The formation of sedimentary strata in an allochthonous shelf environment: The Washington continental shelf

Abstract

Models of fine-scale (<10° m, <10 2 yr) strata formation can enhance geological interpretations of the stratigraphic record. The Washington continental shelf represents a good locality to develop such a model for shelf strata, because it is undergoing sedimentation of material supplied by the Columbia River. Modern sand accumulates on the inner (<40–60 m water depth) and mid-shelf regions, and modern mud (mostly silt) accumulates on the mid and outer (>120 m) shelf regions. The inner shelf sand and mid-shelf sandy silt are the predominant accretionary deposits on the shelf. Important sedimentological observations obtained by boxcoring the upper 1 2 m of these deposits are: (a) progressive decrease in grain size with distance from the Columbia, (b) downward coarsening within the seabed, (c) loss of distinct sedimentary structure (homogenization of sediment) with distance from the Columbia. Non-dimensional parameters based on sediment transport and benthic biological studies can be used to relate rates of sediment mixing (reworking) to rates of accumulation. These parameters have the potential for quantitative prediction of strata formation, from measurements of active processes. Physical and biological mixing can be assumed to remain relatively constant in an alongshelf direction on the Washington shelf, and accumulation rate is known (from Pb-210 geochronology) to decrease away from the Columbia. Together these factors tend to increase the ratio of mixing to accumulation with distance from the Columbia, and predict the sedimentological observations listed above.