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ABSTRACT

A three-dimensional model for a tidal inlet-barrier island depositional system was constructed through examination of 37 vibracores and 10 auger drill holes on Capers and Dewees Islands, South Carolina. Two cycles of southerly inlet migration and subsequent abandonment resulted in beach ridge truncation on the northern ends of both barriers. Historical evidence indicates that these tidal inlets migrated 1.5 km to the south owing to a dominant north-south longshore transport direction. The hydraulic inefficiency of these over-extended inlet channels caused shorter, more northerly-oriented channels to breach through the ebbtidal deltas. After inlet reorientation, large wave-formed swash bars migrated landward closing former inlet channels. Weakened tidal currents through the abandoned channels permitted clay plugs to form thick impermeable seals over active channel-fill sand and shell.

Price and Capers Inlets formed during the onset of the Holocene transgression following submergence of the ancestral Plio-Pleistocene Santee River drainage system. Coarse, poorly sorted inlet-deposited sand disconformably overlies Pleistocene estuarine clay and is capped by a dense clay plug. Shoreline reorientation and landward retreat of a primary barrier island chain occurred between the first and second cycles of inlet-channel migration and abandonment. Beach ridges prograded seaward over the first inlet sequence. A second cycle of inlet migration truncated the northernmost portion of these beach ridges and scoured into the clay plug of the earlier inlet deposit. Abandonment of this channel resulted in deposition of a second abandoned inlet-channel clay plug. Abandoned tidal inlet channels exhibit U-shaped strike and crescentic- to wedge-shaped dip geometries. Basal, poorly sorted inlet sands are sealed beneath impermeable, abandoned-channel silt and clay, washover deposits, and salt marsh. Multiple episodes of inlet migration and abandonment during a rising sea-level deposited stacked inlet-fill sequences within the barrier islands.

The resultant stratigraphy consists of interlayered, fining-upward, active inlet-fill sand overlain by thicker abandoned inlet-fill clay plugs. These clay plugs form impermeable zones between adjacent barrier island sand bodies. Shoreline transgression would remove the uppermost barrier island deposits, sealing the inlet-fill sequences between Pleistocene estuarine clay and shoreface to shelf silt and clay.