CATASTROPHIC COMPLEX LANDSLIDES ON SUMAS MOUNTAIN

In order to determine the geomorphic processes involved in the formation of a 1000m wide fan-shaped deposit on Sumas Mountain in northwestern Washington State, the texture and composition of the sediment deposited was characterized in the field and the morphology of the fan was analyzed using GIS. The fan deposit, which lies at the base of the Collins Creek drainage, is morphologically distinct from typical alluvial fans in its lobate and steep-edged nature; both features are often characteristic of large, catastrophic landslides known as sturzstroms. Lithology of gravels in the fan deposit matches that of numerous 5-10m diameter boulders near the fan apex and of a bedrock scarp at the head of the drainage, which may establish bedrock slope failure at the scarp as the sediment source for the fan. However, the matrix supported nature of the fan sediment indicates grains were transported in a fluid medium, while the poor sorting and angularity of the grains is distinctive of debris flows. Ultimately, these data suggest the fan may be the deposit of a large and complex landslide, in which bedrock failure at the scarp transitions into a debris avalanche that entrains water to become a massive debris flow.