SEDIMENTARY INDICATORS OF CLIMATIC VARIABILITY THROUGH THE LATE PLEISTOCENE IN THE POTOMAC RIVER VALLEY SOUTH OF WASHINGTON DC

Unconsolidated gravel, sand, silt, and mud of late Pleistocene age forms bluffs along the tidal reaches of the Potomac River 15-30 miles south of Washington D.C.. These sediments are interpreted as an assemblage of estuarine deposits, paleosols, loess, aeolian dune deposits, ice-rafted dropstones, and thermokarst features from periods of prolonged ground ice (permafrost). Optically Stimulated Luminesence (OSL) dates from these sediments range between 90 and 30 ka. A series of cores from Hybla Valley, about 10 miles south of Washington DC, contains a similar assemblage of sediments plus carbonaceous muds suggesting marsh deposition. OSL and Accelerator Mass Spectrometry (AMS) ¹⁴C dates range from 117 to 24.5 ka, and palynological evidence suggests a nearly continuous record of climate variability from 150 to 15 ka. Estuarine deposits in the age range of 117 and 50 ka at elevations of 1-6 m above sea level require that the region was uplifted several tens of meters, probably due to a forebulge initiated during the last glacial maximum.

Estuarine deposits in outcrop are comprised of upward-fining successions of gravel and sand with dune-scale cross-bedding grading up to ripple-scale cross-lamination, and common mud drapes both between cross-bedding sets and within cross-bedding. Wave-formed structures also are common. Paleosols are defined by clay-rich sediment with abundant sediment-filled root casts and narrow cracks that become less abundant downward into undisturbed sediment. The paleosols are commonly punctuated by thin beds of sand or mud indicating periods of aggradation. Thermokarst horizons are recognized by bowl-shaped pods of deformed sand and mud (pots), typically overlying muddy silts with floating pebbles and cobbles. Aeolian sands are well-sorted and well-rounded with dune-shapes on exposed surfaces recognizable in LIDAR images. In cores, warm pollen assemblages are associated with marsh deposits and carbon-rich silty muds, whereas cold conditions are associated with upward-fining wave-sorted sands or low-carbon muddy silts. Transition from cold to warm commonly exhibits soft-sediment deformation consistent with thermokarst features observed in outcrop, whereas transition from cold to warm is commonly marked by well-developed soils.