QUATERNARY BASALT GRAVEL DEPOSITS IN THE COLORADO RIVER BASIN NEAR GRAND MESA: IMPLICATIONS FOR CLIMATIC AND GEOMORPHIC EVOLUTION OF WESTERN COLORADO

Evolution of the Colorado River Basin in western Colorado during the Cenozoic has been the focus of various research initiatives. Previous studies range from fluvial activity linked to uplift of the Colorado Plateau, incision of nearby Unaweep Canyon, connections downstream with integration through Grand Canyon to glaciation on Grand Mesa. Unfortunately, little research has addressed coupling of the channel evolution of the Colorado River with deglaciation of Grand Mesa. This study asks what were the role and impact of deglacial episodes of Grand Mesa on channel evolution in the upper reaches of the Colorado River?

Preliminary observations of outcrops surrounding the northern flanks of Grand Mesa suggest mass movement and catastrophic paleo-flooding as the dominant processes responsible for channel evolution. Boulder- to cobble-sized basalt clast deposits in the Plateau Creek Valley, a tributary of the main Colorado River, are sub-rounded to sub-angular in shape. The clasts were derived from the upper region of Grand Mesa and have consistent angles of imbrication. Previous work by Brunk, Giardino, Lee, Rodosovich and McClenning in 2009 and Blakeley and Giardino in 2013 suggest that the mobilization of the sediments was the result of large debris flows. These deposits were subsequently entrained in deglacial floods.

Our study will focus on size distributions, sorting, and orientations of basalt clasts in the Plateau Creek Valley and the Kannah Creek Valley along the northern and the western flanks of Grand Mesa. We will map paleochannels and investigate stratigraphic patterns to refine the distinctions between debris flows and glacial-flood events. Consistent occurrences of the deposits around Grand Mesa will further explain how deglaciation led to the development of the channels and valleys in the upper reaches of the Colorado River Basin.