BIODIVERSITY AND STRATIGRAPHY OF THE BASAL YORKTOWN FORMATION AT KINGS MILL, JAMES CITY COUNTY, VIRGINIA, USA

Yorktown Formation (Pliocene) samples yielded a rich assemblage of molluscs, vertebrates, ostracodes, and miscellaneous invertebrate phyla from samples collected on the James River at Kings Mill, Virginia. The Kings Mill section, now rip-rapped, contained four unconformities: Eastover -Sunken Meadow; Sunken Meadow -Leptopecten leonensis bed; Leptopecten bed -Chama reef; and Yorktown -Pleistocene. Most Sunken Meadow and Leptopecten outcrops show leaching of aragonite, but at Kings Mill aragonitic taxa including micro-mollusks are well preserved, possibly due to buffering effects from the overlying, carbonate-rich Chama reef.

The Sunken Meadow molluscan fauna contains 220 species, of which 208 are found at Kings Mill. Continuing curation of the overlying Leptopecten bed samples has increased the known fauna from 111 to 213 molluscan taxa. Abundant Cyclocardia and diverse Astarte species in both units plus abundant Placopecten in the Sunken Meadow indicated a boreal faunal province, but more complete assemblages indicated warm temperate affinities. Kings Mill Sunken Meadow molluscan genera include 4 presently found only north of Cape Hatteras, 63 genera found both north and south, and 47 found only south of Cape Hatteras, NC. Similarly, the Leptopecten bed contains 3 genera from north of the cape, 66 from both north and south, and 47 found only to the south. Bathymetry estimate was based on the rich otolith assemblages in both units at Kings Mill, suggesting water depths greater than 80 meters.

The Leptopecten bed does not conform to prevailing stratigraphic models for the Yorktown Formation. The bed has three stratigraphically overlapping species of Chesapecten, C. jeffersonius, C. septenarius, and C. madisonius (10 to 14 rib morphology), the same overlap found in Yorktown Unit 3 of Gibson’s 1987 section at the Lee Creek mine, Aurora, NC. We propose that the Leptopecten leonensis bed is an important Early Pliocene biostratigraphic unit traceable from the Rappahannock River through Kings Mill and the Lee Creek section to the Lower Goose Creek Limestone of South Carolina and in the Tamiami Limestone and lower Jackson Bluff Formation of Florida.