EARLY PLEISTOCENE GLACIAL LAKE LESLEY, WEST BRANCH SUSQUEHANNA RIVER VALLEY, CENTRAL PENNSYLVANIA

Laurentide glaciers extended into north central Pennsylvania repeatedl y during at least the last 2 million years. Early Pleistocene glaciati on extended farther south into central Pennsylvania than any subsequen t glaciation, reaching the West Branch Susquehanna River (WBSR) valley . Early Pleistocene ice dammed the northeast-flowing West Branch Susqu ehanna River at Williamsport, forming Glacial Lake Lesley, a 100-km-lo ng proglacial lake. In this paper, we present compelling evidence for the lake and its age. Maximum lake volume (similar to 100 km(3)) was c ontrolled by the elevation of the lowest drainage divide, similar to 3 40 m above sea level at Dir, Pennsylvania Stratified deposits at McElh attan and Linden are used to reconstruct depositional environments in Glacial Lake Lesley. A sedimentary section 40 m thick at McElhattan fi nes upward from crossbedded sand to fine, wavy to horizontally laminat ed clay, consistent with lake deepening and increasing distance from t he sediment source with time. At Linden, isolated cobbles, interpreted as dropstones, locally deform glacio-lacustrine sediment. We use pale omagnetism as an age correlation tool in the WBSR valley to correlate contemporaneous glaciofluvial and proglacial lacustrine sediments. Rev ersed remanent polarity in finely-laminated lacustrine clay and silt a t McElhattan (I = 20.4 degrees, D = 146.7 degrees, (alpha(95) = 17.7 d egrees) and in interbedded silt and sand at Linden (I = 55.3 degrees, D = 175.2 degrees, alpha(95) = 74.6 degrees) probably corresponds to t he latter part of the Matuyama Reversed Polarity Chron, indicating an age between similar to 770 and similar to 970 ka. At McElhattan, a dia micton deformed the finely laminated silt and clay by loading and part ial fluidization during or soon after lake drainage. As a result, the deformed clay at McElhattan lacks discrete bedding and records a diffe rent characteristic remanent magnetism from underlying, undeformed bed s. This difference indicates that the characteristic remanent magnetis m is detrital. An electrical resistivity survey and drill borings defi ne a buried bedrock channel at Bald Eagle near the drainage divide tha t is the proposed spillway for Glacial Lake Lesley. The highest terrac e at Bald Eagle (Qt1(be)) was truncated by the spillway channel. Age o f Qt1(be) is estimated as at least middle Middle Pleistocene to Early Pleistocene by correlation of soil physical properties on Qt1(be) to s oil chronosequences developed for Susquehanna River alluvial terraces, further downstream. This age is generally consistent with the age est imated from paleomagnetism. (C) 1998 Elsevier Science B.V.