LUMINESCENCE DATING AT GOMEZ PIT, VIRGINIA, AND IMPLICATIONS FOR THE MIDDLE TO LATE PLEISTOCENE ATLANTIC COAST HISTORY

There are rare but well documented Pleistocene marine/fluvial/marine successions along the Atlantic Coast, south of the glacial limit. However, their chronology is still so poorly known that their chronostratigraphical and paleogeographical significance has not been yet properly understood. One such site is the Gomez Pit from Southern Virginia where the sedimentary succession records several transgressive and regressive events that presumably span most of the Pleistocene.

Several years ago, a series of sediment samples had been collected with the hope of elaborating an absolute age chronology based on a multiple-dating approach, e.g. U-Th on corals, AAR on mollusc shells, and luminescence dating of marine and fluvial sand. The identification and dating of the Last Interglacial marine units have been achieved using both U-Th and AAR, and the latter has been used as well for detecting presumed penultimate interglacial sediments. IRSL has been of limited help because of severe anomalous fading of feldspar IRSL. Moreover, the natural quartz OSL is found to be in field saturation.

Over the last decade, the phenomenology of AF has been thoroughly investigated. Correction methods for AF have been developed and tested on two units of the Gomez Pit succession: the underlying Pliocene Yorktown and the MIS5a marine sand units. For the former, the correction had to yield a luminescence intensity signal coherent with saturation of the induced IRSL dose response growth curve. For the latter MIS5a, the uncorrected apparent IRSL age was 50% of the absolute age documented with U-Th and AAR. Hence, the application of the correction method had to result into twice the luminescence intensity measured for the natural signal.

The method for which we found agreement between the stratigraphic absolute age and AF-corrected IRSL age is the Dose Rate Correction method (DRC). Therein, the anomalous fading decay curve is extrapolated to such a time for which the laboratory dose rate is similar to the one measured in natural conditions. This DRC method has been successfully applied to the MIS5a and Pliocene units. The entire Gomez Pit sequence is now being dated using the DRC-corrected IRSL method. Results and implications for the Middle Pleistocene coastal history in Southern Virginia will be presented at the conference.