PLIO-PLEISTOCENE PECTINID BIOSTRATIGRAPHY OF VIRGINIA, THE CAROLINAS, AND GEORGIA

Pectinidae are calcitic, large, rapidly speciating and relatively abundant clams exceptionally well suited for biostratigraphic studies. Of 31 species and subspecies of Pectinidae present in the study area, 22 were in the Yorktown-Chowan River-James City basin of Virginia and northern North Carolina, and 23 south of the Cape Fear Arch in the Carolinas and Georgia. Twelve taxa were held in common between the basins. Most abundant were rapidly evolving species of Chesapecten and the bewildering array of stratigraphically restricted variations (subspecies?) of Carolinapecten eboreus.

Both our understanding of Atlantic Coastal Plain Plio-Pleistocene stratigraphy and documented faunas continue to mature. Earliest pectinid species descriptions had locality data only to state or city and Conrad used “Medial Tertiary” before adopting Lyellian epochs. Dall more vigorously applied Lyellian epochs, and cited specific localities, but his work predates modern formational usage. Both Tucker and Mansfield (1916-1944) cited Lyellian epochs, precise localities, and stratigraphic formations. Gibson (1987) reviewed pectinid biostratigraphy for the Miocene and Pliocene of Maryland, Virginia, and the Carolinas. Formation concepts used by Clark, Stephenson, Cooke, and Mansfield defined broad biostratigraphic units often interpreted as single transgressive-regressive pulses. In original concept, the Yorktown Formation included strata now dated from 5 to 2.7 million years. As late as 1988, literature claimed no evidence of internal unconformities within these Atlantic Coastal Plain broadly defined sedimentary packages. Then initial oxygen Isotope sea level curves (Krantz, 1991) documented 9 separate transgressive-regressive cycles during this interval, and J. Carter, M. Campbell, and L. Campbell found evidence for numerous unconformities within this interval, suggesting far greater complexity than that previously documented. We presently document 8 Pliocene transgressive-regressive sedimentary units in the study area. Typically 3 to 5 pectinid taxa form an assemblage within a unit, with a maximum of 10 taxa in a given stratigraphic horizon. Collectively, these assemblages uniquely define a biostratigraphy in approximate conformity with the sea level maxima predicted by the oxygen isotope curves.