A DEBRIS-AVALANCHE ORIGIN FOR THE OLIGOCENE WEST ELK BRECCIA IN THE EASTERN GUNNISON AREA, COLORADO, U.S.A

The Oligocene West Elk Breccia is a volcaniclastic unit in the Gunnison area of the Southern Rockies in west-central Colorado. Unit constituents were largely derived from the West Elk volcano, which developed in the southeastern Piceance Basin on Mesozoic sediments. West Elk breccias represent a broad debris apron off the volcano that infilled a low area between the Laramide Gunnison and Sawatch-Elk Mountain uplifts. Much of the current literature on the West Elk Breccia suggests that the unit was formed largely by lahars, but new work suggests a facies complex with debris-avalanche, fluvial, lacustrine, and lahar components. The possibility of debris-avalanche sedimentation was largely unrecognized until recently, but two breccia subunits from the paleo-valleys exhibit jig-saw blocks, large angular megablocks more than 100 m in long dimension oriented at unusual orientations, and penetrating sand and gravel dikes, supporting the likelihood of debris avalanches from sector collapses on the volcano. Both subunits reflect the backfilling of Oligocene paleovalleys in upstream portions of paleo-Tomichi Creek in the eastern Gunnison area. One subunit is composed almost wholly of gray volcaniclasts in a gray ashy matrix, but includes extremely large, dark megablocks of basaltic andesite, which reflects early eruptions on the volcano. In another subunit, a white, sandy, tuffaceous conglomerate, containing gravels from Precambrian, Paleozoic and Tertiary volcanic rocks, apparently served as a rather mobile matrix for the movement of megablocks of basaltic andesite and of underlying units, the instigating sector collapse having gained access to gravels eroded from nearby Laramide highlands. Evidence at the volcano scale (a preserved scarp at the base of the old volcano), outcrop scale (megablocks surrounded by matrix), and intra-outcrop scale (clast angularity and matrix injected into fractures resulting in jig-saw fractures) suggest that the West Elk subunits filling paleo-Tomichi Creek in the eastern Gunnison area largely reflect debris avalanches that moved downslope into paleovalleys up to 20-km away. More homogenized distal equivalents of these units with rounded to sub-rounded clasts may in fact represent lahars that fed from the debris avalanches.