HIGH POINT PALEOSOL: RECORD OF EARLY WISCONSINAN PEDOGENESIS ON THE POTOMAC RIVER

Loess and water-modified loess are important stratigraphic components of exposed sections along the eastern shore of Chesapeake Bay and parts of the Potomac River estuary. Paleoenvironmental and geoarcheologic information derived from paleosols developed in the loess are integral to development of a late Pleistocene chronostratigraphic framework and a comprehensive paleoclimatic model for this region since the OIS6 glacial maximum. The High Point (HP) paleosol is exposed in a 10-m high cliff face at the juncture of Occoquan Bay and the Potomac River, Fairfax County, Virginia. The 2.28-m-thick aggradational pedon developed in an upward fining sequence of estuarine sand to loess. Particle-size data for the 1.4-m-thick loess indicate 50-55% silt and 10-22% very-fine sand. Subaerial pedogenesis began with emergence resulting from sea-level lowering and(or) uplift. The HP paleosol is relatively uniformly oxidized (7.5YR 4/4-5/4 matrix color) and characterized by weak coarse prismatic structure that parts to weak or moderate medium subangular blocky structure and significant bio-porosity and argillan development. B-horizon textures are dominantly silty clay loam. The E-horizon exhibits dessication cracks and “bowls” characteristic of periglacial processes, and is overlain by eolian fine sand modified by freeze-thaw processes. OSL ages for the estuarine sand below the paleosol and the fluvial sand above the paleosol, 86 ka and 55 ka respectively, restrict the period of loess deposition, landform emergence, and subaerial pedogenesis to late OIS5a through OIS4. Pollen data from nearby cored sediments in Hybla Valley, VA, indicate that at 65 ka the Potomac River valley had an open-boreal to tundra type of environment, in general agreement with the -60 to -70-m range in OIS4 eustatic sea-level.