Abstract

Significant rates of sediment accumulation on modem continental shelves are generally associated with fluvial dispersal systems. These dispersal systems primarily contain fine-grained sediment and form mud (silt and clay) deposits. The mud deposits also contain some sand and, despite low proportions of sand, represent the greatest rates of sand accumulation in modern shelf environments. This paper summarizes recent studies of several shelves on which mud deposits are accumulating, and focuses on the relationships of sand with the predominantly muddy deposits. Generally, for small rivers (<10⁶ tons/yr sediment discharge), modem fluvial sediment is trapped in estuaries, but for intermediate rivers (10⁶ to 10⁸ tons/yr), significant amounts of sediment can escape and form mid-shelf mud deposits. The mid-shelf deposit associated with the Columbia River dispersal system (10⁷ tons/yr) is characterized by relatively slow rates of accumulation (<0.2 to 0.5 cm/yr) and by extensive biological mixing. This leads to homogenizalion of mud and sand. The sand content decreases progressively along the dispersal system as a result of the preferential accumulation of sand. For large rivers (>10⁸ tons/yr), sediment can also accumulate on the inner shelf, forming mud deposits that extend from the shore across the mid-shelf region. The inner-shelf deposit associated with the Amazon River dispersal system (10⁹ tons/yr) is characterized by rapid accumulation (1 to 10 cm/yr) and negligible biological mixing in proximal regions. This leads to preservation of stratified mud and sand. Generally, with increasing distance from proximal regions, accumulation rates decrease, mixing rates increase, mud and sand become homogenized, and sand content decreases. The distal portion of the Huangho dispersal system (10⁹ tons/yr) contains only mud, which accumulates slowly (<0.2 cm/yr) over a transgressive sand layer. At the boundary between the mud and sand, a homogenized deposit of bimodal sediment is present. Viewed on a larger scale, both mid-shelf and inner-shelf mud deposits bury the transgressive sand layer and the bedding structures it contains. The presence of a transgressive sand layer on shelves where mud deposits probably formed during periods when the sea level was high, suggests that these deposits can be “cleaned” (i.e., silt and clay removed, and sand concentration increased) by successive regressions and transgressions of the shoreline.