Deposition of sand in a trench-slope basin by unconfined turbidity currents

Abstract

Site 181 of the Deep Sea Drilling Project is located on the seaward margin of a small basin which formed on the accretionary lower slope of the eastern Aleutian Trench. Sand layers recovered from this basin display an unexpected degree of compositional heterogeneity. Evidently, all of the detritus was not derived from a single point source or submarine channel. Instead, unconfined turbidity currents must have converged on the site of deposition from initiation points which were widely spaced across the hinterland of the Gulf of Alaska. Most of the unconfined flows probably were deflected around larger bathymetric obstacles; however, the bathymetric relief associated with many ridges appears to be insufficient for complete blockage of turbidity currents. Ridge bypassing can also occur by direct upslope flow over the ridge crests, and we believe this mechanism represents an important element of forearc sedimentation.