FIELD AND COSMOGENIC NUCLIDE STUDIES OF POSSIBLE LAST GLACIAL MAXIMUM GELIFLUCTION BENCHES IN NORTHEASTERN PENNSYLVANIA

Relict lobes, benches of poorly sorted colluvium, and barren boulder fields are abundant throughout the unglaciated part of the mid-Atlantic region; the timing and processes associated with their formation are uncertain. A new ~2-km road with exposures up to 10-m high reveals the transformation and down-slope transport of fractured sandstone bedrock to colluvium over an elevation range of 180 m. Using LiDAR, we mapped over 20 lobes and benches 3-10 m high and 50-150 m wide on Chestnut Ridge south of the last glacial maximum (LGM) and north of the penultimate ice margin. For cosmogenic analysis, we posit that colluvial sediment was transported downslope by gelifluction during active layer thaw. Relict thermal contraction polygons provide evidence for continuous permafrost during LGM.

Strata within multiple benches constrain mechanisms of colluvium formation and deposition. Outcrops of fractured bedrock, possibly from high frost-cracking intensities during times of continuous permafrost, consist largely of loose, angular boulders (clast-supported) with overall fabric similar to original bedding. Within sediment lobes and benches, all exposures show slope-parallel m-scale beds of matrix-supported bouldery sediment. In situ cosmogenic ¹⁰Be concentrations in sand and clasts from one bench constrain near surface residence time of the material. Below 4 m, ¹⁰Be concentrations for clasts and matrix are similar (35,000 to 50,000 atoms/g) and 3 to 9X lower than samples above. Shallower than 4 m, in two colluvial beds, nuclide concentrations are similar for clasts and matrix (130,000 to 300,000 atoms/g). Field observations and preliminary analysis of nuclide concentrations are consistent with near-surface exposure during only the last glacial cycle and the relatively rapid erosion and deposition of colluvium via gelifluction during cold-climate conditions. Little reworking of this sediment has occurred since deposition, with exception of some near-surface winnowing of fines. Openwork boulder fields at the surface belie the underlying matrix-supported sediment and its gelifluction origin.