Causes of varied sediment gravity flow types on the Alsek Prodelta, northeast Gulf of Alaska

Abstract

Slope failures and subsequent mass movements have been identified in Holocene glaciomarine sediment on declivities less than 1.3° on the Alsek prodelta, Gulf of Alaska. Isolated collapse features cover less than 10 percent of a nearshore sand deposit, in water depths less than 40 m. In contrast, sediment gravity flow deposits (disintegrative failures) cover more than 95 percent of a clayey silt deposit that is located in water depths between 35 m and 80 m. The morphology of individual disintegrative failures in the prodelta clayey silt indicates an eastward increase in the internal deformation and downslope translation of the failed sediment mass, the most extreme deformations being relatively large linear depressions up to 6‐m deep, 400‐m wide, and 1800‐m long, extending downslope in the easternmost part of the study area. In‐place cone penetration tests show that the nearshore sand is dense and is probably not highly susceptible to cyclic strength degradation and ultimate slope failure. The isolated collapse features are thought to result from the slope failure of more susceptible clayey silt that underlies the sand, sampled in nearby vibracores. The generation of disintegrative failures on the Alsek prodelta involves a drained conversion of the sediment (pore‐water influx) from an in‐place dense condition (State II) to an expanded condition (State I) during storm‐wave loading. Without this conversion, only nondisintegrative failures, typified by limited internal deformation or minor downslope translation of the failed sediment mass are possible. Higher porosity, underconsolidated, clayey silt of the eastern part of the study area is more susceptible to conversion from State II to State I than is the denser, normally consolidated, clayey silt of the western part of the study area. This trend in the porosity and consolidation state of the sediment is expressed as the eastward increase in the internal deformation and downslope translation of disintegrative failures.