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The continental margin west of Oregon consists of a generally convex-upward surface 35 to 60 mi. (nautical) wide. The continental shelf, which forms the upper part of the surface, slopes seaward at less than 1° and ranges irregularly in width from 9 to 35 mi. Several elongate hills or banks rise above the general shelf level. The lower portion of the continental margin (continental slope) increases in average declivity from about 1°-10°, and is modified by numerous ridges, hills, benches, and submarine canyons.
Off the central part of the Oregon coast the shelf widens from 13 mi. at 45°00^primeN. to 35 mi. at 43°58^primeN., and then narrows abruptly to 16 mi. at 43°55^primeN. Two essentially north-trending shoals, Stonewall Bank and Heceta Bank, dominate the topography of the shelf in this area.
The apparent offlap relationship of late Eocene to middle Miocene marine sedimentary rocks along the shore and the occurrence of Pliocene marine sedimentary rocks on the two banks suggest that one or more Tertiary sedimentary basins exist beneath the continental shelf and slope. Gravity measurements indicate that thick sections of sedimentary rocks may be present. From echo soundings, Stonewall and Heceta Banks are interpreted to be the surface expressions of structures associated with the Tertiary basins.
Fine to very fine detrital sands and glauconite-rich silts and clays cover the shelf in areas between the gravel and rock outcrops common on and in the vicinity of the shoals.
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