Diverse, well preserved organic-walled phytoplankton assemblages were recovered from the Tertiary section of the Oak Grove core drilled on the Virginia Coastal Plain. Strata penetrated include the Aquia, Marlboro Clay, and Nanjemoy Formations (Paleocene to Eocene) of the Pamunkey Group and the Calvert and Choptank Formations (Miocene) of the Chesapeake Group. The assemblages are composed of 176 dinoflagellate species and subspecies (82 genera), of which 20 species and subspecies and one combination are new; five acritarch species (two genera) and five chlorophyte species (three genera) are also reported.

The biostratigraphic distribution of the 186 species provides information concerning the ages of the formations and the nature of their lithologic contacts. The organic-walled phytoplankton assemblages corroborate the Paleocene (Thanetian) age indicated by the calcareous nannofossils, Foraminifera, ostracodes, and pollen and spores for the Aquia strata; certain dinoflagellate and pollen species in the lowermost 11.5 m (37.9 ft), however, suggest this basal interval, devoid of other microfossils, could be Paleocene (Danian) in age. An Eocene (Ypresian) age is assigned to the Nanjemoy strata based on the organic-walled phytoplankton, as well as the pollen and spores, calcareous nannofossils, ostracodes, and mollusks. The intervening Marlboro Clay appears to straddle the Paleocene-Eocene boundary based on its dinoflagellate and pollen species. Dinoflagellate species suggest a general Early to Middle Miocene age for the Calvert and Choptank section in the core; diatom samples in the Calvert indicate a late Early to early Middle Miocene age and in the Choptank a middle Middle Miocene age. Lithologic and biostratigraphic evidence suggests the upper and possibly the lower Marlboro Clay contacts represent minor disconformities, in addition to the major disconformity at the Nanjemoy-Calvert boundary; the Calvert-Choptank contact appears conformable.

The ten provisional dinoflagellate zones proposed for the Aquia, Marlboro Clay, and Nanjemoy strata of the core permit correlation with other previously studied Paleocene and Eocene sections of the Virginia and Maryland Coastal Plain. These Lower Tertiary strata are also generally biostratigraphically correlated with numerous coeval European sections, and a composite offshore eastern Canada section.

The dinoflagellate, acritarch, and chlorophyte assemblages are statistically analyzed for paleoenvironmental implications using species diversity indicators (richness, Shannon-Wiener index, evenness) and analyses patterned after previous palynological studies suggesting correlation of various assemblage parameters and characteristics with particular depositional environments. Results are consistent with sedimentological and foraminiferal-based paleoenvironmental assessments of generally inner shelf marine settings for the Aquia, Nanjemoy, Calvert, and Choptank Formations, and a lagoonal or estuarine setting for the Marlboro Clay. Cluster, principal coordinates, and rank-abundance analyses help to objectively define and characterize six major and 12 smaller clusters of samples related by their species associations through time.