OUTBURST FLOOD CREATES OPPORTUNITY TO LOOK INSIDE THE WEST SALT CREEK ROCK AVALANCHE DEPOSIT, MESA COUNTY, COLORADO

On May 27, 2016, a 390-acre-ft sagpond that had been enlarging during spring snowmelt overflowed, initiating an outburst flood that lasted at least six hours and cut downward as much as 32 m into deposits of the May 25, 2014, West Salt Creek rock avalanche. The flood created extensive cross-sectional exposures of parts of the slump block, hummocks, and toe that provide insights about the mechanisms of movement of the rock avalanche. The slump block contains tens of meters thick, dilated rockslide debris, with large blocks of shale that dip as much as 42° south (toward the head scarp). The north side of the slump block contains blocks that dip 20° - 30° either north, east, or west. The base of these deposits rests on a compact, clay-rich diamicton that contains numerous subrounded clasts of locally derived basalt, sandstone and shale. The contact is sharp, somewhat uneven, and slopes about 10° - 15° toward the north and water perches on the diamicton. The hummock cross-sections, as much as 10 m deep, exposed dilated rock slide debris of varying lithology, mainly shale, and with no discernable internal structures or grading. In the toe area, the rock avalanche deposits consist of highly dilated, unsorted shale fragments with a few basalt boulders and logs. A laterally extensive brown layer of clay-rich material containing angular shale fragments with flow bands and a very irregular upper contact forms the basal part of the rock avalanche deposits. Below this is a relatively undisturbed dark-brown organic and clay-rich top soil with an underlying clay-rich B-horizon. These formed on a clayey sand parent material that contains subrounded and subangular clasts of basalt, sandstone and shale. These observations indicate that the rock avalanche entrained moist clay-rich topsoil as it traveled down the valley. Hummocks in the central core began as dilated debris and moved down valley without significantly changing state, consistent with riding on a low-resistance underlying layer. Absence of grading ruled out processes that could have sorted the particles. Presence of clay-rich, water-perching layers beneath the starting and ending points of the avalanche debris along with entrained soft sediment beneath the ending point are consistent with water and water-saturated material contributing to the rock avalanche’s high mobility.