Geologic units in San Juan county, Colorado

Additional scientific data in this geographic area

Intra-ash flow andesitic lavas (Tertiary) at surface, covers 22 % of this area

Intra-ash flow andesitic lavas

Felsic and hornblendic gneisses, either separate or interlayered (Early Proterozoic) at surface, covers 13 % of this area

Includes metabasalt, metatuff, and interbedded metagraywacke; locally contains interlayered biotite gneiss. Derived principally from volcanic rocks

Uncompahgre Fm (Early-Middle Proterozoic) at surface, covers 13 % of this area

Quartzite, slate, and phyllite

Cutler Fm (Permian) at surface, covers 11 % of this area

Arkosic sandstone, siltstone, and conglomerate

Pre-ash-flow andesitic lavas, breccias, tuffs, and conglomerates (Tertiary) at surface, covers 9 % of this area

Includes several named units

Rico and Hermosa Fms (Pennsylvanian) at surface, covers 7 % of this area

Arkosic sandstone, conglomerate, shale, and limestone. Includes at base in some areas siltstone and shale of Molas Fm or Larsen Quartzite

Ash-flow tuff of main volcanic sequence (Tertiary) at surface, covers 6 % of this area

Includes many named units

Middle Tertiary intrusive rocks (Tertiary) at surface, covers 4 % of this area

Intermediate to felsic compositions

Granitic rocks of 1400-m.y. age group (Middle Proterozoic) at surface, covers 3 % of this area

Includes Silver Plume, Sherman, Cripple Creek, St. Kevin, Vernal Mesa, Curecanti, Eolus, and Trimble Granites or Quartz Monzonites; also, San Isabel Granite of Boyer (1962) and unnamed granitic rocks

Glacial drift (Quaternary) at surface, covers 2 % of this area

Includes some unclassified glacial deposits

Morrison, Wanakah, and Entrada Fms (Jurassic) at surface, covers 2 % of this area

Morrison, Wanakah, and Entrada Fms

Granitic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers 2 % of this area

Includes Boulder Creek -M.Y. AGE GROUP (AGE 1650-1730 M.Y.)--Includes Boulder Creek, Cross Creek, Denny Creek, Kroenke, Browns Pass, Powderhorn, Pitts Meadow, Bakers Bridge, and Tenmile Granites, Quartz Monzonites, or Granodiorites; also, unnamed granitic rocks

Eocene prevolcanic sedimentary rocks (Tertiary) at surface, covers 1 % of this area

South-central: Arkosic sand and bouldery gravel of Echo Park Alluvium. Southwest: includes Telluride Conglomerate and Blanco Basin Fm (arkosic mudstone, sandstone, and conglomerate)

Dolores Fm (Triassic) at surface, covers 1 % of this area

Red siltstone, shale, sandstone, and limestone-pellet conglomerate

Gravels and alluviums (Quaternary) at surface, covers 0.5 % of this area

Includes Broadway and Louviers Alluviums

Dakota Sandstone or Group (Cretaceous) at surface, covers 0.5 % of this area

Dakota Sandstone or Group

Landslide deposits (Quaternary) at surface, covers 0.2 % of this area

Locally includes talus, rock-glacier, and thick colluvial deposits

Mancos Shale (Cretaceous) at surface, covers 0.2 % of this area

In northwest and west-central: Intertongues complexly with units of overlying Mesaverde Group or Fm; lower part consists of a calcareous Niobrara equivalent and Frontier Sandstone and Mowry Shale Members; in areas where the Frontier and Mowry Members (Kmfm), or these and the Dakota Sandstone (Kfd) are distinguished, map unit (Km) consists of shale above Frontier Member. In Southwest: Lower part contains Juana Lopez Member (Kmj)

Leadville Limestone, Gilman Sandstone, Dyer Dolomite, and Parting Fm (Devonian-Ordivician) at surface, covers 0.1 % of this area

Parting Fm: quartzite and shale.

Rhyolitic intrusive rocks and flows of late-volcanic bimodal suite (Tertiary) at surface, covers < 0.1 % of this area

Rhyolitic intrusive rocks and flows of late-volcanic bimodal suite

Upper Tertiary intrusive rocks (Tertiary) at surface, covers < 0.1 % of this area

Intermediate to felsic compositions