SIGNIFICANCE OF SILT PELLETS IN MELT-OUT TILLS

The presence of pelletoidal greywacke clasts in iceberg-rafted portions of the Gowganda Formation of Canada have been used to infer the nearby existence of glaciers (Ovenshine, 1970). More recently, silt pellets observed in Quaternary glaciolacustrine deposits in New England have been related to ice rafting of basal ice formed by glaciohydraulic supercooling (Ridge, 2002).

Recent observations of sedimentological processes at the Matanuska Glacier, Alaska, demonstrate that silt pellets and other aggregates occur in melt-out tills developed from debris-rich basal ice. Through the mechanism of glaciohydraulic supercooling, debris becomes distributed within the basal ice in various modes, including as suspended aggregates of various texture that are discontinuously distributed in clear ice and as star-shaped silt aggregates that are concentrated at ice crystal boundaries. Slow, in situ melting of basal ice buried by ice-marginally-released sediment gradually and slowly releases the debris to form melt-out till. Direct observation of the till as it originates from the basal zone shows that silt pellets can be preserved without alteration. Because of the textural and sometimes compositional differences from surrounding material of the diamicton, they are clearly recognizable and, when found in association with laminated melt-out tills, may provide direct evidence for glaciohydraulic supercooling.