SEDIMENTOLOGIC CHARACTERISTICS OF LATE CENOZOIC GRAVEL-ARMORED SURFACES ON THE SOUTHWESTERN FLANK OF GRAND MESA, WESTERN COLORADO

Grand Mesa is a large erosional landform, which has an average elevation of about 10,000 feet, and is capped by a series of 10 Ma basalt flows. During the erosional sculpting of Grand Mesa, three topographic levels of gravel-armored surfaces formed on is southwestern flank, ranging in average elevation from 5,100 to 6,560 feet. Level 1 gravels, which are the highest, locally contain the Lava Creek B ash and are about 640 Ka. Level 2 gravels range in age between 140 and 550 Ka, and Level 3 gravels (lowest) are less than 70 Ka. All the gravel sequences rest on weathered Mancos Shale (Late Cretaceous).

The research reported herein represents a sedimentological examination of 18 of these gravel-armored surfaces between Delta and Kannah Creek, Colorado. A total of 46 field sites were chosen for data collection, which included information on gravel thickness, stratification, clast composition, and maximum clast size. In summary, the thickness of the gravel sequences ranges from 3 to 80 feet, with the higher (older) sequences being thicker (average = 37 feet) than the lower (younger) sequences (average = 17 feet). All of the sequences are poorly stratified, suggesting deposition by a combination of debris-flow, mud-flow, and alluvial processes. Maximum clast size ranges from 0.5 to 10 feet. Gravel clasts are strongly dominated by basalt (95 to 99 %); the remaining clasts (pebbles and cobbles) include sandstone, quartzite, chert, diorite, andesite, schist, gneiss, granite, and pegmatite. Many of these "exotic" clasts could not have come from the bedrock units that constitute Grand Mesa (Mancos, Mesaverde, Wasatch, Green River, and Uinta Formations). They probably represent reworked high-elevation gravels from the ancestral Colorado and/or Gunnison River system, or reworked gravels from an unnamed Miocene (?) unit that locally underlies the basalt cap.