ELONGATED LOW-GRADIENT DEBRIS FANS: JOKULHLAUPS RESULTING FROM TRIBUTARY DAMMING BY PRE-WISCONSINAN GLACIAL ICE LOBES MOVING UP THE BUFFALO CREEK VALLEY IN CENTRAL PENNSYLVANIA

Widespread till and moraines record several excursions of middle Pleistocene ice that moved up-gradient into several watersheds of the Valley and Ridge Province in central Pennsylvania. Pre-Wisconsinan glacial lobes moving up-valley from the West Branch of the Susquehanna River into the Buffalo Creek drainage basin produced a unique assemblage of landforms associated with ice-damming and jokulhlaups emanating from high-gradient mountain watersheds. Extensive low-gradient (avg. 2o slope) gravel surfaces dominate the geomorphology of this region and defy simple explanation. Failure of the ice-dams released sediment-rich water from short-lived lakes, entraining cobbles and boulders which were deposited as major elongated debris fans extending several km downstream from dam bursts. Imbrication is uncommon but occasionally present in these matrix-supported sediments which appear to have been emplaced by hyperconcentrated flood flow. Excavations and geophysical surveys show fan thicknesses average 10m but can exceed 30m in these valley fills. Two distinct episodes of sedimentation are visible in some excavations, apparently associated with separate pre-Wisconsinan glacial events. Clast weathering and soil characteristics are consistent with a middle Pleistocene age, circa 800 ka, from a paleomagnetic date for glacial lake sediments just north of the region (Ramage et al., 1998). Post-glacial stream incision has focused along the margins of many of these fan surfaces, resulting in a topographic inversion, leaving these former valley fills up to 15 m above modern channels. Post-glacial streams apparently find it easier to incise into softer valley-floor bedrock, chiefly shale, along fan margins. Modern streams are hydraulically incompetent to entrain the sandstone clasts in their current hydrologic regime. More dramatic, but similar, topographic inversion characterizes the fill terraces along the margins of the Beartooth Mountains in the Bighorn Basin of Montana where inverted topographic surfaces are capped by crystalline gravels derived from the mountains deposited over weaker shales of the basin (Ritter, 1967).