STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENTS OF THE WHITE CLAY CREEK, SOUTHEASTERN PENNSYLVANIA

We describe sediments from ~10 locations of the White Clay Creek (WCC) to better understand how its geomorphic character evolved through time. Basal deposits typically consist of either matrix- or clast-supported sand and gravel, often overlying metamorphic bedrock and occasionally overlain by lenses of stratified clast-supported pebbles. Some pebbly lenses appear in cross-section as triangular-shaped deposits reminiscent of 10m-long, 0.5m-high gravel bars (now buried). Basal gravels are occasionally overlain by discontinuous, decimeter-thick organic-rich lenses of mud and sand with leaves, seeds, twigs, and buried logs. The pre-settlement ages of these deposits are confirmed by 10 radiocarbon dates with ages from 308 to 3367 yrs. BP. At one cross-section, sand and gravel are overlain by a ~1-m thick, laterally extensive deposit of stems, twigs, and leaves with a muddy matrix and a radiocarbon date of 1724-1876 yrs. BP. Commonly overlying organic-rich deposits (or gravels where the organic-rich deposits are absent) is a meter-thick layer of massive mud and sand with well-developed mottling, occasionally capped by decimeter-thick dark sediments interpreted as a buried A-horizon. These deposits are similar to the uppermost 1-2 m thick unit of sand and mud that typically caps most exposures. Geomorphic mapping and sediment dating indicate that these sediments are currently being deposited during overbank flows, and we therefore also interpret older, similar sediments as fluvial overbank deposits. In localized backwater areas immediately upstream of some colonial mill dams, decimeter to meter-thick units of mm-scale laminated mud and sand are preserved; these overlie buried A horizons, confirming their post-settlement age. We interpret our observations to suggest that the pre-settlement WCC was characterized by a mixture of river channel, floodplain, and wetland environments developed on and adjacent to an older colluvial landscape (some of which may have been emplaced by periglacial mass-wasting processes). Post-settlement deposits represent subaerial floodplain deposition (a process that continues today), accumulation of subaqueous and subaerial deposits in backwater areas behind mill dams, and spatially restricted, laterally accreted floodplain sedimentation.