LATE QUATERNARY TERRAINS OF THE SUGAR LAKE QUADRANGLE, NORTHWESTERN PENNSYLVANIA

Recent surficial geologic mapping of the Sugar Lake 7.5’ quadrangle, using LIDAR and field-based geologic investigations, permits a detailed examination of landform-sediment assemblages (terrains) and evaluation of late Quaternary landscape dynamics. Four terrains were distinguished. Terrain 1 contains the oldest suite of landforms, marking the furthest limit of Wisconsin glaciation in northwestern Pennsylvania. It is comprised of relatively smooth uplands underlain by thin (<2 m.), residual sandy diamicton, colluvium, and occasional bedrock outcrops. Terrain 1 is dissected by deep gorges that dramatically narrow, shallow, and terminate to the outer edge of the Kent “end moraine”, suggesting ice-marginal meltwater as the agent of erosion. Terrain 2 contains the classically defined and widely mapped Kent “end moraine”, which has a strongly constructional, hummocky topography, with kettles and kettle lakes in upland positions. It contains abundant till ridges composed of very poorly sorted boulder to gravel rich diamicton. Kames are abundant within valleys, have a more subdued hummocky topography than upland landforms, and generally lack kettles. In some locations kames are responsible for drainage reorganization, where sediments partly fill deep glacial valleys. Kames include both sorted/stratified and unsorted/unstratified sediments ranging from silt to boulders, that often have internal slump structures. Facies are interpreted to represent fluvial, deltaic, lacustrine, and ablation till origins. The abundance of ice contact features throughout terrain 2 suggests stagnant ice retreat and melt-out from a disintegrating ice margin. The Kent “end moraine” is thus not a true moraine associated with active ice, but a complex suite of landforms related to ice stagnation and ablation. Terrain 2 contains the thickest sediments (12-40 m.). Terrain 3 is dominated by smooth upland surfaces underlain by silty to loamy diamicton comprising a ground moraine, with thicknesses averaging 5-13 m. Terrain 4 consists of late and post-glacial sediments that fill deep, sub-glacially carved valleys. Relatively smooth valley floors contain dead-ice sinks and moats with stratified sand and gravel, wetland peat, and lakes with organic rich mud that has vertically aggraded at least 12 m. during the Holocene.