POST-GLACIAL HOLOCENE WETLANDS DEVELOPED ON PERMAFROST-THAW SEDIMENTS ALONG VALLEY BOTTOMS IN UNGLACIATED PENNSYLVANIA AND MARYLAND

Previous workers (Newell and DeJong, 2011) interpreted the unglaciated, upland New Jersey landscape as a paleo-landscape created by seasonal discharge and mass movement on low slopes underlain by permafrost during Pleistocene cold-climate conditions. Dormant colluvial deposits were viewed as evidence of erosion and transport during active layer thaw, particularly as climate warmed and permafrost degraded. Our research adds to previous work that supports a similar history of landscape development south of Pleistocene full-glacial ice margins in Pennsylvania (PA) and Maryland (MD). Using lidar, orthoimagery, and geomorphic and paleoecological studies, we find the following evidence of past permafrost: 1) extensive networks of thermal contraction polygons; 2) ubiquitous gelifluction sheets and lobes up to 10s of m thick on side slopes and valley bottoms; and 3) retrogressive thaw slumps and other thermokarst features. Gelifluction produced poorly sorted, matrix-supported deposits indicative of mass movement under saturated conditions. These periglacial landforms and associated deposits are less pronounced to the south. A low-relief footslope deposit of fine colluvial material, typically gleyed silt and sand, mantles periglacial rubble and thickens on periglacial tributary fans. We interpret this footslope sediment as the result of downslope transport of fines during permafrost thaw.

Our radiocarbon dating at dozens of sites, combined with paleoseed and other macro-fossil analysis, indicates that post-glacial valley bottom wetlands supplied by groundwater became established on periglacial deposits during the early Holocene. Oldest radiocarbon dates from organic-rich wetland soils typically are ≤11.2 ka, post-dating the Younger Dryas cold period. At one valley bottom location (Great Marsh, PA), calibrated radiocarbon dates of 19.1-19.9 kyrs BP for organic matter in silt immediately below a 12.1-13.2 kyrs BP organic-rich soil indicate a late Pleistocene-Holocene erosional disconformity. Holocene wetland sediments and soils on coarse, wet substrates persisted until European settlement, often preserved beneath historic sediment stored in valley bottoms as a result of upland soil erosion, mill damming, and road building that began circa late 17^th to 18^th c.