Looking for Loess in All the Wrong Places: Characteristics and Geomorphic Inferences of a Late Pleistocene (OIS 5a and4) Paleosol in Silt-Rich Coastal Plain Deposits along the Potomac River in Southeastern Virginia.

Paleosols form an integral and widely-studied component of Pleistocene-age eolian deposits in the central United States where they serve as important stratigraphic markers and paleoclimatic indicators.  In contrast, paleosols in the Atlantic Coastal Plain region have had limited attention and until recently have not been considered stratigraphically or paleoclimatically important.  We described and sampled a late Pleistocene paleosol from a shoreline outcrop at High Point on Mason Neck, on the east edge of Occoquan Bay in VA. The outcrop exposes 8-10 meters of fluvial, estuarine, and eolian deposits. The paleosol, which is informally named the High Point paleosol, occurs in a 2.5 m thick, silt-rich deposit underlain by estuarine sand and clay and overlain by eolian(?) and fluvial sand.  Optically stimulated luminescence ages of the bounding estuarine and fluvial sands are 86 ka and 56 ka respectively. These ages constrain paleosol formation to oxygen isotope stages (OIS) 5a and 4.The paleosol contains an E, Bw, Bt, and C horizon sequence formed during, as well as subsequent to, sediment aggradation. Pedologically, the paleosol is oxidized throughout (7.5YR 4/4-5/4 matrix color), has a dominantly  silty clay loam texture, prismatic and subangular blocky structure, well developed bio-porosity, and clay films. The paleosol’s upper 40 cm includes a massive Bw horizon with bleached E horizon material infused along prism faces and cracks, interpreted as desiccation features.  The E horizon is intermittent and occurs in ‘bowls” about 50 cm across and up to 20 cm deep. The E is silt loam, but contains occasional coarse fragments 5-15 cm in size.  The bowl features and coarse fragments suggest a periglacial setting.  The paleosol’s oxidized color, texture, and aggradational character, along with lack of fluvial sedimentary structures suggests that the sediment is loess. Stratigraphically, the paleosol marks a subaerial-exposure and soil-forming period sandwiched between two subaqueous units.  The bounding units were deposited during regional OIS5-to-OIS3 subsidence events that were probably related to decay of the OIS6 Laurentide glacial forebulge.