A Comparison Between Central Oregon Continental Shelf Sediments and Adjacent Coastal Sediments

A study of sedimentary structures and sediment texture of beach and continental shelf sediments provides a basis for the inter-pretation of the recent history of sedimentation on the Oregon con-tinental shelf. The texture of various sediment types suggests that coastal rivers supply a considerable quantity of very fine sand in addition to modern mud to the continental shelf. Relict sediments remain uncovered on areas of the shelf not under the influence of major river runoff and are mixed or covered by modern mud and very fine sand where rivers influence shelf sedimentation. Primary sedimentary structures show that sands ranging from very fine to medium in mean grain size are capable of being moved at shelf water depths from at least 38 to 88 meters. No net transport of sand, however, can be determined from these structures. Organic structures apparently are controlled by water depth and/or grain size and sediment type which determine faunal composition and concentration. At midshe1f depths fine sand cores are character-ized by shell fragments, worm tubes, and worm trails. Beyond 100 meters water depth worm tubes were not found, but worm trails become abundant, and large burrows appear in the sediment