Abstract

For accretionary shelves, sediment accumulation largely depends on the amount and type of sediment supplied. On a worldwide basis, the largest sources of sediment to shelves are rivers. Other sources (in particular, glacial, biogenic, coastal) may be locally important for sediment supply. Shelves generally have a basal sand layer, which formed during the Holocene transgression, and on which more recent fluvial sediment has accumulated. The sediment discharge of most rivers is greater than 75% mud. Sediment accumulation associated with large fluvial sources (> 10⁸ metric tons/yr) is characterized by inner-shelf mud deposits and the accumulation associated with smaller sources is characterized by mid-shelf mud deposits. Physical processes (primarily surface waves) inhibit net accumulation of mud on inner shelves, and prevent accumulation where fluxes of mud are small. In the latter case, sand accumulates from fluvial sources and the retreat of the coast. Mud deposits overlie the trans-gressive sand layer, whose surface may be plain or built into bed forms (e.g., sand waves). This interface between mud and sand is commonly transitional due to the mixing of mud and sand within a surface layer of the sea bed (surface mixed layer) during the embryonic stages of mud deposit formation. Modern sand accumulates within mud deposits and shows a distinct depletion along dispersal systems. In proximal regions of large dispersal systems (>10⁸ tons/yr), biological mixing is negligible and sediment accumulation is rapid (>2 cm/yr), therefore sand is preserved as interlaminae and interbeds within the mud deposit. In small dispersal systems, biological mixing occurs and sediment accumulation is slow (<1 cm/yr), therefore sand and mud are homogenized before preservation. Distal regions of all dispersal systems contain little sand, because of its preferential accumulation in proximal regions. Within the muddy, distal portions of dispersal systems, progressive fining is not observed, probably because particles are transported as aggregates.