GEOLOGY OF THE GUNNISON AND SIGNAL PEAK 7.5’ QUADRANGLES, GUNNISON COUNTY, COLORADO

The Gunnison and Signal Peak 7.5’ Quadrangles were mapped as part of the Gunnison Project of Colorado’s STATEMAP program. The Gunnison Quadrangle was completed in 2006 and the Signal Peak Quadrangle in 2007. The Gunnison and Signal Peak Quadrangle lie on the southeast margin of the Piceance basin between the Gunnison and Sawatch uplifts and lie north and east of the volcanic centers of the West Elk and San Juan mountains.

The southern margin of the quadrangle exposes Paleoprotozoic basement consisting of portions of the complexly folded Gunnison Annular complex that was intruded into biotite quartzites, biotite schists and amphibolites of the metasedimenatry and metavlocanic Cochetopa succession

The quadrangles are located on the eastern margin of the ancestral Uncompahgre uplift as evidenced by the nonconformity between the Precambrian and overlaying Jurassic Junction Creek Sandstone or Brushy Basin Member of the Morrison Fomation. Mesozoic units include the Junction Creek Sandstone, Morrison Formation, Burro Canyon Formation, Dakota Sandstone and Mancos Shale. The gentle northward and westward dip of Mesozoic strata is the result of tilting on the north flank of the Gunnison Uplift the west flank of the Sawatch uplift. Discrete Laramide structures in include north and northwest trending faults and folds that are mainly parallel to the trend of the basement fabric. Local expressions of Laramide deformation include reverse movement on the southwest dipping Gunnsion Fault and the continuation of the reverse movement on the steeply west dipping Almont fault into a series of anticlines and synclines.

The Paleogene history is history of the area is dominated by (1) erosion from the topographic highs to the north, east, and south that were uplifted during the Laramide orogeny; (2) deposition of volcaniclastic aprons from the West Elk volcanic center to west that disrupted the established drainage systems and turned the region into a depositional basin; (3) deposition of rocks eroded from the Laramide topographic highs and San Juan volcanic centers in alluvial fans into the basin; and (4) the eruption of voluminous ash flow sheets from central and western San Juan volcanic centers that repeatedly disrupted the fluvial system.