SPATIAL AND TEMPORAL CHANGES IN SUBSTRATE CONDITIONS IN THE NEOGENE ATLANTIC COASTAL PLAIN DEPOSITS OF VIRGINIA

The unconsolidated Neogene deposits of the Atlantic Coastal Plain in Virginia (Salisbury Embayment) provide highly fossiliferous samples amenable to paleontological and sedimentological analysis. Bulk field samples were collected from sedimentary units ranging in age from upper Miocene (Cobham Bay Member of the Eastover Formation) to the Pliocene, including the Sunken Meadow (lower Pliocene), Rushmere (upper Pliocene) and Morgart's Beach (upper Pliocene) members of the Yorktown Formation. These members represent three unconformity-bounded sequences and were collected in replicate at a fine-scale resolution of 10 to 50 cm intervals at 8 localities in Virginia.

Faunal relationships among the 85 most abundant bivalve genera (over 77,500 specimens) from 235 bulk samples were explored using multivariate statistical analysis techniques. Multiple ordination techniques on faunal abundance data were investigated, including Detrended Correspondence Analysis (DCA), which has shown to be useful for extracting simple paleoenvironmental gradients. The sedimentary units were deposited during a time of protracted Cenozoic climatic cooling overlain by short-term climatic and sea level fluctuations. Paleocommunity analysis of samples with temporal control suggests that substrate conditions may not simply reflect eustatic sea level change. New sieve data provides additional quantitative insight into changes in spatial and temporal substrate conditions. Over geologic time, the collected samples record a general paleoenvironmental transition from shallow marine silty sand to sand and a reverse trend upwards to muddy sand bottom conditions. In addition, spatial coverage of data suggest that substrate conditions have substantial local variance during deposition of each member, detected not only by sieve data, but also by paleocommunity analysis of fine-resolution samples.