Multiple dating techniques to determine transient knickpoint propagation, terrace formation, and incision of the Allegheny Plateau, Youghiogheny River, Ohiopyle, PA.

Terrestrial cosmogenic nuclide (TCN) exposure, burial, and isochron dating, luminescence dating, and field mapping of terraces along the Youghiogheny River in SW Pennsylvania are used to address long-standing challenges in drainage and landscape evolution associated with early Pleistocene base level fall and formation of the Ohio River. The modern Youghiogheny channel drops ~130 m over 50 km forming a broad knickzone flanked by fluvial, lacustrine, and fan-deltaic terraces of the Carmichaels Fm possibly related Glacial Lake Monongahela. TCN burial ages of 1.4+/- 0.1 Ma and 0.61+/-0.06 Ma on similar-elevation terraces downstream of and within the knickzone, respectively, suggest an upstream knickzone propagation rate of 0.1 m/yr, consistent with modeled channel response times in a detachment-limited stream power model with n=1. An infrared stimulated luminescence age at the downstream site is >295+/-60 ka, consistent with, but not able to replicate, the TCN age. Eight TCN exposure ages on tors, cliff faces, and channel bottoms indicate exposure ages ranging from ~10 to 200 ka, or steady-state erosion rates ranging from ~75 to 3 m/Myr, respectively, consistent with both the slope and erosion process at the sampling site. The mean TCN ¹⁰Be concentration for fluvial boulders sampled at a degraded, colluviated crest of the stratigraphically oldest terrace indicates a steady state erosion rate of ~13 m/Myr. These data contribute to a terrace stratigraphic age model that records unsteady river incision at rates of ~50 m/Myr through the knickzone reach, a model which will be tested by additional TCN burial and isochron ages. Terrace thickness and TCN paleoerosion rates contrast with the low modern sediment flux suggesting climatically-driven, unsteady hillslope erosion rates. However, the longitudinal correlation of the terraces also contains evidence of autogenic meander-loop cutoff and knickpoint retreat driven incision related to the draining of Glacial Lake Monongahela.