ON THE STATISTICAL ASSOCIATION OF THE PLEISTOCENE GLACIAL CLIMATE CYCLE AND STRATH TERRACES/LANDSCAPE EVOLUTION WITHIN THE FREMONT RIVER GORGE, CAPITOL REEF NATIONAL PARK

Forcing mechanisms for changing the longitudinal profile of a fluvial system include base-level change, tectonic changes along the profile (faults and folds), and climate. We herein examine the influence of Late Pleistocene climate change on strath terrace development and the landscape evolution of the Fremont River Gorge as it crosses the Waterpocket Fold at Capitol Reef National Park. The preservation of strath terraces and thereby the downcutting history is likely enhanced in this area of the Park as the Fremont River cuts the relatively resistant sandstones of the Glen Canyon Group. We combine geologic mapping, terrace elevation analysis, Schmidt hammer analysis, and some published cosmogenic absolute age dates, to demonstrate that there appears to be a significant statistical association between terrace populations at given heights above the present stream bed and changes in the Late Pleistocene glacial-interglacial climate cycle. This association may best be explained by a cooler, wetter Utah effectively widening the floodplain of the Fremont River during the last glacial maximum. This explanation assumes that the number of preserved terraces at a given height reflects the original width the the floodplain at the time of terrace formation.

Our data also suggest a downcutting rate through the Fremont River Gorge of 80cm/ky for the past 200ky. These rates are up to three times faster than general rates reported elsewhere on the Colorado Plateau but are consistent with fluvial-carved canyons within high elevation areas that were geographically close to alpine glaciation or major snow fields.