A carbonate build-up exposed in the Intracoastal Waterway in Horry County, South Carolina is estimated to be Pleistocene to Pliocene in age. The carbonate build-up discontinuously spans approximately 12 kilometers of the waterway. Prominent formations in the area consist of the Waccamaw and the Canepatch Formation. A distinct sand layer is present laterally between the build-up and the excavated waterway bank, which consists primarily of well preserved fossil molluscs and echinoderms.
Field observations show that the carbonate build-up is bluish-gray in color and composed of shell fragments, predominantly mollusc, in a mud matrix. Thin sections analyzed, using a petrographic microscope, revealed sparry calcite, calcite fibers and dolomite. The origin of cementation of the build-up is determined to be from seawater/freshwater mixing as well as meteoric diagenesis. The fossil analysis of the thin-sections exposed fossils other than mollusc, such as echinoderms, pelecypods, foraminifera and bryozoans. Many of the fossils show evidence of micritic envelopes as well as micritic filled grains.
The Ca/Mg ratio, using atomic absorption, ranged from 17:1 to 28:1. This ratio can is similar to other recent fine calcareous sand and silt. Also this range is an indication of a slightly dolomitic limestone composition. Isotopic data was collected to provide corroboration of the depositional environment inferred by the Mg/Ca ratio and thin-section analysis. The oxygen values are used to infer the temperature and salinity during the time of deposition. A paleo-temperature range of 13.5°C to 14.5°C with a paleo-salinity of 35‰ was determined from the oxygen isotope data indicating a cool to temperate depositional environment.
The weight percentage of carbonate grains versus siliciclastic grains was also used to aid in determining the paleoenvironment. A percentage of approximately 53-65% carbonate grains were determined by dissolving a sample of the carbonate build-up in 8 M nitric acid. A general trend of decreasing carbonate content can be seen in the samples collected further south down the waterway. The siliciclastic sediment was added by mixing with sand from longshore currents as well as from river sediment entering near the sample site.
The carbonate build-up underwent diagenesis hardening the build-up thus resulting in a coquina. The coquina present here may correlate with the Anastasia Formation in Florida, the Neuse Formation in North Carolina or the Socastee Formation in South Carolina.