Geologic units in Larimer county, Colorado

Additional scientific data in this geographic area

Granitic rocks of 1400-m.y. age group (Middle Proterozoic) at surface, covers 23 % 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

Biotitic gneiss, schist, and migmatite (Early Proterozoic) at surface, covers 18 % of this area

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

Granitic rocks of 1700-m.y. age group (Early Proterozoic) at surface, covers 11 % 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

Felsic and hornblendic gneisses, either separate or interlayered (Early Proterozoic) at surface, covers 11 % of this area

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

Pierre Shale--Upper unit (Cretaceous) at surface, covers 5 % of this area

Pierre Shale--Upper unit

Ingleside Fm and Fountain Fm (Permian-Pennsylvanian) at surface, covers 3 % of this area

Ingleside Fm: Limestone and calcareous sandstone. Fountain Fm: arkosic sandstone and conglomerate

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

Includes some unclassified glacial deposits

Pierre Shale--Lower unit (Cretaceous) at surface, covers 3 % of this area

Sharon Springs Member (organic-rich shale and numerous bentonite beds) in lower part

Dakota, Morrison, and Sundance Fms (Cretaceous-Jurassic) at surface, covers 3 % of this area

Dakota, Morrison, and Sundance Fms

Jelm, Lykins, Lyons, and Satanka Fms (Triassic-Permian) at surface, covers 2 % of this area

Red siltstone, shale, and sandstone

Older gravels and alluviums (Quaternary) at surface, covers 2 % of this area

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

Gravels and alluviums (Quaternary) at surface, covers 2 % of this area

Includes Broadway and Louviers Alluviums

Eolian deposits (Quaternary) at surface, covers 2 % of this area

Includes dune sand and silt and Peoria Loess

Colorado Group (Cretaceous) at surface, covers 2 % of this area

Consists of Niobrara Formation (Kn) and either Benton Shale or Carlile, Greenhorn, and Graneros Fms (Kcg)

Pierre Shale--Middle unit (Cretaceous) at surface, covers 2 % of this area

In Boulder-Fort Collins area, contains Richard, Larimer, Rocky Ridge, Terry, and Hygiene Sandstone Members; elsewhere shale between zones of Baculites reesidei and B. scotti

Modern alluvium (Quaternary) at surface, covers 1 % of this area

Includes Piney Creek Alluvium and younger deposits

White River Fm or Group (Tertiary) at surface, covers 1 % of this area

East: Ashy claystone and sandstone. Includes Castle Rock Conglomerate in region southeast of Denver. Northwest: Ashy claystone in North Park

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

Sandstone, siltstone, and conglomerate; in North Park and Laramie basin

Triassic and Permian rocks (Triassic-Permian) at surface, covers 1 % 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

Pierre Shale, undivided (Cretaceous) at surface, covers 0.7 % of this area

Pierre Shale, undivided

Casper Fm and Lower part of Fountain Fm (Permian-Pennsylvanian) at surface, covers 0.6 % of this area

Casper Fm: sandstone

Volcanic rocks in northwestern Colorado (Tertiary) at surface, covers 0.5 % of this area

Mainly of intermediate compositions

Bouldery gravel on old erosion surfaces in Front Range and Never Summer Mountains (Tertiary) at surface, covers 0.5 % of this area

Bouldery gravel on old erosion surfaces in Front Range and Never Summer Mountains

Laramie Fm (Cretaceous) at surface, covers 0.5 % of this area

Shale, claystone, sandstone, and major coal beds

Ogallala Fm (Tertiary) at surface, covers 0.5 % of this area

Loose to well-cemented sand and gravel

Fox Hills Sandstone (Cretaceous) at surface, covers 0.3 % of this area

Fox Hills Sandstone

Ash-flow tuff of main volcanic sequence (Tertiary) at surface, covers 0.1 % of this area

Includes many named units

Lykins Fm and Lyons Sandstone (Triassic-Permian) at surface, covers 0.1 % of this area

Lykins Fm and Lyons Sandstone

Sherman Granite (Middle Proterozoic) at surface, covers < 0.1 % of this area

In Medicine Bow and Laramie Mountains.

Landslide deposits (Quaternary) at surface, covers < 0.1 % of this area

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

Chugwater Formation or Group (Lower Triassic-Upper Triassic) at surface, covers < 0.1 % of this area

Chugwater Formation (North and Northeast Wyoming) - Red siltstone and shale. Alcova Limestone Member in upper middle part in north Wyoming. Thin gypsum partings near base in north and northeast Wyoming. Chugwater Group or Formation (South Wyomingt) - Red shale and siltstone containing thin gypsum partings near base. Group includes Popo Agie Formation (red shale and red, yellow, and purple siltstone; lenses of lime-pellet conglomerate), Crow Mountain Sandstone (red and gray, thick bedded), Alcova Limestone, and Red Peak Formation (red siltstone and shale). Chugwater Formation includes as members all the units listed above. Includes overlying Jelm Formation in Shirley and Seminoe Mountains and at northern end of Laramie Basin. Jelm Formation - Red sandstone.

Mesozoic rocks (Lower Cretaceous-Triassic) at surface, covers < 0.1 % of this area

Mainly Lower Cretaceous, Jurassic, and Triassic Fms

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.

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

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

Southwest Wyoming: South end of Wind River Range--Siliceous, arkosic, and locally radioactive sandstone, claystone, and conglomerate. Fission-track age about 27 Ma. Recent work suggests that part of these deposits may be of Eocene age. Pliocene and Miocene (as originally defined 2) South Pass Formation. Saratoga Valley--White to greenish-gray tuffaceous sandstone, siltstone, and claystone; locally conglomeratic. North Park Formation; Central Wyoming: Arkosic sandstone, conglomerate, and siltstone; some light-colored tuffaceous radioactive claystone and white cherty limestone. North of Sweetwater River in Granite Mountains--Light-colored tuffaceous radioactive claystone, siltstone, sandstone, and arkose. Moonstone Formation; East Wyoming: Light-colored tuffaceous claystone, sandstone, and conglomerate. Ogallala Formation in Denver Basin.

Casper and Fountain Formations (Upper and Middle Pennsylvanian-Permian) at surface, covers < 0.1 % of this area

Casper Formation - Gray, tan, and red thick-bedded sandstone underlain by interbedded sandstone and pink and gray limestone. May include some Devonian(?) sandstone along east flank of Laramie Mountains. Fountain Formation - Arkose and red sandstone.

Cloverly, Morrison, and Sundance Formations (Middle Jurassic-Lower Cretaceous) at surface, covers < 0.1 % of this area

Cloverly Formation - Rusty sandstone at top, underlain by brightly variegated bentonitic claystone; chert-pebble conglomerate locally at base. Morrison Formation - Dully variegated claystone, nodular limestone, and gray silty sandstone. In southern Yellowstone and Jackson Hole areas the presence of Morrison is questionable. Sundance Formation (Js) - Greenish-gray glauconitic sandstone and shale, underlain by red and gray nonglauconitic sandstone and shale.

White River Formation (Oligocene) at surface, covers < 0.1 % of this area

White to pale-pink blocky tuffaceous claystone and lenticular arkosic conglomerate.

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

Intermediate to felsic compositions

Jelm and Chugwater Formations, Forelle Limestone, and Satanka Shale (Permian-Lower Triassic) at surface, covers < 0.1 % of this area

Chugwater Group or Formation (South Wyoming) - Red shale and siltstone containing thin gypsum partings near base. Group includes Popo Agie Formation (red shale and red, yellow, and purple siltstone; lenses of lime-pellet conglomerate), Crow Mountain Sandstone (red and gray, thick bedded), Alcova Limestone, and Red Peak Formation (red siltstone and shale). Chugwater Formation includes as members all the units listed above. Includes overlying Jelm Formation in Shirley and Seminoe Mountains and at northern end of Laramie Basin. Jelm Formation - Red sandstone. Forelle Limestone - Thin-bedded limestone. Locally is a member of the Goose Egg Formation. Satanka Shale - Red shale.

Alluvium and Colluvium (Pleistocene-Holocene) at surface, covers < 0.1 % of this area

Clay, silt, sand, and gravel in flood plains, fans, terraces, and slopes.

Chugwater and Goose Egg Formations (Permian-Upper Triassic) at surface, covers < 0.1 % of this area

Chugwater Formation (North Wyoming) - Red siltstone and shale. Alcova Limestone Member in upper middle part in north Wyoming. Thin gypsum partings near base in north and northeast Wyoming. Goose Egg Formation - Red sandstone and siltstone, white gypsum, halite, and purple to white dolomite and limestone. Chugwater Group or Formation (South Wyoming) - Red shale and siltstone containing thin gypsum partings near base. Group includes Popo Agie Formation (red shale and red, yellow, and purple siltstone; lenses of lime-pellet conglomerate), Crow Mountain Sandstone (red and gray, thick bedded), Alcova Limestone, and Red Peak Formation (red siltstone and shale). Chugwater Formation includes as members all the units listed above. Includes overlying Jelm Formation in Shirley and Seminoe Mountains and at northern end of Laramie Basin. Jelm Formation - Red sandstone.

Forelle Limestone and Satanka Shale (Permian) at surface, covers < 0.1 % of this area

Forelle Limestone - Thin-bedded limestone. Locally is a member of the Goose Egg Formation. Satanka Shale - Red shale.