IDENTIFICATION OF A PALEO-TIDAL INLET USING GROUND-PENETRATING RADAR AND SEDIMENT CORES, DEBIDUE ISLAND, SOUTH CAROLINA

An historical inlet investigation was performed to settle a land boundary dispute between two contiguous parcels of land along Debidue Island, South Carolina. Property boundaries were based on the location of “New Inlet” in the early 1800's, which has since closed. This location was subject to controversy for many years, as the plats used for delineating the paleo-inlet were questionable. An approximate position of the former inlet was identified using historical maps, plats, and aerial photographs.

The exact position of the paleo-inlet was determined from 1.8-km of ground penetrating radar (GPR) profiles take along the barrier. The reflectors in the GPR sections were identified from 13 sediment cores and the analysis of 46 sediment samples. Grain size analyses provided the basis for identifying sedimentary units and or distinguishing tidal inlet fill sequences. Four samples containing organic carbon were dated to provide a chronology for the barrier evolution and tidal inlet development. Trees were also age-dated to corroborate the geological findings.

A paleo-inlet was identified along the central portion of Debidue Island, 280-m south of the current property boundary. The GPR profiles revealed a 3.4-m deep, 60-m wide channel cut-and-fill structure. Grain size data indicate that the paleo-channel is floored by a lag deposit containing moderately sorted, medium to coarse-grained sand with quartz pebbles and shell hash. Geomorphic data and hydrodynamic analysis suggest that “New Inlet” was a small, ephemeral, storm-generated inlet connected to a “cat-eye pond” within the barrier, rather than to a network of backbarrier tidal creeks.

A four-stage model of the evolution of the southern half of Debidue Island envisions initial spit elongation from central Debidue Island prior to 1000 ybp. Next, beach ridge accretion widened the island eventually forming a large swale, as younger beach ridges prograded in a more southeasterly direction. Breaching of the younger beach ridge complex generated New Inlet in the mid-1700’s. Finally, “New Inlet” closed in the early 1800's as the "cat-eye pond" area filled with sediment decreasing the inlet tidal prism.