Oregon Continental Shelf Sedimentation: Interrelationships of Facies Distribution and Sedimentary Processes

Abstract

The nature and distribution of sedimentary facies on the Oregon continental shelf are controlled by several factors: (1) river discharge and sediment input, (2) estuarine circulation system, (3) wave dimensions and direction, (4) subsurface and bottom currents, (5) density stratification of the water column, and (6) reworking by benthic organisms. Most of these factors were probably operating during the Holocene transgression of the sea which deposited a basal sand facies over the shelf. As sea level approached its present position, mud (silt and clay) began to accumulate slowly on the mid-and outer-shelf. Through the reworking by benthic organisms, this mud was mixed into the underlying basal sand creating a mixed mud and sand facies. A mud facies developed in the vicinity of the Columbia, Umpqua, and Rogue Rivers where the rate of mud deposition exceeded the reworking activity. The drowned mouths of coastal estuaries produce a sediment trap for fluvial and marine sand and the silts and clays that come in contact with the intruding salt wedge. The grain size modes of the modern shelf sediments suggest that the very fine sand (3-4Ø) and finer material are presently transported in suspension through estuaries with a high fresh-water discharge and a large sediment input. This material diffuses through the turbulent surf zone and emerges as three distinct turbid layers on the continental shelf: (1) a surface layer at the seasonal thermocline, (2) a mid-water layer at the permanent pycnocline, and (3) a bottom turbid layer. During the winter, long-period surface waves from the southwest stir the bottom to water depths of 200 m and, when combined with bottom currents associated with internal and surface tides and northward flowing subsurface currents, provide the mechanism to transport very fine sand across the 45-km wide shelf off the Columbia River. This stirring process resuspends previously deposited bottom sediments which are then carried to the lower continental slope in low-density flows. The bulk of the silt and clay, as well as the biogenic debris in the water column, probably bypasses the shelf in the surface and mid-water layers and is deposited on the lower continental slope. Summer waves have shorter periods and are from the northwest, with bottom stirring occurring only to depths of 90 m on the average. These conditions favor deposition of silts and clays on the mid- and outer-shelf. While the fluvial sediment supply from small, coastal rivers is substantially decreased at this time, the discharge from the Columbia River reaches a maximum during the onset of these moderate wave conditions. The sand facies, particularly on the inner shelf, consists of relict sand and gravel or mixtures of these relict deposits and modern sands. The modern mud facies is a transient feature in some areas of the shelf; its presence is largely controlled by the resuspension processes and sediment input on the continental shelf.