Abstract: Depositional Model of Mesotidal Back-Barrier Environment

Ian A. Fischer, Larry G. Ward, Christopher H. Ruby

Most of the southeastern coast of the United States is characterized by extensive back-barrier environments consisting of muddy tidal flats and associated oyster beds, sandy tidal-creek point bars, and well developed salt marshes. These areas are the primary depositional sites of fine-grained sediment and organic material in the nearshore environment. A three-dimensional model, developed from analysis of vibrocores, channel cutbanks, and surficial sediment distribution, details the stratigraphic relations among subtidal sands, intertidal mudflats and salt marshes, and migrating tidal channels.

The model was developed at Kiawah Island, a mesotidal barrier island typical of the South Carolina coast. The island is separated from the Pleistocene mainland by an extensive back-barrier and lagoonal area. Relict beach ridges are scattered throughout the system. Fine-grained, extensively rooted and burrowed salt marshes occur in the highest parts of the intertidal zone and on the flanks of the relict beach ridges. Muddy tidal flats, containing a high percentage of shell hash derived from oyster beds, are present between the marsh and subtidal channels. The subtidal channels are generally sandy with lag deposits of shell hash and armored mud balls, derived from the slumping of cutbanks.

The general stratigraphic sequence consists of a basal sand unit underlying a shell-rich muddy tidal flat. Oyster beds commonly are preserved in life position, usually near the sand/mud interface. Tidal-flat bedding is horizontal but usually disrupted by burrowing. Highly rooted and burrowed marsh muds overlie the tidal-flat deposits and in places interfinger with the relict beach sands. Meandering tidal creeks commonly cut out the sequence and replace it with channel-fill sands and rooted slump blocks. These channel-fill deposits comprise most of the back-barrier environments.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma