GEOCHRONOLOGY OF QUATERNARY COASTAL UNITS, ALBEMARLE EMBAYMENT, NORTH CAROLINA COASTAL PLAIN

The geochronology of Quaternary units within the Albemarle Embayment, North Carolina coastal plain, is based on limited isotopic data (radiocarbon and U-series), plus a growing database of amino acid racemization (AAR) results. Although less quantitative, AAR results at NC latitudes (temperatures) are useful over a time scale ranging back to ~1.5 million years. Over the past four years, a series of rotosonic drill holes has been obtained from 18 sites on the Outer Banks (OBX holes) and 5 sites on the mainland, primarily in Dare and Hyde counties (MLD holes), where the Quaternary section is thickest (up to ~70 meters). AAR data (obtained by gas chromatography) for seven amino acids are used to identify individual aminozones or clusters of D/L values. Rapidly-racemizing amino acids such as aspartic acid (ASP) or alanine (ALA) are better for resolving age differences within the late Quaternary, while slower racemizing amino acids such as valine (VAL) or leucine (LEU) are more useful for estimating ages throughout the Quaternary record, albeit with somewhat lower age resolution. Seismic sections along various back-barrier transects, along with lithologic and biostratigraphic information, both supplement and constrain various age models that are useful for AAR age estimation. Age estimates based on two similar models, either a “non-linear” model (Wehmiller et al., 1988) or a “modified exponential” model (Manley et al., 2000) yield similar results suggesting that the combined aminozonation for the Albemarle Embayment represents at least six depositional episodes between early Pleistocene (~1.8 million) and last interglacial (~100 kyr). In no core have all six of these zones been observed in a single superposed section, but in at least seven cores two or more Pleistocene superposed aminozones are evident. The Dare/Hyde county sections contain the thickest record and the most aminozones, with aminostratigraphically correlative units being observed in several OBX holes between Nags Head and Buxton. The “aminostratigraphic basement” for the region is a racemic unit that is observed in five holes – it is either the Chowan River Formation or the Yorktown Formation.