GEOMORPHIC EVOLUTION 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. The basalt formed from a series of volcanic eruptions approximately 10 Ma. The lava ponded and cooled in low areas and, because of its hardness, has subsequently protected the underlying softer sedimentary rocks. Unprotected strata adjacent to the basalt cap have been stripped away over the last 10 million years to produce the valleys of the Colorado River, Gunnison River, North Fork of Gunnison River, Plateau Creek, and Kannah Creek. 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 (terraces) 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. At least six major terrace levels, ranging in elevation from 1,387 and 2,812 m have been mapped to date. The highest terraces project about 400 m above present-day base level. The Lava Creek B (LCB) ash, which erupted from the Yellowstone area approximately 640,000 has been tentatively identified at two locations on the southwest flank of Grand Mesa: Petrie Mesa and Paradox Mesa. The ash layer is usually 0.5 to 3 meters thick and is interstratified with the terrace gravels. It is well cemented with calcite and appears to have accumulated in low-energy depositional settings.