PLEISTOCENE DRAINAGE REVERSALS AND GORGE CUTTING AT THE LAURENTIDE ICE MARGIN IN NORTHWESTERN PENNSYLVANIA INTERPRETED FROM NEW COSMOGENIC EXPOSURE AND LUMINESCENCE AGES

The Laurentide ice margin in Pennsylvania has long been argued to have locally blocked north-flowing, pre-glacial drainages, resulting in proglacial lakes that overflowed divides, carving deep, narrow gorges; however, the timing of gorge cutting and incision rates are poorly known. New terrestrial cosmogenic nuclide (TCN) ¹⁰Be exposure and optically stimulated luminescence (OSL) ages from the Slippery Rock Gorge, a 120 m deep, 9 km long canyon cut through Pennsylvanian clastic sedimentary rocks, indicate initial cutting in the middle Pleistocene, and deepening during the late Pleistocene as one or more proglacial lakes formed and drained. A kame delta perched at the gorge lip near its deepest reach marks the former location of glacial Lake Prouty. An OSL age from this delta of 140±23 ka suggests that Lake Prouty formed and overflowed the former drainage divide at Cleland Rock during the Illinoian (Titusville; MIS 6) ice advance. The overflow channel rapidly lowered and retreated northward, capturing portions of the formerly north-flowing Slippery Rock Creek, and reversing that drainage to the south. We model ages of four ¹⁰Be concentrations in a way that accounts for local topographic shielding and a slow background erosion rate of 1 m/Myr, consistent with findings of nearby published studies. We find that the model exposure ages are relatively insensitive to initial assumptions of sample thickness, density, and erosion rate, and become younger towards the river ranging from 12±1.3 and 27±2.5 ka near the gorge lip to 6.0±0.7 ka at a mid-canyon position, to 1.7±0.3 ka at modern river level. The long profile of Slippery Rock Creek has two major knickzones related to the base level fall created by the drainage reversal and increase in drainage area created by stream capture, respectively. The long-term mean rate of incision at the location of the kame delta is 0.7 mm/yr (0.7 m/ka), but this is not representative of the current rate given that the TCN data shows that gorge formation has been episodic. Collectively, these results indicate that the gorge was already present by the late Pleistocene, but that it was deepened and its walls widened during and since the last glaciation, supporting the published hypothesis of multiple phases of gorge cutting, with at least one resulting from the overflow of a proglacial lake.