MAPPING THE LAST PERMAFROST MAXIMUM BOUNDARY SOUTH OF THE LAST GLACIAL MAXIMUM ICE MARGIN, EASTERN US APPALACHIAN REGION (Invited Presentation)

Relict landforms and sediments beyond the Last Glacial Maximum (LGM) ice margin are used to deduce paleoclimatic conditions and geomorphic processes that produced and transported frost-weathered sediment. We focus on landforms diagnostic of continuous permafrost to reconstruct the maximum permafrost extent, the Last Permafrost Maximum (LPM), south of the LGM ice margin in the eastern US Appalachian region. Ground cracking (aka “frost cracking”) during cold conditions associated with modern continuous permafrost (<-6°C MAAT) commonly produces thermal contraction polygons (TCPs) along the boundaries of which ice or sand wedges develop. We mapped thousands of TCPs in shale bedrock throughout Pennsylvania and Maryland, from the LGM ice margin southward ~400 km. Others have mapped TCPs in coastal plain sediments to the east over a similar latitudinal distance. Using lidar digital elevation data and field work, we map another ubiquitous landform associated with frost weathering and periglacial conditions, relict lobes of bouldery colluvium up to several m in height and 10s of m in width, along sandstone ridges adjacent to shale hills within the same ~400 km zone. Lobe frequencies decrease from >18 to ~9 lobes/km² over a distance of 250 km from the LGM ice margin, diminish markedly to ~3 lobes/km² at 300 km, and disappear completely south of ~ 400 km. We note that periglacial conditions suitable for formation of TCPs also produced the sand-boulder colluvium that mantles hillslopes in this region, and further propose that these features are diagnostic of continuous permafrost in this setting. Detailed stratigraphic and sedimentologic work at several relict lobes exposed in large, new roadcuts near the LGM ice margin in Pennsylvania further supports our hypothesis that large lobe formation occurs in association with permafrost, in particular thawing of the active layer that results in intermittent downslope motion of an ~1-2 m thick layer of frost-weathered sediment. Previously, we used OSL to determine that the sandy infill of one TCP was deposited about the time of the LPM, and we are completing cosmogenic isotope analysis of sediment within a relict lobe near the LGM ice margin. From regional mapping and field work at individual outcrops, we conclude that TCPs and lobes are cogenetic and diagnostic of continuous permafrost.