SEDIMENTARY RECORDS OF PLEISTOCENE CLIMATE CHANGE IN OUTCROPS AND CORES ALONG THE TIDAL POTOMAC RIVER SOUTH OF WASHINGTON DC

Outcrops of late Pleistocene sediments occur within 5-10 m high eroded terraces along the tidally influenced Potomac River in Maryland and Virginia southwest of Alexandria, VA. These outcrops consist of fluvio-estuarine sediments (channel and bar deposits are prominent) intercalated with paleosols and eolian silt and sand. Optically Stimulated Luminescence (OSL) dating of the exposures thus far has produced ages equivalent to Marine Isotope Stage(MIS) 2 through the end of MIS 5. An auger survey in the 1970's documented varying thicknesses of Pleistocene sediment unconformably overlying an irregular Cretaceous topography. The thickest occurrences of Pleistocene sediment (39 m) occur within terraces in Hybla Valley adjacent to the Potomac River. Continuous core was obtained for 35.5 m depth which along with four shorter cores, and OSL dating produced MIS 2 through late MIS 6 ages through the cored sediments. The Hybla Valley cores are dominated by muddy sediment reflecting organic-rich wetlands and shallow standing water that alternate with paleosols. Fossil pollen from outcrops are limited to a few reduced horizons but the cores provide a nearly continuous record of climate change of unprecedented resolution.

Based on the pollen record, the Hybla Valley sedimentary record is more continuous with smaller hiatus intervals than in the outcrops. The Hybla Valley was an area of more rapid subsidence and removed from channel cut and fill. Whereas channel sands in outcrops are associated with warmer pollen assemblages, wave-formed sands in the core represent cooling pollen assemblages as the wetlands dried out. Loess beds stand out in contrast to the channel deposits in outcrop, but they are more homogenized into paleosols in Hybla Valley. Although the core provides detailed information on climate change through the pollen record, the outcrops provide more information about elevation changes in response to the glacial bulge during the last glacial maximum.