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Kiawah Island, South Carolina, is a mesotidal barrier island composed of prograding beach ridges backed by extensive salt marshes, tidal creeks, and muddy tidal flats. The salt marsh-tidal creek complex is well developed along the length of the island and between bifurcating beach ridges. The major subenvironments of the back barrier are sandy high marshes adjacent to relict beach ridges, rooted and burrowed low marshes, tidal flats and associated oyster bars, active tidal creeks with sandy point bars, and inactive tidal-creek channels (cutoffs) being filled with fine-grained sediment.
Four stratigraphic models based on examination of 60 vibracores penetrating up to 6 m, 20 box cores, numerous channel cutbanks and surficial sediment distributions have been developed to describe the relations of Kiawah's back-barrier environments. (1) The active tidal-channel model consists of a coarse, cross-bedded shell lag underlying muddy-sand point-bar deposits. Bioturbated muddy sand containing shell hash and organic material commonly overlies the point-bar deposits and is capped with rooted, highly burrowed, fine-grained low marsh deposits. The basal unit of this entire sequence and of most cores is a lagoonal-bayfill mud containing Rangia. (2) The cutoff channel model contains a fining-upward sequence developed as a result of decreasing flow through the abandoned channel. Low marsh may also cap this sequence. (3) The tidal-flat model is best developed in shallow, open lagoonal areas. In this sequence, active channel-fill and point-bar deposits are capped by thick tidal-flat sediments. (4) The "mature" marsh model is developed where beach ridges are absent and low marsh is predominant. This sequence consists of channel-fill, fine-grained point-bar deposits, tidal-flat deposits, and very thick, rooted and burrowed low marsh sediments. The predominance of sandy point-bar and channel-fill deposits in this low-energy back-barrier area is significant and can be related to the reworking of beach ridges by meandering tidal creeks.
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