RADIOLARIANS AND DIATOMS FROM THE POLLACK FARM SITE, DELAWARE: MARINE–TERRESTRIAL CORRELATION OF MIOCENE VERTEBRATE ASSEMBLAGES OF THE MIDDLE ATLANTIC COASTAL PLAIN1

The Pollack Farm Site near Cheswold, Delaware, is named for a borrow pit excavated during highway construction. The excavation exposed a portion of the Cheswold sands of the lower Miocene Calvert Formation. Two sand intervals (Cheswold C-3 and C-4) yielded a diverse assemblage of land and marine vertebrate remains and more than 100 species of mollusks. An isolated occurrence of a sandy silt (the radiolarian bed) stratigraphically between the two macrofossil-bearing units yielded only siliceous microfossils—radiolarians, diatoms, and sponge spicules. Radiolarians from the radiolarian bed identify the Stichocorys wolffii Zone, which has an age estimated between 17.3 and 19.2 Ma. This is compatible with the strontium-isotope age estimate of 17.9±0.5 Ma on mollusks from the lower shelly sand bed (Cheswold C-3 sand) at the site. Both age determinations are compatible with the early Hemingfordian North American Land Mammal Age that was assigned to the land mammal fossils recovered from the C-3 sand. The lower Miocene diatom Actinoptychus heliopelta from the radiolarian bed identifies East Coast Diatom Zone (ECDZ) 1. This taxon also occurs within “Bed 3A” of the Calvert Formation, the older part of the highly diatomaceous silty clays of Bed 3 that crop out in the Coastal Plain of Maryland and Virginia. Correlation of borehole geophysical logs between Delaware and New Jersey places the vertebrate and molluscan assemblages collected from outcrops of the Shiloh marl of the lower Kirkwood Formation of New Jersey stratigraphically below those from the Pollack Farm Site. Estimates between about 19 and 20 Ma are calculated from a published Sr-isotope ratio for one mollusk shell from the Shiloh site. This indicates a possible unconformity between the Shiloh-equivalent beds (Cheswold C-2 sand, not exposed) and the ~18-Ma lower shell bed (Cheswold C-3 sand) at the Pollack Farm Site. The vertebrate assemblage from the Pollack Farm Site is of early Hemingfordian age, but vertebrates from the Shiloh site are not age-diagnostic. Both assemblages occur about 150–200 feet above the base of the Miocene section. The Farmingdale vertebrate fossils of the northeastern Coastal Plain of New Jersey occur just above the base of the Miocene section (Kirkwood Formation) with an estimated age of 20.5–22.6 Ma based on published strontium-isotope data from boreholes in the vicinity. This age is compatible with the revised interpretation of late Arikareean for the age of the Farmingdale land mammal fossils. The Popes Creek vertebrate assemblage of Maryland is from beds nearly stratigraphically equivalent to the Pollack Farm Site exposures, which agrees with the close temporal correlation of vertebrate remains from both sites. The Barstovian-age vertebrate assemblages from the Calvert Formation and basal part of the overlying Choptank Formation at the Calvert Cliffs exposures of Maryland are from units equivalent to the early middle Miocene Dorcadospyris alata Radiolarian Zone. Radiolarian criteria that are used to indicate the degree of neritic versus oceanic conditions for the Miocene of the middle Atlantic Coastal Plain show increased neritic influence for the Pollack Farm Site and nearby exposures of beds of the S. wolffii Zone in Delaware, as compared with the more oceanic influence interpreted for other occurrences of the zone to the south and west in Maryland. This is consistent with the regional deltaic influence indicated for Delaware and New Jersey during the early Miocene and with the shallow inner neritic to intertidal paleoenvironments interpreted by other contributors to this volume for the Pollack Farm Site. 1 In Benson. R.N. , ed., 1998, Geology and paleontology of the lower Miocene Pollack Farm Fossil Site, Delaware: Delaware Geological Survey Special Publication No. 21, p. 5–19.