A Sedimentary Model of the Continental Margin Off Oregon: ABSTRACT

The continental margin west of Oregon consists of a generally convex-upward surface 35 to 60 nautical miles wide. The continental shelf, which forms the upper part of the surface, slopes seaward at less than one degree and ranges irregularly in width from 9 to 35 miles. Several elongate hills or banks rise above the general End_Page 336------------------------------ shelf level. The lower part of the continental margin (continental slope) increases in average declivity from about 1° to 10°, and is modified by numerous ridges, hills, benches, and valleys. Stratigraphic, structural, and geophysical data suggest that during the Tertiary as much as 20,000 feet of sediment accumulated off the central coast of Oregon in the area of the present continental shelf. Recent sediments in this region consist of well-sorted, fine to very fine, detrital sands on the inner shelf, grading to poorly-sorted glauconite-rich clayey silts on the outer shelf. Continental slope sediments are primarily clayey silts containing small percentages of Foraminifera, radiolarians, diatoms and sponge spicules. Lithologic and faunal similarities of the Recent sediments to sedimentary rocks exposed along the coast and on the shelf and slope indicate that deposition during late Tertiary time occurred in shelf and slope environments. The fossil faunas also indicate that parts of the continental margin may have been uplifted as much as 4,000 feet since the late Tertiary, and that there has been a general westward shifting of the sites of sediment accumulation. End_of_Article - Last_Page 337------------