Eocene biostratigraphy of South Carolina and its relationship to Gulf Coastal Plain zonations and global changes of coastal onlap
An assimilation of mega-invertebrate assemblages in South Carolina with Gulf Coast biozones allows for the subdivision of the middle and late Eocene into four regional zonations that can be compared with proposed global sequences controlled by eustatic changes of sea level. The middle Eocene can be divided, in ascending stratigraphic order, into the Cubitostrea perplicata zone , C. lisbonensis zone , and C. sellaeformis zone. The late Eocene is represented by the Crassatella alta–Chlamys cawcawensis zone. These zones are separated by Type 1 or Type 2 unconformities that bracket global sequence boundaries (unconformity types after Vail and Todd, 1981). The following sequences are proposed for the middle and upper Eocene: TE2.1, Cubitostrea perplicata zone, followed by a Type 2 unconformity; TE2.2, C. lisbonensis zone, followed by a Type 2 unconformity; TE2.3, C. sellaeformis zone, followed by a Type 1 unconformity; and TE3, Crassatella alta–Chlamys cawcawensis zone, followed by a Type 2 unconformity. Although not recognized in the downdip areas of South Carolina, the TE3 sequence can be further subdivided into a TE3.1 sequence and TE3.2 sequence, separated by a Type 2 unconformity. This regional break lies at the Gosport Sand-Lower Moodys Branch Formation/Upper Moodys Branch Formation contact in the Gulf Coastal Plain and at the base of the Tivola Tonque in Georgia.