Studies of fine-grained sediment transport processes and products in the California Continental Borderland

Abstract

Summary The California Continental Borderland contains a series of margin basins which are sediment traps at increasing distance from the principal terrigenous and biological sediment sources located in the northern coastal areas. Terrigenous sediments are contributed during a short winter season from small steep coastal drainages. Sand content can be as much as 30–40% of the total contributed load. Silts and clays are sorted out of the river input by coastal wave action and distributed offshore by the seasonal ocean circulation systems. Suspended sediment moves to the basin floors via nepheloid plumes that are generated in the well mixed coastal waters and move offshore with the currents as surface, mid-depth and bottom turbid plumes. During times of low suspensate content in the dry season, lateral transport is dominant. At times of high sediment input and higher concentrations of fine suspensates in the coastal waters, cascading to successively deeper pycnoclines becomes an important process. Resuspension of fine sediments from the outer shelf substrate and the axes of submarine canyons by internal waves and tidally generated currents also contributes to the load of the bottom nepheloid plumes. Large semi-fixed seasonal eddies in the Borderland surface circulation hold the suspended load in the northern sector of the borderland for several days to weeks. Zooplankters have ample opportunity to filter out this material and aggregate it into faecal pellets which rapidly fall to the local basin floors. The removal is sufficiently effective so as to deposit a minimum of 60% of the continental contribution within the northern inner zone of the borderland. High sedimentation rates in these basins promote mass-movement and also decrease the already low oxygen contents of basin floor water as a result of oxidation of the organic fraction of the suspended load. Primary sedimentation structures are preserved in these anaerobic basin floor environments. Mass-movements initiate mud turbidity currents that move to the central basin floors. Mass-movements include creep and sliding of thin sheets from low angle slopes. These effects are amplified following exceptional floods at intervals of around 30 years.