CRETACEOUS STRATIGRAPHY IN THE UPDIP PORTION OF THE U.S. ATLANTIC COASTAL PLAIN, CHESTERFIELD COUNTY, SOUTH CAROLINA

Two cores in the updip part of the U.S. Atlantic Coastal Plain (Chesterfield County, South Carolina) reveal details of strata mapped as the Cretaceous Middendorf Formation. The Patrick core (CTF-320) is from the Carolina Sandhills State Forest near the Middendorf type section, started at a ground elevation of 112.8 m above sea level (ASL), and reached 85.3 m total depth within the Middendorf Formation. The Cheraw core (CTF-081) is from Cheraw State Park approximately 25 km northeast of the Patrick core, started at a ground elevation of 58.8 m ASL, and reached a contact of the Middendorf Formation on Paleozoic schist at 66.4 m depth (and total depth of 74.4 m in schist). Pebbles of smoky quartz and milky quartz (up to 5 cm diameter) are present at the base of the Middendorf Formation.

In both cores, the strata are predominantly sandstone to pebbly sandstone, except in the Patrick core at 38.1 to 26.8 m depth where clay is the predominant lithology. Fining upward sequences and cross-bedding are visible in some sandstone units. Other lithologies include 0.2 to 5.0 m thick beds of clay to silty clay, and <1.5 m thick units of alternating sand and clay laminations. An assemblage of coarse to very fine sand with disrupted bedding, mottled texture, and downward tapering vertical structures (interpreted as a paleosol) is present in the Patrick core at 85.3 to 81.7 m depth (27.5 to 31.1 m ASL), and a similar assemblage is present in the Cheraw core at 38.1 to 36.3 m depth (20.7 to 22.5 m ASL). In both cores, clay beds are more common above this paleosol, and gravel is more common below. Additional work may show that this paleosol denotes a regional unconformity.

Below the paleosol in the Cheraw core, silty clay at 65.5 m depth (6.7 m below sea level) has yielded Late Cretaceous palynomorphs including abundant fern spores (Cicatricosisporites, Gleicheniidites, Foveotriletes), as well as gymnosperm pollen (Taxodiaceae and Pinuspollenites), angiosperm pollen (Momipites), and fresh water algae (Schizosporis). In modern environments, Taxodiaceae does not tolerate high salinity, and is typically found on intermittently flooded, poorly drained flood plains. These pollen data are consistent with the overall interpretation of the strata in both cores as fluvial sediments that accumulated during base level fall (lowstand) and subsequent early transgression.