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Sand-dominated intertidal environments in St. Helena Sound, South Carolina, a mesotidal estuarine system, can be divided into a continuum of barlike deposits dominated by tidal flows and sheetlike deposits of sandy tidal flats that are influenced by both waves and currents. A large sand flat attached to a marsh-island complex in the central interior of St. Helena Sound resembles a large flood-tidal delta and has been reworked by waves and migrating ebb-dominated tidal channels. The lower tidal flat (flood ramp) is composed of coarse to medium sand mixed with shell material. Large-scale planar cross-beds are formed by flood-oriented sand waves. Middle tidal-flat deposits consist of fine to very fine sand where burrowing by intertidal fauna disrupts structures of intermedia e to small-scale bed forms generated by both waves and tidal currents. The muddy, fine-grained sands of the upper tidal flat are reworked by wave-generated small-scale ripples and are partially bioturbated. A salt marsh-chenier complex, landward of the upper tidal flat, has prograded over older portions of the sand flat during earlier regressive phases.
Evolution of the St. Helena Sound sand flats began with a transgressive phase marked by a transgressive lag deposit dated at 4,200 yr B.P., overlying Pleistocene estuarine, mud-flat, and salt-marsh deposits. Sediment for buildup of the sand flats was derived from reworking of surrounding barrier-island sands by migrating tidal channels. Sand was introduced into the lower tidal flat by swash-bar accumulation or strong flood-tidal currents. Building of the sand-flat sequence and development of the overlying salt marsh-chenier complex occurred episodically due to minor fluctuations in sea level. At present, the sand-flat sequence is in a transgressive phase and is being reworked by migrating tidal channels and the seawardmost chenier is subject to frequent overwashing.
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