Results of clay mineral analyses of surface sediments from the Oregon continental margin are combined with estimates of continental sediment influx to determine the relations between major source regions and dominant dispersal pathways of fine grained sediment in offshore areas. Both the clay assemblages and the sediment supply estimates indicate that the rivers of the northern California Coast Range, the Klamath Mountains and the Columbia Watershed are the principal sediment suppliers, providing over 90 percent of the silts and clays to the Oregon margin and Cascadia Basin. In order of importance, the major sediment sources appear to be, in l0 6 tonnes/year, the Eel (24.8), Columbia (14.3), Klamath (10.9), Rogue (4.7 estimated), Umpqua (3.2) and Mad (2.5) rivers. Trends in the clay mineral distributions suggest that deposition of fine grained sediments on the Oregon continental shelf is controlled by seasonal meteorological and oceanic factors such as periodic peak river discharges, winter storms and summer coastal upwelling. A lobe of montmorillonite-rich clays on the northern Oregon shelf may be caused either by southward extension of the Columbia River Plume in May-June, or more likely, from discharges of local Coast Range streams and northward transport during the winter months. Montmorillonite-rich clays found in submarine canyons and channel systems suggest that much of the Columbia load is channelled to abyssal fans off Washington and northern Oregon. On the continental slope, clays relatively rich in chlorite indicate that a portion of the massive influx from the northern California and southern Oregon coastal streams is carried northward, probably via the California Undercurrent. The imprint of this poleward flow can be traced to marginal areas off the Washington coast. Clays in the Cascadia Basin appear to be a mixture of Columbia River and Klamath/California Coast Range type sediments, suggesting that the northward flow may not be confined to the continental slope.

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