ice were also mimicked in the diamiction except for the long axes having a lower angle of dip. Other sedimentary features of the basal debris and ice were also preserved in the diamiction. A melt-out till sequence observed at Skaftafjellsjokull, Iceland in March 2001 compares in terms of process and property to that at the Matanuska. Our study shows that slow melting of debris-rich basal ice can produce a stratified diamicton that preserves basal zone properties with little disruption to the original ice and debris characteristics.