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Depositional environments were surveyed in a large sand-dominated estuary on the south-central South Carolina coast. The estuarine system is characterized by abrupt lateral variations in sediment texture, composition, and physical and biogenic sedimentary structures. Sand-dominated intertidal environments include point bars, ebb sand spits trailing seaward from marsh headlands, tidal sand ridges, and sandy tidal flats. Morphologically, point bars, trailing spits, and tidal ridges are in a continuum of linear to sinuous depositional forms 1 to 3 km long.
An idealized vertical section through these sand bodies is a fining upward sequence beginning with channel lags of coarse sand and gravel followed by interbedded and bioturbated sands and muds interpreted as channel-fill deposits. A gradation between high-angle, large-scale cross-bedding and low-angle, small-scale cross-beds is typical of bar platform and vertical accretion bar sands. Capping the sequence are muddy sands to laminated muds grading into salt-marsh deposits.
Sand flats that prograde from marsh islands typically display a fining-upward trend beginning with shell material concentrated in medium to coarse sand where high-angle cross-bedding is the dominant sedimentary structure. In the upper part of the tidal-flat sequence, biogenic structures increase and disrupt low-angle, small-scale cross-bedding. Wave-deposited laminated sands, capped by rooted, bioturbated marsh muds, top the idealized sequence. Delta-like deposits within the estuary are morphologically gradational between tidal-sand ridges and true flood-tidal deltas and include physical and biogenic structures common to both.
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