Early Pleistocene Glacial Lake Lesley, West Branch Susquehanna River valley, central Pennsylvania

Abstract

Laurentide glaciers extended into north central Pennsylvania repeatedly during at least the last 2 million years. Early Pleistocene glaciation extended farther south into central Pennsylvania than any subsequent glaciation, reaching the West Branch Susquehanna River (WBSR) valley. Early Pleistocene ice dammed the northeast-flowing West Branch Susquehanna River at Williamsport, forming Glacial Lake Lesley, a 100-km-long proglacial lake. In this paper, we present compelling evidence for the lake and its age. Maximum lake volume (∼ 100 km 3) was controlled by the elevation of the lowest drainage divide, ∼ 340 m above sea level at Dix, Pennsylvania. Stratified deposits at McElhattan and Linden are used to reconstruct depositional environments in Glacial Lake Lesley. A sedimentary section 40 m thick at McElhattan fines upward from crossbedded sand to fine, wavy to horizontally laminated clay, consistent with lake deepening and increasing distance from the sediment source with time. At Linden, isolated cobbles, interpreted as dropstones, locally deform glacio-lacustrine sediment. We use paleomagnetism as an age correlation tool in the WBSR valley to correlate contemporaneous glaciofluvial and proglacial lacustrine sediments. Reversed remanent polarity in finely-laminated lacustrine clay and silt at McElhattan ( I = 20.4°, D = 146.7°, α 95 = 17.7°) and in interbedded silt and sand at Linden ( I = 55.3°, D = 175.2°, α 95 = 74.6°) probably corresponds to the latter part of the Matuyama Reversed Polarity Chron, indicating an age between ∼ 770 and ∼ 970 ka. At McElhattan, a diamicton deformed the finely laminated silt and clay by loading and partial fluidization during or soon after lake drainage. As a result, the deformed clay at McElhattan lacks discrete bedding and records a different characteristic remanent magnetism from underlying, undeformed beds. This difference indicates that the characteristic remanent magnetism is detrital. An electrical resistivity survey and drill borings define a buried bedrock channel at Bald Eagle near the drainage divide that is the proposed spillway for Glacial Lake Lesley. The highest terrace at Bald Eagle (Qt1 be) was truncated by the spillway channel. Age of Qt1 be is estimated as at least middle Middle Pleistocene to Early Pleistocene by correlation of soil physical properties on Qt1 be to soil chronosequences developed for Susquehanna River alluvial terraces, further downstream. This age is generally consistent with the age estimated from paleomagnetism.