Fish Canyon Tuff

Olson, J.C., Hedlund, D.C., and Hansen, W.R., 1968, Tertiary volcanic stratigraphy in the Powderhorn-Black Canyon region, Gunnison and Montrose Counties, Colorado, IN Contributions to general geology, 1967: U.S. Geological Survey Bulletin, 1251-C, p. C1-C29. Pg. A77-A78. Fish Canyon Tuff. Erupted from the La Garita caldera near the center of the San Juan volcanic field. Present in Saguache and Rio Grande Counties (San Luis basin), in Mineral and Hinsdale Counties (San Juan Mountain province), and in Gunnison County (Piceance basin). Includes the intracaldera rocks previously assigned to the Alboroto Rhyolite† (herein abandoned) by Larsen and Cross (1956, USGS Prof. Paper 258, pl. 1); in outflow areas along north side of volcanic field, includes only the upper Alboroto of Larsen and Cross (1956, p. 137). Is a distinctive crystal-rich quartz latite ash-flow tuff; characterized by abundant phenocrysts of plagioclase, sanidine, quartz, biotite, and hornblende, and accessory mineral sphene. Is largest and most extensive ash-flow tuff in San Juan field; unit had an original area of at least 15,000 sq km, and a volume in excess of 3,000 cubic km. Includes (in ascending order) an outflow member, La Garita Member (revised), and Phoenix Park Member. Age is Oligocene (27.8 m.y.), based on K-Ar methods (citing Lipman and others, 1970, GSA Bull., v. 81, no. 8, p. 2339-2340). Lipman, P.W., 1975, Evolution of the Platoro caldera complex and related volcanic rocks, southeastern San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper, 852, 128 p. Erupted 27.8 m.y. ago from the La Garita caldera which is 35-40 km in diameter. Includes the intracaldera La Garita Member. Rests on Masonic Park Tuff and is overlain by Carpenter Ridge Tuff. Locally Huerto Andesite intervenes between Fish Canyon and Carpenter Ridge. Accumulated within the caldera complex in the low northeastern part of Platoro caldera moat. Is nearly 1 km thick in the headwaters of the South Fork of the Rio Grande where it forms a single thick cooling unit. Outside Mount Hope caldera it is typically 20-200 m thick. Is the most widespread (at least 15,000 sq km) and voluminous (more than 3,000 cubic km) single ash-flow sheet in the San Juan volcanic field. Detailed lithologic description. Areal extent map shows it in parts of San Juan Mountain province, San Juan basin, Piceance basin, and San Luis basin. Has normal remanent magnetic polarity. Areal extent of Fish Canyon Tuff in southwestern Colorado shown on distribution map; extends from Rio Grande and Saguache Counties, (San Luis basin) into Mineral and Hinsdale Counties (San Juan Mountain province), Archuleta County (San Juan basin), and Gunnison County (Piceance basin). Derived from the La Garita caldera, the largest caldera of the San Juan volcanic field and among the great calderas of the world. Had an estimated volume of more than 3,000 cubic km. Age is 27.8 m.y. Is ash-flow sheet within LaGarita caldera in central San Juan Mountains of CO, San Juan Mountain province. High-resolution 40/39 Ar geochronology (unpublished data, M. Lanphere, USGS) yields age of 27.75 Ma [late Oligocene]. For more information, please contact Nancy Stamm , Geologic Names Committee Secretary.