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A palentologic and geochemical analysis of mixed-salinity environment faunas in the upper Yorktown Formation (Late Pliocene), Williamsburg, Virginia
ODonnell, Matthew James
ODonnell, Matthew James
Abstract
Abundant Rangia clathrodonta and Corbicula densata, bivalves indicative of brackish- to freshwater environments, occur in the upper Yorktown Formation (late Pliocene) at three localities in Williamsburg, Virginia. Published models of Yorktown deposition do not fully explain the presence of these brackish taxa in an otherwise marine setting. This study investigates these mixed-fauna bed and attempts to reconstruct the paleoenvironments that resulted in this unique faunal assemblage. Macrofaunal bioturbation complicates the stratigraphy. Statistical analyses of macrofaunal abundances revealed that a mixed freshwater-brackish-marine faunal assemblage distinct from normal marine faunal assemblages is present in the upper Yorktown. Ostracode abundance analysis indicated shifts in environmental conditions between marine and restricted environments that correlated with changes in lithology and macrofaunal abundances. Strontium, oxygen, and carbon isotope analyses of modern freshwater and fossils brackish and marine molluscan shells showed that three distinct salinity environments (freshwater, brackish, and marine) are represented in these beds. A depositional model with rapidly fluctuating environments in a shallow restricted basin accounts for the series of very thin deposits of disparate environments overlying each other in quick succession, resulting in a depositional unit seemingly representing one single environment.
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2003-01-01
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Geology