Microstructures of glacigenic sediment-flow deposits, Matanuska Glacier, Alaska

Microstructures of glacigenic sediment gravity-flow deposits formed at the terminus of the Matanuska Glacier, Alaska, were analyzed to characterize flow type. These sediment flows have been classified into four types based primarily on water content and sedimentological characteristics (Lawson, 1979a, 1982). Thin sections of flow deposits show a variety of micro- and mesoscale characteristics that vary according to water content of the source flow. Wet-type flow deposits are characterized in thin section by a well-defined parallel and imbricated microclast fabric and thin laminations resulting from laminar to plastic flow regimes. Dry-type flow deposits are characterized in thin section by bi- or polymodal or random microclast fabrics, greater textural heterogeneity, and deformational microstructures associated with plastic to brittle flow regimes. Thin laminations and a "laminar flow fabric" in wet-type flow deposits may be characteristic of sediment gravity flow in a glacial environment. Characterization of these microstructures supports the contention that micromorphological analyses can be used to elucidate sediment flow genesis and the conditions of the flow just prior to deposition. Thus, micromorphology may also be useful for differentiating sediment-flow type in Pleistocene diamictons in other locations.