Geologic units in Routt county, Colorado

Additional scientific data in this geographic area

Browns Park Fm (Tertiary) at surface, covers 18 % of this area

Sandstone and siltstone; west of Park Range

Mancos Shale (Cretaceous) at surface, covers 16 % 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)

Lewis Shale (Cretaceous) at surface, covers 10 % of this area

Lewis Shale

Granitic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers 9 % 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

Williams Fork Fm (Cretaceous) at surface, covers 7 % of this area

Sandstone, shale, and major coal beds

Felsic and hornblendic gneisses, either separate or interlayered (Early Proterozoic) at surface, covers 6 % of this area

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

Iles Fm (Cretaceous) at surface, covers 6 % of this area

Sandstone and shale. Trout Creek Sandstone Member at top; coal beds in upper half

Lance Fm (Cretaceous) at surface, covers 5 % of this area

Shale, sandstone, and minor coal beds; Fox Hills equivalent at base

Glacial drift (Quaternary) at surface, covers 3 % of this area

Includes some unclassified glacial deposits

Granitic rocks of 1400-m.y. age group (Middle Proterozoic) at surface, covers 2 % 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

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

Gabbro and mafic diorite and monzonite

Modern alluvium (Quaternary) at surface, covers 2 % of this area

Includes Piney Creek Alluvium and younger deposits

Dakota Sandstone or Group (Cretaceous) at surface, covers 2 % of this area

Dakota Sandstone or Group

Biotitic gneiss, schist, and migmatite (Early Proterozoic) at surface, covers 1 % of this area

Locally contains minor hornblende gneiss, calc-silicate rock, quartzite, and marble. Derived principally from sedimentary rocks

Upper Tertiary intrusive rocks (Tertiary) at surface, covers 1 % of this area

Intermediate to felsic compositions

Wasatch Fm (Tertiary) at surface, covers 1 % of this area

Claystone, shale, and sandstone

Landslide deposits (Quaternary) at surface, covers 1.0 % of this area

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

Older glacial drift (Quaternary) at surface, covers 0.9 % of this area

Older glacial drift

Gravels and alluviums (Quaternary) at surface, covers 0.9 % of this area

Includes Broadway and Louviers Alluviums

Basalt flows and associated tuff, breccia, and conglomerate of late-volcanic bimodal suite (Tertiary) at surface, covers 0.9 % of this area

Includes basalts of Hinsdale Fm in San Juan Mountains - Servilleta Fm in San Luis Valley and many other occurrences

Minturn Fm in west-central and south-central and other units (Pennsylvanian) at surface, covers 0.7 % of this area

Arkosic sandstone, conglomerate, shale, and limestone. Includes Madera Fm and Sharpsdale Fm of Chronic (1958) in Sangre de Cristo Range and Gothic Fm of Langenheim (1952) in Elk Mountains. Other units of Middle Pennsylvanian age.

Dakota, Morrison, and Sundance Fms (Cretaceous-Jurassic) at surface, covers 0.7 % of this area

Dakota, Morrison, and Sundance Fms

Morrison, Curtis, and Entrada Fms (Jurassic) at surface, covers 0.7 % of this area

In extreme southwestern Moffat County, includes thin wedge of Carmel Fm (red siltstone and sandstone) beneath Entrada

Volcanic rocks in northwestern Colorado (Tertiary) at surface, covers 0.6 % of this area

Mainly of intermediate compositions

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

Includes Slocum, Verdos, Rocky Flats, and Nussbaum Alluviums in east, and Florida, Bridgetimber, and Bayfield Gravels in southwest

Chinle And State Bridge Fms (Triassic-Permian) at surface, covers 0.4 % of this area

Red siltstone and sandstone

Triassic and Permian rocks (Triassic-Permian) at surface, covers 0.4 % of this area

Red siltstone, shale, and sandstone. Includes various combinations of Nugget, Jelm, Popo Agie, Chugwater, Red Peak, Forelle, Satanka, and Goose Egg Fms near Wyoming border

Chinle and Chugwater Fms (Triassic) at surface, covers 0.4 % of this area

Chinle and Chugwater Fms

Mesaverde Fm, undivided (Cretaceous) at surface, covers 0.2 % of this area

In northwest and west-central: Major coal beds in lower part; Rollins Sandstone Member at base in Delta, Gunnison, and Pitkin Counties. In southwest: sandstone and shale.

Middle Tertiary intrusive rocks (Tertiary) at surface, covers 0.2 % of this area

Intermediate to felsic compositions

Mesozoic rocks (Lower Cretaceous-Triassic) at surface, covers 0.2 % of this area

Mainly Lower Cretaceous, Jurassic, and Triassic Fms

Fort Union Fm (Tertiary) at surface, covers 0.2 % of this area

Shale, sandstone, and local coal beds

Maroon Fm (Permian-Pennsylvanian) at surface, covers 0.2 % of this area

Arkosic sandstone, siltstone, conglomerate, and local limestone

Basalt flows (Quaternary) at surface, covers 0.1 % of this area

Basalt flows

Morrison Fm and Curtis Fm (Jurassic) at surface, covers 0.1 % of this area

Morrison Fm: Variegated claystone, mudstone, sandstone, and local beds of limestone. Curtis Fm: glauconitic sandstone and limestone

Mesozoic and Paleozoic rocks (Mesozoic-Pennsylvanian) at surface, covers < 0.1 % of this area

Mainly as in Mesozoic unit (MZ) plus Permian and Pennsylvanian Fms

Sawatch Quartzite (Cambrian) at surface, covers < 0.1 % of this area

Locally includes Peerless Fm

Coalmont Fm (Tertiary) at surface, covers < 0.1 % of this area

Arkosic sandstone, conglomerate, and shale; coal in lower part; in North Park

Dakota, Purgatoire, Morrison, Ralston Creek, and Entrada Fms in southeast. Dakota, Morrison, and Entrada Fms in central mountains. Dakota, Burro Canyon, Morrison, Wanakah, and Entrada Fms in Gunnison River area. Dakota, Morrison, Curtis, And Entrada Fm (Cretaceous-Jurassic) at surface, covers < 0.1 % of this area

Dakota, Purgatoire, Morrison, Ralston Creek, and Entrada Fms in southeast. Dakota, Morrison, and Entrada Fms in central mountains. Dakota, Burro Canyon, Morrison, Wanakah, and Entrada Fms in Gunnison River area. Dakota, Morrison, Curtis, And Entrada Fm

Weber Sandstone and Maroon Fm (Permian-Pennsylvanian) at surface, covers < 0.1 % of this area

Weber Sandstone and Maroon Fm

Morrison Fm (Jurassic) at surface, covers < 0.1 % of this area

Variegated claystone, mudstone, sandstone, and local beds of limestone

Miocene Rocks (Miocene) at surface, covers < 0.1 % of this area

Southwest Wyoming: Southern Rock Springs uplift--Pale-green to tan tuffaceous sandstone and claystone of Miocene(?) age. Conglomerate of uncertain correlation locally at base. Saratoga Valley and west and southwest to Colorado--White massive soft tuffaceous sandstone and lesser amounts of white marl; lower part conglomeratic. Underlies North Park Formation in Saratoga Valley. To the west and southwest is called Browns Park Formation. Rawlins area--White massive soft tuffaceous sandstone; Central Wyoming: White soft tuffaceous sandstone. Locally derived conglomerate in upper and lower parts of sequence; in places lower conglomeratic sequence may be of Oligocene age. In Granite Mountains K/Ar age of tuff in lower part of sandstone sequence about 17 Ma and fission-track age of lower conglomerate about 24 Ma.

Chinle Fm (Triassic) at surface, covers < 0.1 % of this area

Red siltstone, sandstone, and limestone-pellet conglomerate

Granitic rocks of 1,700-Ma Age Group (Early Proterozoic) at surface, covers < 0.1 % of this area

Granitic rocks of 1,700-Ma Group

Metasedimentary and metavolcanic rocks (Early Proterozoic) at surface, covers < 0.1 % of this area

Sierra Madre--Granite gneiss, felsic gneiss, amphibolite, and metavolcanic rocks. Medicine Bow Mountains--Granite gneiss, felsic gneiss, amphibolite, and hornblende gneiss. Laramie Mountains--Pelitic schist, marble, granite gneiss, layered amphibolite, and felsic gneiss. Black Hills--Pelitic schist; includes minor amounts of granite and amphibolite.

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

Mesaverde Formation or Mesaverde Group (Upper Cretaceous) at surface, covers < 0.1 % of this area

Mesaverde Formation (north Wyoming) - Light-colored massive to thin-bedded sandstone, gray sandy shale, and coal beds. In Jackson Hole locally contains gold-bearing quartzite conglomerate. North of North Fork Powder River east of the Bighorn Mountains, consists solely of the Parkman Sandstone Member. Mesaverde Group (South Wyoming) - Includes Almond Formation, Ericson Sandstone, Rock Springs and Blair Formations in Rock Springs uplift; Almond Formation (white and brown soft sandstone, gray sandy shale, coal and carbonaceous shale), Pine Ridge Sandstone (light-gray sandstone and thin coal beds), and Allen Ridge (gray sandstone, shale, and thin coal beds) and Haystack Mountains (gray marine sandstone and shale) Formations in Rawlins uplift; Pine Ridge Sandstone (light-gray sandstone and thin coal beds) and Rock River Formation (soft sandstone and sandy shale) in Laramie Basin.

Frontier Sandstone and Mowry Shale Members of Mancos Shale, and Dakota Sandstone (Cretaceous) at surface, covers < 0.1 % of this area

Locally includes, at base, Burro Canyon Fm (shale and sandstone) or, in western Moffat County, Cedar Mountain Fm (conglomerate and shale)

Mafic intrusive rocks (Early Proterozoic) at surface, covers < 0.1 % of this area

Sierra Madre--Gabbro of Elkhorn Mountain; age 1,800 Ma. Medicine Bow Mountains--Mullen Creek 5 and Lake Owens 7 Mafic Complexes; older than 1,700 Ma.

Chugwater Fm (Triassic) at surface, covers < 0.1 % of this area

Red sandstone, siltstone, shale, and local limestone and gypsum

Basalt flows and intrusive igneous rocks (Miocene) at surface, covers < 0.1 % of this area

Basalt flows and intrusive igneous rocks