The significance of dinoflagellates in the Miocene Choptank Formation beneath the Midlothian gravels in the southeastern Virginia Piedmont

The Fall Line (formally "Tidewater Fall Line") separates the more resistant igneous, metamorphic, and consolidated sedimentary rocks of the Piedmont from the typically unconsolidated deposits of the Coastal Plain of Virginia. Widespread but now discontinuous patches of a deeply weathered sand and gravel are found west of the Fall Line, capping the highest hilltops. Near the community of Midlothian, Virginia, the gravels are underlain by fine-grained marine silts that bear an informative assemblage of fossil dinoflagellate cysts (dinocysts). In situ dinocysts belong to middleMiocene zone DN7, which is calibrated to ~12-13 Ma. These deposits are assigned to the upper part of the Choptank Formation, which crops out ~ 25 km(15 mi) to the east at an elevation ~ 60m(200 ft) lower. The dinocyst assemblage suggests that the maximum extent of this Choptank transgression probably covered a significant expanse of the Virginia Piedmont. The Choptank marine silts constrain the age of the unconformably overlying Midlothian gravels to younger than the latter part of the middle Miocene. Previous work has indicated that these gravels also are older than the Pliocene Yorktown Formation. Rare, reworked dinocysts in these Choptank outcrops west of the Fall Line are sourced from older deposits of more than one age. The source could be older updip strata of the lower Eocene Nanjemoy Formation, now erosionally removed. Alternatively, the source could be material referable to the upper Eocene Exmore Formation that resulted from the Chesapeake Bay impact event.