RELATIVE AGES AND ORIGINS OF LATE CENOZOIC PEDIMENTS ON THE SOUTH FLANK OF GRAND MESA, COLORADO

Grand Mesa, at 3,000 m, is a distinctive erosional landform in western Colorado. Its history started as volcanic eruptions poured several hundred meters of basaltic lava into a paleo-valley system approximately 10 ma. Subsequently, these flows became a resistant cap for the underlying softer sedimentary units. Over the last 10 ma, as the Gunnison River lowered local base level, erosion created an array of lower surfaces carved into the Mancos Shale. Each surface is armored by up to 25 m of poorly stratified gravel consisting mostly of basalt. These remnant gravels also contain exotic clasts (e.g., granite and quartzite) and locally beds of Lava Creek B ash (640 ka). This ash and the date of the Grand Mesa basalt, allow gross calibration of Gunnison River incision rates, plus provide a means to estimate the surface ages. The exotic clasts provide evidence for river activity before and after the basalt deposition.

The goal of this research was to map and characterize the gravel-capped surfaces on the south flank of Grand Mesa (total area of about 330 km2), and to find a scheme for age correlation. Using location, elevation, and gradient, the surfaces were grouped into three levels. Level 1 is coincident with the Lava Creek B ash and is about 640 ka. Level 2 surfaces range in age between 140 and 550 ka, whereas Level 3 surfaces are less than 70 ka. Positions of the Gunnison River during various stages of surface formation were determined using mathematical models based on the present-day topographic profiles of the surfaces. This information provides a clearer view of landscape evolution since the Grand Mesa volcanism. In addition, future predictions of incision rates can be made based on the mathematical models that were created from the data set.