FORMATION, DEFORMATION, AND INCISION OF COLORADO RIVER TERRACES UPSTREAM OF MOAB, UTAH

The late Quaternary history of the Colorado River in Grand County, Utah, can be reconstructed through detailed analysis of terraces and their deposits. The goals of this study are to establish a geochronologic record of the Colorado River in Grand County in order to address issues of terrace formation, regional tectonics, and to measure deformation of terraces due to salt tectonism.

Rivers are typically thought to deposit fill terraces in restricted (canyon) reaches and strath terraces in open (valley) reaches. However, reconnaissance has shown that Colorado River terraces between Dewey Bridge and Castle Valley display the opposite spatial pattern, with thick (~13 m) fill terraces found in broad Professor Valley and thin (2-3 m) strath terraces found in the canyon reach upstream. Field observations have also shown that terraces in Professor Valley have been tilted where they are located on the flanks of the Cache Valley graben, a collapsed structure that intersects the river corridor and results from salt tectonism of the Pennsylvanian Paradox Formation. Salt deformation in this area is distinctly different from that downstream in Canyonlands National Park, where grabens and horsts have developed in response to salt flow and unloading by the Colorado River, and there is measureable uplift attributed to diapirism focused along the river corridor. Optically stimulated luminescence is being used to date sediments of the lower flight of 3-4 terraces, and terraces are being surveyed using RTK GPS to quantify potential deformation. These data provide incision rates, which have not been determined in this area, and will further elucidate regional patterns of erosion and salt deformation in southeastern Utah during the late Pleistocene-Holocene epochs.

Initial data suggest that: (1) the bedrock incision rate of the Colorado River at Dewey Bridge is 300 m/m.y. or more, consistent with strong isostatic feedback; and (2) salt tectonism has measurably deformed Quaternary terraces above the Cache Valley graben through subsidence rather than uplift.