Abstract

Elevation, longitudinal profile, and Schmidt hammer data indicate that the two lowermost strath terrace levels mapped in the Fremont River drainage of Capitol Reef National Park correlate in elevation and relative age to the two terrace levels of the smaller Pleasant Creek drainage located approximately 16 km (10 mi) to the south. This correlation suggests that drainage development in this area of the Colorado Plateau was strongly dependent upon a regional-scale forcing mechanism (e.g., climate) rather than strictly independent fluvial drainage-scale processes. Our data demonstrate that because the Fremont River drainage has numerous additional strath terrace levels at elevations much higher than observed in the Pleasant Creek drainage, the Fremont River drainage is much older than the Pleasant Creek drainage.

The number of strath terraces at given elevations (terrace population data) indicates that glacial-interglacial climate forcing played an important role in the landscape evolution of both drainages and by inference, the broader Colorado Plateau. We compared terrace elevation data with published cosmogenic ages for several terrace deposits within the Fremont River drainage east of our study area. This comparison suggests that highest concentrations of preserved terraces may be time-correlative with discrete isotopic stages associated with glacial-maximum and/or deglacial conditions (or phase-shifted from them). Our data also demonstrate that during the past approximately 200 kyr in this area of the Fremont River, incision rates are on the order of 85 cm/kyr (34 in/kyr), which is at the high end of reported rates from other studies on the Colorado Plateau.

Finally, the Fremont River and Pleasant Creek display a vertically stepped profile as they cut down through the relatively resistant sandstone of the Glen Canyon Group within the Waterpocket Fold. In contrast, Sulphur Creek, an upstream tributary to the Fremont River, stepped laterally during downcutting through the mudstone-rich Moody Canyon Member of the Moenkopi Formation. We conclude that the style of landscape evolution created by fluvial processes varies in this area with the relative hardness of exposed bedrock.