Dewatering of Sediments along the Cascadia Margin: Evidence from Geotechnical Properties

Abstract

The lithologic composition and bulk physical properties of sediments recovered by Kasten coring at 29 sites in the Cascadia Margin give insight into the processes of tectonic dewatering from the Pleistocene to the present. Shear strength and water content, in particular, reveal the degree to which tectonic influence has modified the sediments. The undeformed sediments of the abyssal plain have low shear strength and high water contents throughout the maximum cored depths of 3 m. In contrast, sediments at the base of the slope are characterized by extremely high shear strength and low water contents. Between these two end members, sediments from the deformation ridges on the continental margin show intermediate shear strength and water content values. There is a close relationship between the water content/porosity and the degree of sediment diagenesis. High water contents are restricted to silty to clayey intervals of minor consolidation, whereas low water contents appear in the cemented sandy to silty layers. Patterns of fluid migration in the near-surface section are inferred from these data and from pore water chemistry. Calculated rates of fluid expulsion at the seafloor are 2 × lO~4 L/m2/day (2.3 × I0 12 m3/m2/s). The results suggest that fluid expulsion is higher off central Oregon compared to the area off southern Washington. Based on the bulk mineralogical composition of the sediments examined and some grain-size analyses, I conclude that generation of the consolidated layers is closely related to the dewatering processes, a widespread phenomenon along the Cascadia Margin evidenced in vent fields.