THE PITS OF VIRGINIA BEACH - INSIGHTS INTO QUATERNARY SEA LEVEL GEOCHRONOLOGY AND DETAILS OF ATLANTIC COASTAL PLAIN STRATIGRAPHY

The geochronology of Quaternary coastal units of southeastern Virginia has been studied for at least the past four decades. Radiocarbon, uranium-series, amino acid racemization (AAR), electron spin resonance (ESR), and thermoluminescence (TL) methods have been applied to units at one or more sites in the region, often supplementing stratigraphic and paleoenvironmental analyses of various exposed units. Gomez Pit (36.7824N 76.1958W), an active commercial excavation for nearly 18 years, has been the focus of many of these studies, including a 1995 multi-investigator comparison of multiple dating methods.

Two provocative observations derive from the combined geochronologic data: 1) an abundance of U-series dates (by both alpha and TIMS methods, on corals) in the range of 65-85 kyr, a time when eustatic sea level was at least 15 m below present as inferred from isotopic ice volume records; 2) an apparent hiatus in the depositional record, representing at least one interglacial/glacial cycle, instead of the "single transgression" model inferred from most outcrop interpretations. The coral U-series dates first became apparent in the late 1960's (samples from Toy Avenue and Womack Pits); later analyses from New Light and Gomez Pits (Szabo, 1985) replicated these results, as have more recent TIMS analyses on Gomez samples. Although questions about diagenetic alteration remain, there is no doubt about the consistency of the U-series results. Mollusk AAR data (Belknap, 1979) from Toy Avenue, Womack, and New Light pits suggested the existence of two superposed aminozones with an age difference of 100kyr or more, but lack of good exposures prevented reliable confirmation of these conclusions and skeptics remained unconvinced. However, extensive exposures at Gomez Pit have permitted hundreds of AAR analyses (Mirecki, 1990) that provide convincing evidence for these two superposed aminozones. TL data also support the two aminozone interpretation, although the "absolute" TL ages all appear to be too young. Continuing study now focuses on geochemical issues that can affect fossil preservation and geochronological results.