UTILIZING THE MOLLUSK FAUNA OF THE ATLANTIC COASTAL PLAIN TO RECONSTRUCT ENVIRONMENTAL CONDITIONS ACROSS THE PALEOCENE-EOCENE THERMAL MAXIMUM (PETM)

The molluscan (bivalve and gastropod) fauna from the upper Paleocene through lower Eocene of the Atlantic Coastal Plain provides constraints on the environmental conditions before, during, and after the Paleocene-Eocene Thermal Maximum (PETM) within this region. We studied ~1500 mollusk specimens from the upper Paleocene (NP zone 9) Aquia Formation and the lower Eocene (NP zone 10) Nanjemoy Formation in Maryland and Virginia. The assemblages are dominated by few taxa, and are significantly less diverse and less even than those in time-equivalent horizons in the Gulf Coastal Plain, which are thought to represent shallow marine, normal salinity conditions. Both the Aquia and Nanjemoy Formations contain fossil mammals, fresh water gar, and crocodilians. The combined faunal assemblage suggests that these units were fluvially-influenced, perhaps representing estuarine and tidal environments. The Aquia Formation mollusk assemblage is less diverse and less even than that of the Nanjemoy Formation, suggesting that it may have been more proximal to a river.

The Mattawoman Creek core (MCC), drilled in the same geographic area as the fossiliferous Aquia and Nanjemoy outcrops, preserves the PETM within the Marlboro Clay, between the Aquia and Nanjemoy Fms. Rare larval mollusk shells occur in the Marlboro Clay in the MCC, both as original aragonitic shell material and as pyritized casts. No adult mollusks are known from the Marlboro Clay, indicating that environmental conditions prevented the larval mollusks from developing. Based on the lack of unconformities at the base of the Marlboro Clay and the inferred estuarine environments stratigraphically above and below it, we infer that the Marlboro Clay in the Mattawoman Creek core represents fluvially-dominated muds. Because some mollusks can live within high sedimentation/high runoff deltaic environments, we suggest that dysoxic-anoxic conditions during the deposition of the Marlboro Clay may explain the lack of adults and rarity of larval shells. The Bass River and Wilson Lake cores from New Jersey similarly preserve only larval mollusk shells within the PETM, suggesting that river runoff and low oxygen conditions were a prevalent feature along the Atlantic Coastal Plain during the PETM.