EVOLUTION OF UPLAND GRAVEL DEPOSITS ADJACENT TO THE UNCOMPAHGRE RIVER VALLEY NEAR MONTROSE, COLORADO

Four distinct levels of Quaternary gravel deposits are found on mesas, ridges, and uplands along the eastern edge of the Uncompahgre River valley near Montrose, Colorado. The deposits consist of debris-flow facies (poor sorting, mud-matrix supported clasts, large boulders, boulder trains), alluvial facies (well-rounded, clast-supported pebble gravels, sand lenses), or both. The oldest and uppermost deposit, G4, forms an extensive cover of rounded hilltops that extend to Cimarron Ridge. Previously interpreted as glacial till or landslide deposits, evidence suggests that it was deposited as a large, coalescing debris fan. It contains some rounded clasts of Quartzite sourced by the Eocene Telluride Conglomerate. Deposit G3 caps isolated mesas that are somewhat lower than the just below the G4 surface. It is a complex deposit that contains both mainstream alluvial and debris fan facies. It appears to be part of the Bostwick Park-Shin Park paleovalley system that extended from the San Juan Mountains to the Black Canyon of the Gunnison. The presence of the Lava Creek B ash within this deposit establishes it as being 640 ka in age. Deposit G2 caps both broad and finger-like mesas. It contains a debris-flow facies with well-rounded cobbles and pebbles in a mud matrix, and occupies areas where the higher G3 gravel was breached and eroded. This gravel was confined to triangular-shaped basins and narrow valley reaches. The youngest gravel deposit, G1, caps a series of finger-like mesas. It contains debris-flow and alluvial facies with larger boulders and fewer rounded clasts than G2 and was primarily sourced by gravel G4. It was confined to basins and valleys similar to G2. An alluvial sand in the distal part was optically dated at 25.2 ± 3.1 ka. These gravels record the cannibalization, dissection, and reworking of older gravel deposits and redeposition into progressively younger gravel deposits. The Mancos Shale is the primary source of mud in the debris-flow deposits. We envision that the original valley walls of soft shale that flanked the G1-G3 channels have worn away, resulting in inverted topography. Holocene valley-fill deposits that flank these ridges and mesas consist almost entirely of mud. Of the four levels of gravels, only G3 contains intervals of mud-free gravel with abundant quartzite clasts that could be quarried.