MIDDLE MIOCENE DINOFLAGELLATES DEMONSTRATE THE PRESENCE OF UPPER CHOPTANK MARINE BEDS BENEATH THE MIDLOTHIAN GRAVELS IN SOUTHERN VIRGINIA

The Midlothian gravels are an outlying upland deposit of Coastal Plain sand and gravel located in the eastern Virginia Piedmont about 10 miles (16 km) west of the Tidewater Fall Line. These deposits, which comprise a coastal delta of the ancestral James River, are extremely leached and lack fossils useful for age dating. However, muddy marine channel-fill sands beneath these gravels do contain dinoflagellates. The presence of Cannosphaeropsis passio in one sample places at least a part of these sands in dinoflagellate zone DN7, and thus at least some of the channel-fill sands are correlative in age with the upper Choptank Formation of the Coastal Plain. This constrains the age of the overlying gravels to late middle Miocene or younger. Sea level curves indicate that global sea level dropped significantly after the middle Miocene and has remained relatively low ever since. Thus, any age for the Midlothian gravels younger than middle Miocene is very unlikely. The Midlothian gravels appear to be an upper Choptank-equivalent deposit that accumulated on a scoured surface underlain by marine sands as the Choptank sea regressed. The muddy channel sands also contain < 1% reworked pre-Miocene dinoflagellates. Most of these reworked taxa are long ranging (Areoligera complex, small peridinioid species), but several are restricted in Virginia and Maryland to the early Eocene Nanjemoy Formation (Wilsonidium tabulatum, “Wetzeliella” meckelfeldensis, Dracodinium varielogitudum). This indicates that 1) a transgressive event during the early Eocene deposited Nanjemoy marine sediments far west of the present Nanjemoy outcrop belt across part or all of the eastern Piedmont Province, and 2) these lower Eocene deposits capped parts of the eastern Piedmont Province as recently as the middle Miocene but since have been almost entirely removed by erosion.