LATE WISCONSIN LANDFORM-SEDIMENT ASSEMBLAGES INDICATE ICE STAGNATION AND DISINTEGRATION IN NORTHWESTERN PENNSYLVANIA

The Kent end moraine complex (EMC) marks the earliest and most extensive Late Wisconsin Laurentide Ice Sheet (LIS) advance into Northwestern Pennsylvania. Surficial geologic mapping along the ice margin allowed an identification of sediment-landform assemblages and interpretation of early deglaciation. The Kent EMC has a strongly constructional, hummocky topography, with uplands marked by kettles and ridges composed of very poorly sorted gravelly to bouldery diamicton. Valleys within the EMC contain abundant kames, which have a less-pronounced hummocky topography than upland morainal ridges. Kame landforms generally lack kettles, include both sorted/stratified and unsorted/unstratified sediments ranging from silt to boulders, and often have internal slump structures. Facies are interpreted to represent fluvial, deltaic, lacustrine, and ablation till environments. In some locations kames are responsible for drainage reorganization, where sediments partly fill glacially over-deepened valleys. In other locations, the same valleys contain dead-ice sinks and moats filled with stratified sand and gravel, wetland peat, and organic rich lacustrine mud. The abundance of ice-contact features in association with the EMC in northwestern Pennsylvania suggests stagnant ice retreat and melt-out from a disintegrating LIS margin. Thus, the majority of the Kent EMC is not a moraine associated with active ice, but a complex suite of landform-sediment assemblages related to ice stagnation and disintegration. This view is in contrast to the long-held view of the Kent moraine being composed of a series of till sheets associated with an active LIS margin.