GEOCHRONOLOGIC CONSTRAINTS ON THE AGE OF THE CARMICHAELS FORMATION, ASSOCIATED LACUSTRINE DEPOSITS, RATES OF LANDSCAPE CHANGE, GLACIAL ASSEMBLY OF CONTINENTAL DRAINAGE, AND DIFFERENTIAL CRUSTAL WARPING, MIDDLE-LOWER YOUGHIOGHENY WATERSHED, OHIOPYLE STATE PARK, SOUTH-WESTERN PENNSYLVANIA

New terrestrial cosmogenic nuclide (TCN) data sampled from river terraces and lacustrine deposits along the Youghiogheny River in and downstream of Ohiopyle State Park shed new light on the age and genesis of the Carmichaels Formation, its relation to glacial Lake Monongahela, and the assembly of continental drainage by the Laurentide ice sheet. Advance of one or more pre-Illinoian ice sheets blocked a formerly north-flowing drainage west of the Appalachian continental divide forcing a southerly overflow of proglacial Lake Monongahela and a new, southerly course for the Ohio River with a base level fall of ~45 m. Weathered lacustrine and deltaic sediment at an elevation of 277-292 m (909-958 ft) near Cedar Creek Park in the valley of the Youghiogheny River was deposited ~1.8 Ma based on two TCN burial ages with overlapping uncertainties (CCPTCNS 1,811+95/-126 Ka; SRY-BV1 1,745+143/-146 Ka). Unweathered lacustrine deposits exposed beneath a tributary fan at 300 m (980 ft) in the Chestnut Ridge reach of the Youghiogheny gorge 30 km upstream have a burial age of 784.8+157/-144 (YOUGH-CCW) suggesting two or more phases of Lake Monongahela, consistent with previously published paleomagnetic data. Further upstream the river falls ~25 m in < 4 km but has also deposited several terraces underlain by ~3-10 m of Carmichaels Fm sand and gravel. The Qt4 terrace has TCN burial and isochron ages with overlapping uncertainties of 305+/- 57 ka and 368+87/-73 ka (OPCLS and OPPL1-4) respectively. A TCN burial age from Qt3 nearby results in an age of ~609+75/-94 ka (OPC-soil). The Youghiogheny River has persisted in its tight meander path for 10⁵ yrs, incising at a rate of ~50 m/Ma. This rate is twice as fast for comparable flights of terraces both upstream and downstream of Ohiopyle, and ~3 times faster than the steady-state rate of hillslope erosion of ~16 m/Myr determined from TCN exposure ages on alluvial boulders sampled from Qt1. In contrast, an exposure age on a weathered, pitted bedrock ridgetop exposure at the Turtlehead Rock tor has an apparent, no-erosion exposure age of 248±5 ka, or a steady-state erosion rate of ~3 m/Ma. We use these results to apportion Youghiogheny incision to downstream base level fall, upstream changes in climate, and possible broad differential crustal warping inferred from an inversion of fluvial topography across the Laurel highlands of southwestern PA.