Shallow Open-Sea Environments—Shelf Seas

The present-day shelves and their sediments reflect conditions of the Last Glacial period and probably the former low sea level stands at depths of over 100 m. Large bodies of wind-blown, river-transported, and nearshore marine sand that accumulated on the Pleistocene coastal plains, beaches, and river mouths, and were drowned by the Holocene rapid sea-level rise. These sediments are only partially covered by modem sediments, resulting in very complex shelf morphologies. A (near-) equilibrium state between hydraulic processes and sediment distribution in which the shelf has been completely covered 

The climate controls broad latitudinal sediment distribution on the shelf. In tropical, rainy climates, mud that contains high proportion of clay minerals or biogenic carbonate sediment in areas not diluted by mud deposition predominates. By contrast, in hot, dry climates, wind-blown sand and silt accumulate, and in polar climates sediment containing few clay minerals and much coarse-grained material (gravel) prevails. 

The basic classificahon of modem shelf environments is based on tire nature of the hydraulic regime (Johnson and Baldwin, 1986), and may be further subdivided by the distribution of fhe major shelf sedimenf fypes in each of these environments (Swift et al., 1971). The main shelf fypes are fhose dominated by meteorological, tidal, and oceanic currents. Meteorological currents may further be divided into wave-induced currents, mostly confined to inner, nearshore shelves, and storm-induced currents which affect the shelves to considerable depths. Density currents formed by suspended sediments at river mouths and seismically induced waves (tsxmamis) which occur on shelves facing seismically active regions may locally significantly affect specific shelf environments (Coleman, 1969). 

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