DIATOM BIOSTRATIGRAPHY AND PALEOECOLOGY OF VERTEBRATE-BEARING MIOCENE LOCALITIES IN VIRGINIA

Siliceous microfossil samples were obtained from sediment collected with cetacean remains from three eastern Virginia localities: the Carmel Church Quarry in Caroline County (CCQ), Westmoreland State Park in Westmoreland County (WSP), and the Rappahannock River in Richmond County (RMC). While the WSP deposits have been correlated with the upper part of the middle Miocene Calvert Formation based on past studies of diatoms and macroinvertebrates, the assignment of RMC and CCQ to the upper Calvert has been based primarily on lithostratigraphy and (in the case of CCQ) land mammal biostratigraphy. Among these samples, CCQ exhibited the greatest diatom diversity with 28 species from 19 genera. At RMC 12 species from 10 genera were identified, while 19 species from 11 genera were found at WSP, which had the greatest abundance of diatoms. In addition, a single silicoflagellate taxon, Dictyocha crux, was identified at each site. At CCQ, the co-occurrence of Stephanopyxis grunowii and Delphineis biseriata indicates a correlation with Bed 15 of the Calvert Formation. At RMC, the co-occurrence of D. biseriata and D. penelliptica also indicates a correlation with Bed 15. Useful marker diatoms were rare at WSP. Even so, the co-occurrence of D. penelliptica and D. novaecaesaraea combined with the absence of D. biseriata suggests a correlation with Calvert Beds 12-15. Published reports based on mollusks from WSP indicate that this unit correlates with Beds 14-15. All three sites contained abundant specimens associated with or tolerant of brackish water, including Coscinodiscus rothii at CCQ and WSP, Hyalodiscus laevis at CCQ and RMC, and Paralia sulcata at all three sites. There was a mix of both warm- and cool-water taxa (Actinoptychus senarius, P. sulcata) at all three sites. All three sites also included benthic taxa, suggesting that water depths throughout the area were no greater than 20 meters.