Late Cenozoic sedimentation on the Washington continental slope

Abstract

The Washington continental slope, which is situated between the continental shelf on the east and Cascadia Basin on the west, consists of an upper and lower slope. The lower slope of borderland is composed of north to northwest trending, en echelon, anticlinal ridges and intervening sediment-filled basins. The indurated mudstone and limestone ridges represent older Cascadia Basin turbidites that have been accreted onto the continental margin within the last two million years as a result of the underthrusting of the Juan de Fuca plate beneath North America. The ridges ages and the borderland structure indicate that deformation has progressed in a westward direction with time and has probably continued up to the present. Folding and thrusting of the outermost or marginal ridges is presently occurring at a rate of 0.7 cm/yr. The depressions between these ridges, which may be filled with hemipelagic and turbidite sediment, often form valleys that drain the submarine canyons notching the upper slope. Since all the borderland ridges are probably less than 2 million years old, the deposition of postdeformational sediment between the ridges must have occurred during the Pleistocene. Approximate Pleistocene sediment accumulation rates range from less than 0.5m/1000 yrs to more than 1m/1000 yrs. Most of this sediment, trapped in the borderland basins, was probably deposited by turbidity currents during periods of glaciation on the adjacent continent. Although deformation of the lower slope has continued throughout the Pleistocene, there is no evidence in the seismic reflection profiles that the major routes of sediment dispersal have shifted significantly since the beginning of postdeformational sedimentation. The sediment in the borderland piston cores consists of interlayer turbidite and hemipelagic deposits. The sediment is divided into glacial and postglacial sections based on a change from foraminiferan-rich, gray lutite to radiolarian-rich, olive gray lutite. This faunal change, which begins at about 10,500 yrs B.P. on the lower slope, is time transgressive in the northeast Pacific and may separate geologic-climate units. It cannot be used to define the Pleistocene-Holocene time-stratigraphic boundary. The postglacial sediment deposited between 10,500 yrs B.P. and the present can be divided into early and late postglacial sections based on the lowest horizon of Mazama glass deposited about 6,750 yrs. B.P. During the last glacial period turbidity currents played a major role in deposition of sediments. The lower continental slope was apparently deluged with turbidity currents enroute to the deep-sea fans of Cascadia Basin. All the canyons notching the upper slope were probably active until about 10,500 yrs. B.P. With the onset of postglacial conditions the importance of hemipelagic sedimentation increased. All the canyons were probably less active and turbidity currents apparently occurred less frequently, were composed of finer grained material, and tended to be depositional in nature. Over the last 6,750 years Astoria, Grays, and Juan de Fuca Canyons have been filling up with hemipelagic mud and some sand/silt turbidites. Willapa Canyon remained an active avenue for nondepositional or erosional turbidity currents until about 5,000 yrs B.P. when it too began to fill up. All these canyons presently appear to be filling up with fine-grained material, which is probably derived primarily from the Columbia River. Quinault Canyon apparently acts as a funnel for a large amount of material that is moving northward along the continental shelf. This material accumulates in the head of the canyon for about 500 years before it is flushed down the canyon as a large turbidity current that flows down lower Quinault Canyon, out lower Willapa Canyon and into Cascadia Basin.