GRAND MESA: AN IMPORTANT CONTROL POINT FOR ESTABLISHING THE LATE CENOZOIC HISTORY OF WESTERN COLORADO

Grand Mesa provides significant insight to the earliest history of the upper Colorado River system. Located between the present-day Colorado and Gunnison Rivers, this basalt-capped landform provides insight into the land surface as it existed approximately 10 million years ago. The present-day basalt field has a surface area of approximately 53 mi2 and has a “Y-shaped” outline. The Flowing Park and Palisade lobes form the branches of the “Y” with the neck defined by Crag Crest, which is a narrow rib of basalt bounded by large east-west trending slumps. Elevation ranges from 11,189 feet along Crag Crest to about 9,800 feet at the westward limit of the Palisade lobe, and approximately 9,990 feet at the westward termination of Flowing Park lobe. Strata exposed on the flanks of Grand Mesa include the Late Cretaceous Mancos, Iles, and Williams Fork Formations, the Paleocene-Eocene Wasatch, Green River, and Uinta Formations, and a Miocene (?) unnamed unit. Small, high-elevation (~11,320 ft) remnants of 10 Ma (?) basalt also occur 15 mi east of Crag Crest in the Crater Peak area. Fissure-like vents for the basalt were probably located in the Electric Mountain area, with flow movement from east to west. The original basalt field may have had a surface area of about 750 mi2.

The Colorado and Gunnison Rivers and their tributaries have incised approximately 5,450 feet along the margins of the basalt field. This equates to an average incision rate of approximately 0.56 ft per 1,000 yrs. On the flanks of Grand Mesa, are a series of basalt-gravel-capped terraces (alluvial fans, outwash plains, and pediments), carved primarily into the Mancos Shale. These surfaces, which range in age from less than 70 Ka years to possibly 3 Ma, provide a detailed record of the erosional history of Grand Mesa during the Neogene. These surfaces can be grouped into four levels. Level-2 gravels occasionally include interbeds of Lava Creek B ash (640 Ka). Gravel sequences can be up to 80 feet thick and the clasts are strongly dominated by basalt (95 to 99 %); sandstone, quartzite, chert, diorite, andesite, schist, gneiss, granite, and pegmatite make up the remainder. The igneous and metamorphic clasts suggest that through-going rivers, possibly part of the earliest Colorado River system, were present in the Grand Mesa area prior to volcanism. These clasts may also have been derived from the unnamed Miocene (?) unit.