CHARACTERIZATION OF GRAVELS INTERBEDDED WITH OLIGOCENE ASH-FLOW TUFFS NEAR GUNNISON, COLORADO: IMPLICATIONS FOR ANCIENT SURFACE WATER HYDROLOGY AND VOLCANO-FLUVIAL INTERACTION

Deposits of fluvial gravels are interbedded with mid-Oligocene ash-flow tuffs (~28 to 27 Ma) near Gunnison, CO. These deposits are found on the flanks of the West Elk Volcano (30 Ma) as much as 600 meters above the present Gunnison River. We believe that the gravels represent deposits from an ancestral Gunnison river system.

The gravel deposits range from <1 to >300 meters thick and cover areas of <100m² to >1km². The deposits consist of partially deflated piles of rounded clasts that consist of sand to meter sized boulders of Precambrian granite, gneiss, amphibolite and other metasedimentary rocks, Paleozoic and Mesozoic sediments and sporadic clasts of Oligocene tuffs. Correlation of gravels suggests multiple periods of channel development and erosion from sources in the Sawatch highlands. Mapping of individual channel deposits indicate a decrease in elevation to the west that is consistent with a westward flowing drainage, which parallels the modern Gunnison River. Other gravels composed predominately of lava clasts are oriented transverse to the heterolithic gravels and are derived from the West Elk Volcano.

We believe that the position and orientation of the ancestral Gunnison drainage was partially controlled by repeated damming of the river system by the eruption of pyroclastic flows from the San Juan Mountains. We believe that the volcanic dominated gravel was deposited from minor drainages off the West Elk Volcano. By the analysis of these two gravel types and their respective drainages, the paleo-geomorphology of the Gunnison area can be determined.