LATE CENOZOIC EROSIONAL HISTORY OF GRAND MESA, WESTERN COLORADO

Grand Mesa, with an average elevation of about 3,050 m, is a distinctive basalt-capped plateau located between the Colorado and Gunnison Rivers near Grand Junction, Colorado. The basalt cap is 50 to 207 m thick and has been dated (K-Ar) at about 10 mya. Tertiary and Late Cretaceous sandstones and mudrocks underlie the basalt. Maximum topographic relief between the top of Grand Mesa and the present-day positions of the Colorado and Gunnison Rivers in the study area is 1,650 m. This suggests an average incision rate for these rivers of approximately 0.17 m per 1,000 years. The late Cenozoic erosional history of Grand Mesa is documented by an impressive array of discontinuous gravel-capped surfaces that occur on its flanks and grade towards the Colorado and Gunnison Rivers. These surfaces, which are carved primarily into the soft Mancos Shale (Cretaceous), were generated by a variety of colluvial, alluvial, and pedimentation processes. The gravel deposits (mainly basalt clasts) range in thickness from 0.2 to 45 m, and individual surfaces range in length from 0.9 to 26.9 km.

Mapping and topographic profiling of the gravel-capped terraces around the western escarpment of Grand Mesa has documented at least six major levels, ranging in elevation from 1,387 and 2,812 m. Mathematical projection of the remnant surfaces by use of polynomial equations to their estimated toe-points provides a method to estimate their ages. The highest terraces project about 400 m above present-day base level. Use of the average incision rate discussed above suggests that the highest surfaces are in excess of two million years old. The youngest surfaces, which project to only a few tens of meters above the present river levels, are Pleistocene in age (probably Pinedale). Currently, efforts are underway to date the exposure ages of basalt clasts on representative surfaces by use of cosmogenic techniques.