Submarine landslide geomorphology, US continental slope

Abstract

The morphometric analysis of submarine landslides in four distinctly different tectonic environments on the continental slopes of Oregon, central California, Texas, and New Jersey provides useful insight into submarine process, including sediment transport mechanisms and slope stability. Using Geographic Information System (GIS) software, we identify landslides from multibeam bathymetric and GLORIA sidescan surveys based solely on surficial morphology and reflectivity. This method provides useful data in a time- and cost-efficient manner. We measure various aspects of the failures, including landslide area, runout distance, and headscarp height, along with the slope gradient of the runout zone, the failure's scar, headscarp, and adjacent slopes. The largest failures of the four study areas occur in the Gulf of Mexico, adjacent to Mississippi Canyon, and between salt withdrawal basins. Smaller landslides occur within the basins, and at the base of the Sigsbee Escarpment. These smaller landslides tend to have higher headscarps than the larger ones, and often have cohesive material at the base, suggesting a stronger rheology. Oregon has the steepest local slopes, but surprisingly few large failures for a seismically active margin (especially in the north), implying that slope angle and seismic activity may not be the most important slope stability controls. The California continental slope is heavily incised, which makes failure isolation difficult. Most of the landslides occur within the larger canyons (Vizcaino, Pioneer, Monterey) and adjacent to a pock mark field in the Point Arena basin. The majority of landslides offshore New Jersey occur on the open slope between Lindenkohl and Carteret Canyons. Morphometric statistics give us insight into where mass movements occur, how big they are likely to be, their relative importance as sediment transport mechanisms, and the overall slope stability of a given margin. Most landslides occur on slopes less than 10°. Curiously, the steepness of the slope adjacent to the failure tends to be inversely proportional to the runout length. In both California and Oregon, slope failures tend to make the local slope steeper, whereas failures in the Gulf of Mexico and offshore New Jersey will tend to make the local slope less steep. Landslides with rubble beneath the scar are mostly smaller than those without, are deep seated, and make the slope steeper. We use the ratio of headscarp height to runout length as a measure of the failure's dynamic rheology. This ratio in the submarine case is orders of magnitude less than subaerial landslides. Hydroplaning of the failed mass may be responsible for the very long runout lengths. These morphometric relationships give us important insight into landslide dynamics and process in different sedimentary and tectonic environments. (C) 2000 Elsevier Science B.V.