Faunal isolation near primary strandlines has been studied quantitatively in Miocene, Pleistocene, and Recent sedimentary units in the Carolinas and Georgia. The contact between sediments deposited in marine and continental environments is sharp in most areas. The marine sequence consists of a basal transgressive sand dipping abruptly from the contact zone and flattening seaward, about 45 ft below the interpreted maximum mean sea level. The contained megafauna consists of (1) taxa living within the sea during the transgression; (2) taxa living within landward estuaries and swamps being eroded during the transgression; and (3) taxa derived from previous stratigraphic units. At the base of the sand, (1), (2), and (3) are present, (2) and (3) comprising most of the paleobiotal assemblage. A few feet above the unconformity, (2) and (3) comprise less than 5 percent of the assemblage, but a few forms are in place. Littoral and sublittoral sand-silt gradationally overlies the basal transgressive sand seaward and upward. Microfaunal content rises from less than 1 percent (by weight) in the basal sand to 3-5 percent in the overlying sand-silt. The species consist almost entirely of calcareous shallow-marine Foraminifera.
Few pelagic forms are present. Faunal preservation progressively becomes better, and exogenic forms are rare. Overlying the sand-silt facies is bar and barrier island, generally unfossiliferous sand. Intervening areas, isolated from the open ocean, accumulate silt-clay. The fauna consists of less than 0.5 percent shallow-marine Foraminifera, abundant oyster species, and marine and shoreline grasses. The assemblages are controlled by tidal drainage.
Landward from the primary strandlines, inundated river valleys contain few fossils either in bed-load or floodplain deposits. Major valleys near the primary strandlines contain extensive fresh-water swamps, bearing grasses, shrubs, and herbs characteristic of Recent marsh environments together with vertebrate bones and teeth. Estuarine environments as reflected by oysters, and mixed marine and fresh-water assemblages are of minor aerial extent, and usually are buried by sediments deposited in a deltaic environment.
Considering Miocene and Pleistocene sequences up to 20 mi seaward of the primary strandline, only about 10 percent of their associated rock units were formed in continental environments. Bedload and floodplain environments are represented by less than 1 percent. Estuarine environments comprise 3–5 percent and deltaic environments about 5–7 percent. Approximately 90 percent of the associated rock units were formed in marine environments of which 20–25 percent is basal transgressive sand, 15–20 percent littoral-sublittoral sand-silt, and 45–55 percent barrier island and marsh. Considering the Recent, the continental shelf is floored by a basal transgressive sand; little sand-silt and few barrier-island-marsh sequences are present.