LATE HOLOCENE ALLUVIATION IN NORTHWESTERN PENNSYLVANIA: MORE EVIDENCE FOR REGIONALLY SIMILAR FLUVIAL RESPONSE?

Radiocarbon dating of multiple floodplain stratigraphic sequences within the French Creek watershed in northwestern Pennsylvania suggest that vertical accretion has been the dominant mode of deposition for approximately the past 3-4 ka. These late Holocene vertical accretion facies unconformably overlie middle Holocene deposits which are dominated by lateral accretion facies, suggesting major changes in fluvial activity. Regional proxies suggest that the late Holocene, prior to Euro-American settlement, was a time of cool and wet climate under which a pine/hickory/beech forest predominated. This paleoenvironmental setting would have been conducive to relatively frequent, low energy overbank flood events and concomitant soil development.

Late Holocene overbank sediments and associated, buried floodplain soils have radiocarbon dates that place their time of formation into the Little Ice Age. These changes must have been driven by an increased magnitude and frequency of flood events. The impact of Euro-American settlement, beginning in the late 1700’s and intensifying through the 1800’s, is preserved as a cap of vertically-accreted post-settlement alluvium (PSA) that sharply overlies the pervasive prehistoric soil-sediment sequence and as numerous abandoned meanders. Observations from northwestern Pennsylvania compare favorably with other datasets from the Eastern United States, thus suggesting regionally similar fluvial responses to environmental change during the past 3-4 ka.