THE MCBEAN FORMATION IN CALHOUN, LEXINGTON, AND ORANGEBURG COUNITES SOUTH CAROLINA: HOW IS IT RELATED TO THE SANTEE LIMESTONE AND ORANGEBURG DISTRICT BEDS?

Detailed 1:24k-scale geologic mapping has defined the areal and three-dimensional distribution of nine geologic units ranging from Late Cretaceous to Miocene in the South Carolina Upper Coastal Plain in Calhoun, Lexington, and Orangeburg Counties. Mapping was based on observation of cuttings from power-auger holes, surface exposures, and soil types. The Eocene stratigraphy of the mapping area consists of the Middle Eocene Congaree, Warley Hill, and McBean Formations, the Upper Middle Eocene Orangeburg Districts beds, and the Upper Eocene Dry Branch Formation and the Tobacco Road Sand.

Prior mapping did not recognize the McBean Formation occurring in the study area; however, our mapping clearly depicts the extent of the McBean in and around the map area. It is a pale greenish yellow, very fine-grained sandy clay to clayey sand and medium, well sorted quartz sand. Scattered chert fragments, some containing manganese dendrites, and interlayers of light gray to white limestone are common. Previous work proposed that the Santee Limestone is the down-dip equivalent of the McBean (Powell and Baum, 1982) and the Orangeburg District beds are correlative to the upper portion of the McBean (Dockery and Nystrom, 1992).

Our interpretations concur with both ideas in that: 1) The McBean is an up-dip equivalent of the Santee Limestone; and 2) The Orangeburg District beds should be included as an upper member of the McBean Formation. Under these premises, the deposits associated with McBean represent an associated transgression-regression sequence, where the Santee Limestone and McBean Formation display transgressive properties and the Orangeburg District Beds display regressive characteristics. In the map area, the McBean serves as an important marker formation, as its unique lithology allows easy recognition of stratigraphic position in in the field.