The Washington continental slope: Quaternary tectonics and sedimentation

Abstract

An analysis of the tectonics and sedimentation of the lower continental slope off the Washington coast indicates that although deformation has been relatively continuous throughout the Pleistocene, present rates of compression may be much less than the 2.3-cm/year rate derived from apparent plate motions. Over the last 0.5 m.y.B.P. or since the formation of the late Pleistocene discontinuity in Cascadia Basin, folding and thrusting of the outermost or marginal ridge of the lower slope have accommodated an average compression rate of about 0.7 cm/year. Microfossil ages of samples from several lower slope ridges confirm that the entire lower slope has been formed during the last 2 m.y.B.P. with deformation progressing in a westward direction. This Pleistocene deformation of Cascadia Basin sediment against the continental margin, resulting from the subduction of the underlying Juan de Fuca plate beneath North America, has produced a lower slope or borderland composed of north-trending, en-echelon, anticlinal, mudstone ridges. These anticlinal ridges have subsequently controlled sediment dispersal patterns and trapped both hemipelagic and turbidite sediments, which have accumulated during the Pleistocene at rates of less than 0.5 to more than 1 m 1000 years . A comparison of the sediment volume within the borderland basins and on Nitinat Fan indicates that at least 2 3 of the sediment originating from the upper slope canyons during the Pleistocene has bypassed the Washington slope and has been deposited in Cascadia Basin. During the last glacial period the borderland was apparently deluged with turbidity currents en route to the deep-sea fans of Cascadia Basin. With the onset of postglacial conditions about 10,500 B.P., turbidity currents occurred less frequently, contained finer-grained material, and tended to be depositional. Over the last 5000 years all of the major canyons have been filling with hemipelagic mud and sand/silt turbidites except for Quinault Canyon which flushes about every 500 years.