High Sediment Input Research Articles

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

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.

The anatomy of a Lower Palaeozoic accretionary forearc: the Southern Uplands of Scotland

Summary The Southern Uplands of Scotland consists of an accretionary prism which had a prolonged (over 45 Ma) development off the SE coast of ancient North America (Laurentia) during the late Ordovician and Silurian. To the north, a forearc basin occupied the Midland Valley. An arc massif of older metamorphic rocks in the Grampian Highlands, and capping calc-alkaline volcanics, supplied much of the sediment to the trench. In its early development (late Ordovician) the accretionary complex incorporated slivers of ocean-floor material as well as thick turbidites into a steep lower trench slope. Later, in the Silurian, a trench slope break emerged shedding sediment north into the Midland Valley forearc basin. By that time only turbidites and black shales were being accreted. The Southern Uplands is dominated by coherent strata, and, despite intense imbrication, is devoid of mélange. We conclude that the accretionary prism was the result of high sediment input to the trench and very slow, oblique, subduction of the Iapetus Ocean eastwards below Scotland. Décollement surfaces developed during accretion preferentially utilised a highly incompetent black shale layer near the base of the subducting sequence. Prior to the inception of accretion (in the Caradoc), we postulate sediment subduction, and possibly subduction erosion, during the Cambrian and early Ordovician.