Geologic units in South Dakota (state in United States)

Additional scientific data in this geographic area

Pierre Shale (Upper Cretaceous) at surface, covers 49 % of this area

Blue-gray to dark-gray, fissile to blocky shale with persistent beds of bentonite, black organic shale, and light-brown chalky shale. Contains minor sandstone, conglomerate, and abundant carbonate and ferruginous concretions. Thickness up to 1,000 ft (305 m).

Hell Creek Formation (Late Cretaceous) at surface, covers 7 % of this area

Tan to brown, light- to dark-gray, "somber beds" of shale. Interbedded with brown to red carbonaceous shale, gray and brown bentonitic silty shale, and gray, brown and yellow siltstone, sandstone, and claystone-pebble conglomerate. Thickness 260-600 ft (79-183 m).

Niobrara Formation (Upper Cretaceous) at surface, covers 6 % of this area

White to dark-gray argillaceous chalk, marl, and shale. Weathers yellow to orange. Contains thin, laterally continuous bentonite beds, chalky carbonaceous shale, minor sand, and small concretions. Thickness up to 150 ft (46 m).

Fox Hills Sandstone (Upper Cretaceous) at surface, covers 5 % of this area

Bluish-green to green, white to dark-gray, and yellow to tan, carbonaceous and iron-stained, cross-bedded, very fine- to coarse-grained, glauconitic sandstone and siltstone. Interbedded with gray and green to brown shale and siltyshale. Thickness up to 200 ft (61 m).

Carlile Shale (Upper Cretaceous) at surface, covers 4 % of this area

Dark-gray to black, silty to sandy shale with several zones of septarian, fossiliferous, carbonate concretions. Contains up to three sandstone units in the upper portion of the formation and sandy calcareous marl at the base. Thickness up to 330 ft (100 m).

Alluvium (Quaternary) at surface, covers 4 % of this area

Clay to boulder-size clasts with locally abundant organic material. Thickness up to 75 ft (23m).

Eolian Deposits (Quaternary) at surface, covers 3 % of this area

(loess and sand dune) Silt to medium-grained sand. Deposited as sand sheets and barchan, linear, and dome-like dunes and as veneer on uplands. Thickness up to 300 ft (91m).

Terrace Deposits (Quaternary) at surface, covers 3 % of this area

Clay to boulder-size clasts deposited as pediments, paleochannels, and terrace fills of former flood plains. Thickness up to 75 ft (23m).

Arikaree Group (Oligocene to Eocene) at surface, covers 3 % of this area

Includes: Rosebud Formation (Miocene)- Pink siltstone with channel sandstone and concretions. Thickness up to 250 ft (76 m). Harrison Formation (Miocene)- Gray, silty sandstone and reworked volcanic ash with calcareous siltstone and marl. Thickness 180 ft (55 m). Turtle Butte Formation (Miocene)- Light-green to gray siltstone with sandstone channels containing claystone pebbles. Thickness 65 ft (20 m). Monroe Creek Formation (Oligocene)- Tan to grayish-tan, massive sandy siltstone and reworked volcanic ash. Thickness 100 ft (30 m). Sharps Formation (Oligocene)- Pink siltstone and claystone with concretionary layers, paleochannels, and beds of reworked volcanic ash. Thickness 360 ft (110 m).

White River Group (Oligocene to Eocene) at surface, covers 3 % of this area

Includes: Brule Formation (Oligocene)- White, pink, light-green, and light-brown, massive to thin-bedded, bentonitic claystone, tuffaceous siltstone, and well-bedded, calcareous, tuffaceous quartz sandstone. Thickness up to 150 ft (46 m). Chadron Formation (Eocene)- Upper beds are gray, light-brown to maroon bentonite, claystone, siltstone tuffaceous fine-grained sandstone, and local, silicified carbonate lenses. Basal portion consists of poorly cemented, white, coarse-grained arkose and conglomerate. Thickness up to 160 ft (49 m). Chamberlain Pass Formation (Eocene)- Pale olive to pale red, mottled mudstone containing white, cross-bedded channel sandstone with basal conglomerate. Thickness up to 32 ft (10 m). Slim Buttes Formation (Eocene)- White, grayish- to yellowish-orange,pale-red to pink siltstone, clayey siltstone, bentonitic claystone,medium- to fine-grained sandstone, and conglomerate. Thickness up to 48 ft (15 m).

Ludlow Formation (Paleocene) at surface, covers 2 % of this area

White, tan, yellow, and gray, cross-bedded, fine- to medium-grained,silty sandstone interbedded with locally bentonitic, gray siltstone, claystone, and sandy to silty claystone. Characterized by uranium-bearing lignite beds and "clinker" beds fromed by burning coalseams. Thickness up to 420 ft (128 m).

Sioux Quartzite (Lower Proterozoic) at surface, covers 2 % of this area

Pink and reddish to tan, siliceous, fine- to coarse-grained, iron-stained orthoquartzite with minor metamorphosed conglomerate and mudstone layers. Estimated thickness greater than 1,000 ft (305 m).

Ogallala Group (Early Pleistocene to Miocene) at surface, covers 2 % of this area

Includes: Ash Hollow Formation- White, tan, and gray, well-cemented, calcareous sandstone and silty limestone often referred to as "mortar beds". Thickness 90-250 ft 27-76 m) Valentine Formation- Gray, unconsolidated, fine- to coarse grained, fluvial siltstone, channel sandstone, and gravel derived from western sources. Thickness 175-225 ft (53-69 m). Fort Randall Formation- Pink and gray claystone with interbedded sandstone. Also includes green to gray orthoquartzite, bentonitic clay, and conglomerate. Thickness up to 130 ft (40 m).

Cretaceous, undifferentiated (Upper to Lower Cretaceous) at surface, covers 0.9 % of this area

Black opaline spiculite, gray to black shale, yellow-brown to gray chalk, gray silty clay, and pink quartz-rich sandstone. Includes the Split Rock Creek Formation and other near-shore facies of the Dakota Formation, Graneros Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, and Pierre Shale. Thickness up to 400 ft (122 m).

Minnelusa Formation (Permian to Pennsylvanian) at surface, covers 0.6 % of this area

Variegated, yellow to red, gray to brown, pink to purple, and black, interbedded sandstone, siltstone, shale, limestone, dolomite, calcarenite, chert and brecciated beds. Thickness 394-1,175 ft (120-358 m).

Cannonball Formation (Paleocene) at surface, covers 0.6 % of this area

Gray and tan siltstone, sandy to silty claystone, and fine-grained, calcareous clayely to silty sandstone, and abundant, round to lenticular carbonate concretions. Thickness up to 180 ft (55 m).

Madison Group (Mississippian to Devonian) at surface, covers 0.6 % of this area

Includes: Pahasapa Limestone (Mississippian)-White, light-gray to tan, fine- to medium-crystalline limestone and dolomite containing brown to gray chert. Solution features including collapse breccia, sinkholes, and caves are prevalent. Thickness 300-630 ft (91-192 m). Englewood Formation (Mississippian to Dovonian)- Pink, lavender to light-gray, thin- to medium-bedded, finely crystalline, argillaceous, dolomitic limestone. Thickness 30-63 ft (9-19 m).

Inyan Kara Group (Early Cretaceous) at surface, covers 0.4 % of this area

Includes: Fall River Formation- Variegated brown, red, gray to purple, calcareous, well-sorted, fine-grained sandstone, siltstone, and shale containing mica flakes. Thickness 100-200 ft (30-61 m). Lakota Formation- Yellow, brown, red-brown, gray to black silty shale, pebble conglomerate, and massive to thin-bedded, cross-bedded sandstone. Locally interbedded with fresh-water limestone and bituminous coal beds. Thickness 35-500 ft (11-152 m).

Dakota Formation (Upper to Lower Cretaceous) at surface, covers 0.4 % of this area

Light- to reddish-brown, medium- to coarse-grained quartz and minor feldspar sandstone grading upward to a fine- to medium-grained, quartz and minor feldspar sandstone. Contains a middle, gray silty clay unit, and interbeds of gray to dark-gray shale in the upper portion. Thickness up to 450 ft (137 m).

Greenhorn Formation (Upper Cretaceous) at surface, covers 0.3 % of this area

Gray shale, mudstone, marl, calcarenite, and shaly limestone grading upward into light-gray to tan, alternating marl and thin-bedded, fossiliferous limestone. Thickness up to 40 ft (12 m).

Spearfish Formation (Triassic to Permian) at surface, covers 0.3 % of this area

Red sandy shale, siltstone, sandstone, and minor limestone. Interbedded with abundant gypsum. Thickness 328-559 ft (100-170 m).

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

Dark-gray, noncalcareous, pyritic, poorly fossiliferous shale, with numerous sandstone layers at the base. Thickness up to 110 ft (36 m).

Tongue River Formation (Paleocene) at surface, covers 0.2 % of this area

White, gray, and tan, massive, cross-bedded sandstone with interbedded gray, brown, and green claystone, bentonitic claystone, clayely siltstone, carbonate concretions, and lignite. Thickness up to 285 ft (87 m).

Mowry Shale, Newcastle Sandstone, and Skull Creek Shale (Early Cretaceous) at surface, covers 0.2 % of this area

Mowry Shale- Black to gray, siliceous, fissile shale and siltstone containing bentonite layers, and sparse sandstone dikes and sills. Thickness 125-250 ft (38-76 m). Newcastle Sandstone- Gray, light-brown to yellow, discontinuously distributed siltstone, claystone, sandy shale, and fine-grained sandstone. Thickness up to 290 ft (88 m). Skull Creek Shale- Dark-gray to blueish-gray shale containing ferruginous, and carbonate concretions. Thickness 150-275 ft (46-84 m).

Belle Fourche Shale (Late Cretaceous) at surface, covers 0.2 % of this area

Dark-gray to black bentonitic shale containing minor limestone lenses, bentonite layers, fossiliferous calcarenite, and large, ferruginous, carbonate concretions. Thickness 150-350 ft (46-107 m).

Minnekahta Limestone and Opeche Shale (Permian) at surface, covers 0.2 % of this area

Minnekahta Limestone-Purple to gray, finely crystalline, thin- to medium-bedded limestone with varying amounts of red shale. Thickness 30-50 ft (9-15 m). Opeche Shale- Red siltstone, argillaceous sandstone and shale interbedded with caliche layers. Thickness 85-130 ft (26-40 m).

Metamorphosed Shale (Early Proterozoic) at surface, covers 0.2 % of this area

Gray to dark-gray phylite, slate, and mica schist. Estimated thickness at least 5,000 ft (1,524 m).

Metaquartzite (Early Proterozoic) at surface, covers 0.2 % of this area

Light-tan quartzite, siliceous schist, and minor chert. Thickness 800-5,000 ft (244-1,524 m).

Metagraywacke (Early Proterozoic) at surface, covers 0.2 % of this area

Light- to dark-gray, silceous mica schist and impure quartzite. Differentiated where possible into three primary tongues or lenses. (Xgw1, Xgw2, and Xgw3) Thickness from 1,000 ft to over 5,000 ft (305-1,524 m).

Whitewood Limestone, Winnipeg Formation, and Deadwood Formation (Ordovician to Cambrian) at surface, covers 0.2 % of this area

Whitewood Limestone (Ordovician)- Mottled, tan, gray to lavender, fine- to medium-crystalline, sparsely fossiliferous limestone and dolomite. Thickness up to 70 ft (21 m). Winnipeg Formation (Ordovician)- Grat and light-green, fissile shale, and tan, calcareous siltstone, sandy shale, and limestone lenses. Thickness up to 110 ft (34 m). Deadwood Formation (Ordovician to Cambrian)- Variegated, yellow to red, brown, gray, and green, glauconitic, conglomerate, sandstone, shale, dolomitic limestone, and dolomite. Thickness 4-400 ft (1-122 m).

Morrison Formation, Unkpapa Sandstone, Sundance Formation, and Gypsum Springs Formation (Late Jurassic to Middle Jusassic) at surface, covers 0.2 % of this area

Morrison Formation (Late Jurassic)- Light-gray to green and variegatedred, brown, yellow, or lavender, silceous claystone, shale, and siltstone containing interbedded sandstone and fresh-water limestone lenses. Thickness up to 150 ft (46 m). Unkpapa Sandstone (Late Jurassic)- White, massive to thin-bedded, fine-grained, argillaceous sandstone. May be variegated to banded red, yellow, brown, or lavender. Thickness up to 267 ft (81 m). Sundance Formation (Late to Middle Jurassic)- Greenish-gray, yellow, tan, red to orange, and white, variegated, interbedded, fine- to coarse-grained sandstone, siltstone, clay, and limestone. Thickness 250-350 ft (76-107 m). Gypsum Springs Formation (Middle Jusassic)- Massive white gypsum and minor maroon siltstone and shale. Thickness up to 40 ft (12 m).

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

Landslide, slump, and collapsed material composed of chaotically mixed boulders and finer grained rock debris. Thickness up to 180 ft (55m).

Metamorphosed Black Shale (Early Proterozoic) at surface, covers < 0.1 % of this area

Dark-gray biotite schist, biotite-muscovite schist, pyritic biotite schist, and local massive chert beds. Thickness approximately 2000-4000 ft (610-1,219 m).

Tertiary, undifferentiated (Pliocene to Miocene) at surface, covers < 0.1 % of this area

Light greenish-brown and light yellowish-tan to orangish-brown clay, silty clay, and fine sand; light-brown to pink siltstone; green, massive, orthoquartzite conglomerate; and multicolored quartz- and feldspar-rich sand and gravel. Includes equivalents of the Ogallala Group Ash Hollow, Valentine, and Fort Randall Formations. Also includes the Thin Elk, Bon Homme, Herrick, Medicine Root, and "western derived" gravels. Thickness up to 70 ft (21 m).

Metamorphosed Tuffaceous Shale (Early Proterozoic) at surface, covers < 0.1 % of this area

Light-gray to light-tan, muscovite schist and muscovite phylite. Thickness approximately 1,000-3,000 ft (305-914 m).

Harney Peak Granite (Early Proterozoic) at surface, covers < 0.1 % of this area

Pink to tan, finely crystalline to pegmaticic, peraluminous, muscovite granite and pegmatite containing accessory biotite, garnet, apatite, and tourmaline. Batholith body is dome-shaped with numerous smaller dikes and sills.

Upper Metagraywacke (Early Proterozoic) at surface, covers < 0.1 % of this area

Light- to dark-gray, quartz-mica-feldspar schist, quartz-mica schist, staurolite and garnet-rich schist, metaconglomerate, calc-silicate gneiss, and cummingtonite-quartz schist. Thickness up to 14,000 ft (4,267 m).

Metamorphosed Siltstone (Early Proterozoic) at surface, covers < 0.1 % of this area

Medium-gray to dark-greenish-gray phyllite, slate, and biotite schist containing minor chert and amphibolite. Locally intruded by thin metagabbro sills. Thickness 1,000-3,000 ft (305-914 m).

Pahasapa Limestone, Englewood Formation, Whitewood Limestone, Winnipeg Formation, and Deadwood Formation (Mississippian to Cambrian) at surface, covers < 0.1 % of this area

Note: see individual descriptions

Lower Metagraywacke (Early Proterozoic) at surface, covers < 0.1 % of this area

Light- to dark-gray, medium- to thick-bedded, quartz-mica schist containing calc-silicate lenses and ellipsoidal masses. Thickness up to 7,000 ft (2,133 m).

Gravel Deposits (Tertiary) at surface, covers < 0.1 % of this area

Clay to boulder-size clasts primarily from igneous and metamorphic rocks of the central Black Hills. Also includes Phanerozoic lithic clasts and rare vertebrate fossils. Thickness up to 60 ft (18 m).

Metagraywacke Unit 3 (Early Proterozoic) at surface, covers < 0.1 % of this area

Upper Xgw- Light- to dark-gray, silceous mica schist and impure quartzite.

Latitic Intrusive Rocks (Eocene to Paleocene) at surface, covers < 0.1 % of this area

Dark-gray to greenish-gray laccoliths and sills of latite, latitic andesite, and quartz latite. Contains phenocrysts of andesine, oligoclase, biotite, hornblend, and sphene in a finely crystalline andesine-biotite-quartz groundmass.

Metamorphosed Conglomerate (Early Proterozoic) at surface, covers < 0.1 % of this area

Gray to grayish-brown, conglomeratic biotite phyllite, siliceous biotite phyllite, mica schist, quartzite, and iron-formation. Thickness up to 2,000 ft (610 m).

Outwash, Terrace (Pleistocene-Upper Wisconsin) at surface, covers < 0.1 % of this area

Heterogeneous clay to gravel of glaciofluvial orgin. Thickness up to 60 ft (18m)

Metabasalt (Early Proterozoic) at surface, covers < 0.1 % of this area

Dark-green amphibolite, actinolite schist, and greenstone. Interflow units consists of graphitic schist, chert, and carbonate- and silicate-facies iron-formation. Thickness of individual flows 50-400 ft (244-1,524 m).

Ogallala Group or Formation (Miocene) at surface, covers < 0.1 % of this area

Silt, sand, sandstone, gravel and conglomerate. Predominantly interfingered fine- to coarse grained, poorly sorted, arkosic, fluvial deposits of light-gray, light-olive-gray, and grayish-green calcareous silt and sand, and locally poorly consolidated conglomerate, sandstone, and siltstone.

Trachytic Intrusive Rocks (Eocene to Paleocene) at surface, covers < 0.1 % of this area

Tan to reddish-brown, iron-stained stocks, laccoliths, sills, and dikes of trachyte, quartz trachyte, and alkalic rhyolite. Contains phenocrysts of sanidine, orrthoclase, anorthoclase, aegirine-augite and biotite in a finely crystalline orthoclase-quartz biotite groundmass.

Metamorphosed Carbonaceous Shale (Early Proterozoic) at surface, covers < 0.1 % of this area

Dark-gray to gray, siliceous biotite phyllite and schist. Thickness greater than 2,500 ft (762m).

Rhyolitic Intrusive Rocks (Eocene to Paleocene) at surface, covers < 0.1 % of this area

Light-tan to light-gray stocks and small laccoliths of rhyolite. Contains phenocrysts of oligoclase, quartz, and biotite in a finely crystalline orthoclase or sanidine-quartz groundmass.

Milbank Granite (Upper Archean) at surface, covers < 0.1 % of this area

Pink to dark-red, coarse-grained granite composed of orthoclase, quartz, and biotite.

Metagraywacke Unit 1 (Early Proterozoic) at surface, covers < 0.1 % of this area

Lower Xgw- Light- to dark-gray, silceous mica schist and impure quartzite.

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

Clay to boulder-size clasts forming rubble residuum and talus. Thickness up to 30 ft (9m).

Metaconglomerate and Metaquartzite (Early Proterozoic) at surface, covers < 0.1 % of this area

Light-gray to gray, conglomeratic and feldspathic schist, biotite schist, taconite, and phyllite. Individual conglomerate and fanglomerate tongues from 100-500 ft (30-152 m) thick. Total thickness over 10,000 ft (3,048 m).

Metabasalt (Early Proterozoic) at surface, covers < 0.1 % of this area

Alkalic basalt, greenstone, and actinolite schist. Includes metamorphosed volcanoclastic rocks and iron-rich schist.

Metaconglomerate (Early Proterozoic) at surface, covers < 0.1 % of this area

Tan to light-gray, conglomeratic siliceous schist, feldspathic schist, and minor marble. Thickness locally over 6,000 ft (1,829 m).

Phonolitic Intrusive Rocks (Eocene to Paleocene) at surface, covers < 0.1 % of this area

Gray to greenish-gray sills, laccoliths, and small stocks of phonolite, trachyphonolite, and trachyandesite. Contains phenocrysts of andesine, feldspathoids, aegirine-augite, biotite, and sphene in a finely crystalline plagioclase-biotote-feldsphoid groundmass.

Metagraywacke Unit 2 (Early Proterozoic) at surface, covers < 0.1 % of this area

Middle Xgw- Light- to dark-gray, silceous mica schist and impure quartzite.

Metagabbro (Early Proterozoic) at surface, covers < 0.1 % of this area

Dark-green sills of amphibolite, actinolite schist, greenstone, and serpentine. Thickness of sills variable, up to 1,000 ft (305 m).

Niobrara Formation (Late Cretaceous) at surface, covers < 0.1 % of this area

Argillaceous chalk, limestone and shale. Chalk is medium gray to white, interbedded with thin layers of chalky shale; contains many fossil clams, oysters, and formanifera. Limestone is light gray to medium gray and yellowish gray, interbedded with medium-gray chalky shale; also contains fossil clams, oysters, and formanifera. Bedding plans commonly marked by thin layers of gypsum and locally a thin bed of red flint occurs at top of unit. Approx. max thickness 570 ft.

Till, Ground Moraine (Pleistocene-Upper Wisconsin) at surface, covers < 0.1 % of this area

Heterogeneous, clay with silt to boulder-size clasts of glacial orgin. A geomorphic feature that is characterized by smooth, rolling terrain. Composite thickness of all Upper Wisconsin till may be up to 300 ft (91 m).

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

Tan to green, calcareous siltstone, claystone, channel sandstone, conglomerate, and arkose. Thickness up to 50 ft (15 m).

Metamorphosed Dolomite (Early Proterozoic) at surface, covers < 0.1 % of this area

Light-gray to light-tan marble, phyllite, and calcareous phyllite. Thickness 60-300 ft (18-91 m).

Fort Benton Group (Upper Cretaceous, upper Cenomanian-Turonian) at surface, covers < 0.1 % of this area

Interval includes “Graneros” Shale, Greenhorn Formation, Carlile Shale. Primary Lithologies: shale, medium to dark gray, variably silty, calcareous to very calcareous. Secondary lithologies: chalk, marl, argillaceous limestone, with skeletal (inoceramid) packstones; shale, gray, silty, noncalcareous. Minor: siltstone; calcite and siderite concretions. Maximum thickness 265 ft (80 m).

Iron-Formation (Early Proterozoic) at surface, covers < 0.1 % of this area

Banded, dark-green, reddish-brown, and white iron-formation, ferruginous chert, and minor mica schist. Includes three or more ages of oxide-, carbonate-, silicate-, and sulifide-facies iron-formation and interbedded tuffaceous rocks. Thickness 20-500 ft (6-152 m).

Granite (eastern bedrock) (Upper Archean) at surface, covers < 0.1 % of this area

Felsic coarse-grained granite, includes white, silty clay weathering residuum which grades downward to a pink, white, and gray siltstone.

Metagraywacke (Early Proterozoic to Late Archean ?) at surface, covers < 0.1 % of this area

Gray, siliceous mica schist and impure quartzite. Thickness undetermined.

Dakata and Windrow Formations (“Mid” Cretaceous; upper Albian-upper Cenomanian) at surface, covers < 0.1 % of this area

Dakota Formation widespread in western Iowa, lower sandstone-dominated Nishnabotna Member, upper mudstone/shale-dominated Woodbury Member. Correlative Windrow Formation found as erosional outliers in northeastern and north-central Iowa. Primary lithologies: sandstone, quartzose, very fine to medium grained; mudstone/shale, light to dark gray, variably silty-sandy, noncalcareous (Woodbury Mbr.). Secondary lithologies: sandstone, medium to very coarse grained, part pebbly to gravelly, locally cemented by iron oxides (Nishnabotna Mbr., Windrow Fm.), gravel, quartz and chert clasts; siltstone; mudstone, red, pink, yellow-brown, black (carbonaceous). Minor: lignite; siderite (concretions, pedogenic sphaerosiderite pellets, cemented siltstone); massive iron ore, silty to sandy (Windrow Fm.). Maximum thickness Dakota Fm. 500 ft (150 m), commonly 100-300 ft (30-90 m); Windrow Fm. 40 ft (12 m).

Metamorphosed Carbonaceous Shale (Early Proterozoic) at surface, covers < 0.1 % of this area

Dark-gray to gray, siliceous biotite phyllite, calcareous biotite phyllite, and schist. Minimum thickness 1,500 ft (457 m).

Corson Diabase (Lower Proterozoic) at surface, covers < 0.1 % of this area

Black, intermediate- to coarse-grained diabase composed of plagioclase, pyroxene, and olivine.

Pierre Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Dark-gray shale; marine offshore sediment; as thick as 700 metres (2,300 feet).

Carlile Shale (Late Cretaceous) at surface, covers < 0.1 % of this area

Shale, limestone, and sandstone. At top, locally 5 feet of gray to pale-yellowish brown siltstone or very fine grained sandstone. Upper 200 feet of shale is drak gray to medium gray; locally contains ironstone concretions, and interbedded with thin siltstone. Lower 80 feet of shale is medium gray, calcareous, and contains many very thin bedded, fossiliferous, shaly limestone and calcareous shale layers. Approx. max thickness 300 ft.

Older Metasedimentary Rocks (Late Archean) at surface, covers < 0.1 % of this area

Gray phyillite, mica schist, and biotite-plagioclase schist. Total thickness unknown; approximately 500 ft (152 m) exposed.

Pierre Shale (Late Cretaceous) at surface, covers < 0.1 % of this area

Mostly medium to dark-gray, brownish-gray, and black, fissle clay shale. Locally grades to thin beds of calcareous, silty shale or claystone, marl, shaly sandstone, and sandy shale. Locally contains thin seams of gypsum and sparse selenite crystals. Approx. max thickness 1970 ft.

Undifferentiated (Cenomanian to Campanian) at surface, covers < 0.1 % of this area

Conglomerate, sandstone, mudstone, shale, marlstone, siltstone, and minor lignite, deposited in marine and non-marine settings; likely Cenomanian to Campanian age. Unit outline is the product of contouring the stratigraphic top and base, from which an isopach grid was created. Because the distribution is patchy, unit boundaries were drawn from the gridded data to represent locations where more than 25 feet (8 meters) of thickness occurs. As a result, many areas outside of the unit boundaries may be overlain by thin Cretaceous strata and the unit is depicted without a contact line.

Carlile Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Dark-gray shale; marine offshore sediment; as thick as 120 metres (400 feet).

Dakota Group (Early Cretaceous) at surface, covers < 0.1 % of this area

Upper part is white, light-gray, brownish-gray, yellow, redish-brown, and red sandstone and shale. Sandstone is very fine to coarse grained, friable, micaeous, crossbedded, and lenticular; locally contains gravel near base. contains numerous zones of ironstone and siltstone concretions of variable thickness. Middle part is light-gray, yellow, red brown and dark-gray, sandy carbonaceous shale; commonly contains a zone of concretions near top. Lower part is sandstone similar to that in upper except there are zones of siderite concretions and, locally a basal zone of chert pebbles. Approx. max thickness 600 ft.

Granite (Late Archean) at surface, covers < 0.1 % of this area

Pink and gray, strongly foliated, medium- to coarse-crystalline, locally pegmatitic, biotite-muscovite granite and gneissic granite.

Dam (Unknown) at surface, covers < 0.1 % of this area

Dam

Belle Fourche Shale, Mowry Shale, Newcastle Sandstone,and Skull Creek Shale (Late Cretaceous to Early Cretaceous) at surface, covers < 0.1 % of this area

Note: see individual unit descriptions

Pierre Formation (Cretaceous) at surface, covers < 0.1 % of this area

Dark gray, partly silty shale with abundant bentonite beds and zones of gray, calcareous concretions. Marine. Thickness as much as650 m (2,133 ft). Only upper 50 m (164 ft) exposed.

Foliated to gneissic tonalite, granodiorite, and diorite (Neoarchean) at surface, covers < 0.1 % of this area

Foliated to gneissic tonalite, granodiorite, and diorite. Includes the Lookout Mountain tonalite (~2,718 Ma) of the Giants Range batholith and other intrusions within batholithic complexes defined by a low-to-moderate gravity signature with magnetic foliation apparent from aeromagnetic maps.

Fox Hills Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Olive-brown sand, shale, and sandstone; marine shoreline and off-shore sediment; as thick as 120 metres (400 feet).

Greenhorn Limestone and Graneros Shale (Late Cretaceous) at surface, covers < 0.1 % of this area

Greenhorn Limestone- medium- to light-gray limestone interbedded with argillaceous limestone, marl and calcareous shale; contains Inoceramus fossils. Upper and lower contacts gradational. Approx. max thickness 30 ft. Graneros Shale- medium- to dark-gray, partly calcareous shale. Interbeds of siltstone, sandstone, and carbonaceous shale, and thin bentonite layers in upper part. Approx. max thickness 60 ft.

Trail City Member of Fox Hills Formation (Cretaceous) at surface, covers < 0.1 % of this area

Yellowish orange, wavy-bedded siltstone and black shale with calcareous concretion zone. Thickness 10 m (33 ft).

Greenhorn Formation and Belle Fourche and Mowry Shale (Lower Cretaceous) at surface, covers < 0.1 % of this area

Greenhorn Formation - Light-colored limestone, marl, and limy sandstone interbedded with gray concretionary shale. Belle Fourche Shale - Black soft bentonitic concretionary shale. Mowry Shale (Kmr) - Silvery-gray hard siliceous shale containing abundant fish scales and bentonite beds.

Hell Creek Formation (Cretaceous) at surface, covers < 0.1 % of this area

Light gray, bentonitic claystone that alternates with gray to brown sandstone interbedded with carbonaceous shale. Laterally equivalent to Lance Formation. Fluvial and flood plain. Thickness as much as 335 m (1,100 ft).

Granite to granodiorite (Neoarchean) at surface, covers < 0.1 % of this area

Granite to granodiorite. Variably magnetic.

Minnelusa Formation (Lower Pennsylvanian-Permian) at surface, covers < 0.1 % of this area

Buff and red limy sandstone; some thin limestone beds, solution breccias, and gypsum.

Niobrara Formation and Carlile Shale (Upper Cretaceous) at surface, covers < 0.1 % of this area

Niobrara Formation (Kn) - Light-colored limestone and gray to yellow speckled limy shale. Carlile Shale (Kcl) - Dark-gray sandy shale; Sage Breaks Member at top; Turner Sandy Member in middle.

Niobrara Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Light-brown to dark-gray calcareous shale; marine offshore sediment; as thick as 75 metres (250 feet).

Cloverly and Morrison Formations or Inyan Kara Group (Upper Jurassic-Lower Cretaceous) at surface, covers < 0.1 % of this area

Cloverly Formation (North and South Wyoming) - Rusty sandstone at top, underlain by brightly variegated bentonitic claystone; chert-pebble conglomerate locally at base. Cloverly Formation (Northeast Wyoming - Hartville Uplift) - Rusty to light-gray sandstone containing lenticular chert-pebble conglomerate interbedded with variegated bentonitic claystone. Morrison Formation (North and South Wyoming) - Dully variegated claystone, nodular limestone, and gray silty sandstone. In southern Yellowstone and Jackson Hole areas the presence of Morrison is questionable. Morrison Formation (Northeast Wyoming) - dully variegated siliceous claystone, nodular white limestone, and gray silty sandstone. Inyan Kara Group (Northeast - Black Hills) - Rusty to light-gray sandstone containing lenticular chert-pebble conglomerate interbedded with variegated bentonitic claystone. Includes Fall River and Lakota Formations.

Granitic intrusion (Neoarchean) at surface, covers < 0.1 % of this area

Granitic intrusion. Includes the Sacred Heart (~2592, 2,603 Ma) and Ortonville granites, the Shannon Lake Granite (~2,674), and other intrusions having low gravity and magnetic signatures.

Pierre Shale (Upper Cretaceous) at surface, covers < 0.1 % of this area

Dark-gray concretionary marine shale; contains several bentonite beds.

Timber Lake Member of Fox Hills Formation (Cretaceous) at surface, covers < 0.1 % of this area

Yellowish orange to gray, fine- to medium grained, noncalcareous, hummocky-bedded sandstone. Thickness 15–22 m (50–72 ft).

Sioux Quartzite (Lower Proterozoic) at surface, covers < 0.1 % of this area

Crops out in northwestern Lyon County; buried beneath Cretaceous strata to south and east. Primary lithology: quartzite, fine to coarse grained, pink, red, purple (low-grade metamorphic rock). Minor: conglomerate; claystone/argillite, red. Estimated thickness <500 ft (150 m).

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

Greenschists and amphibolites; banded iron formation; stretched pebble conglomerates; metabasalt; serpentinite; felsic tuff; mylonite; intermediate felsic plutonic rocks; migmatite; layered gneiss.

Newcastle Sandstone and Skull Creek Shale (Lower Cretaceous) at surface, covers < 0.1 % of this area

Newcastle Sandstone - Gray sandstone and sandy shale containing some bentonite and coal. Skull Creek Shale - Black soft fissile shale.

Ludlow Member of Fort Union Formation (Tertiary) at surface, covers < 0.1 % of this area

Gray and brown shale, siltstone, silty or bentonitic claystone, sandstone, and coal. Alluvial plain with marine-influenced tongues. Thickness as much as 230 m (755 ft).

Granitic orthogneiss and migmatite (Mesoarchean to Paleoarchean) at surface, covers < 0.1 % of this area

Granitic orthogneiss and migmatite. Geophysical map patterns imply this unit intruded other gneisses.

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

Gravel, sand, silt, and clay deposits of stream and river channels, and floodplains.

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.

Metabasalt (Early Proterozoic to Late Archean ?) at surface, covers < 0.1 % of this area

Dark-green amphibolite and amphibolite schist. Thickness of individual flows 50-200 ft (15-61 m).

Sundance and Gypsum Spring Formations (Middle Jurassic-Upper Jurassic) at surface, covers < 0.1 % of this area

Sundance Formation (Js) - Greenish-gray glauconitic sandstone and shale, underlain by red and gray nonglauconitic sandstone and shale. Gypsum Spring Formation - Interbedded red shale, dolomite, and gypsum. In north Wyoming wedges out south in T. 39 N.

Ekalaka Member of Fort Union Formation (Tertiary) at surface, covers < 0.1 % of this area

Yellowish orange or tan, fine- to medium-grained sandstone interbedded with mudstone and thin shale and coal beds. Estuarine. Thickness as much as 55 m (180 ft).

Greenhorn Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Dark-gray calcareous shale; marine offshore sediment; as thick as 45 metres (150 feet).

Spearfish Formation (Permian-Upper Triassic) at surface, covers < 0.1 % of this area

Red shale, red siltstone, and white gypsum beds; gypsum beds especially abundant near base.

Mafic metavolcanic rocks (Neoarchean) at surface, covers < 0.1 % of this area

Mafic metavolcanic rocks. Includes minor volcaniclastic and hypabyssal intrusive rocks metamorphosed to lower greenschist to lower amphibolite facies; includes the Ely Greenstone (~2,722 Ma).

Pegmatite (Early Proterozoic to Late Archean ?) at surface, covers < 0.1 % of this area

Light-tan to pink pegmatite

Hell Creek Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Gray sand, silt, clay, and sandstone; river sediment; as thick as 150 metres (500 feet).

Volcaniclastic rocks of felsic to intermediate composition (Neoarchean) at surface, covers < 0.1 % of this area

Volcaniclastic rocks of felsic to intermediate composition.

Sioux Quartzite (Paleoproterozoic) at surface, covers < 0.1 % of this area

Quartzite, mudstone, and local conglomerate of fluvial and marine origin (~1,760 to 1,630 Ma; 1,902 ± 55 Ma rhyolite pebble in basal conglomerate).

Niobrara Formation (Upper Cretaceous; ?Santonian, lower Campanian) at surface, covers < 0.1 % of this area

Primary Lithologies: shale, gray, silty, calcareous to marly. Estimated maximum thickness 50 ft (15 m). Present only in Lyon County.

Groat sandstone bed of Pierre Formation (Cretaceous) at surface, covers < 0.1 % of this area

Gray, ferruginous and glauconitic, fine- to coarse-grained sandstone, siltstone, and sandy to silty gray shale. Offshore marine. Pinches out laterally. Thickness as much as 100 m (328 ft).

Landslide deposits (Pleistocene-Holocene) at surface, covers < 0.1 % of this area

Locally includes intermixed landslide and glacial deposits, talus, and rock-glacier deposits.

Pahasapa and Englewood Limestones (Upper Devonian-Lower Mississippian) at surface, covers < 0.1 % of this area

Pahasapa Limestone - Gray massive dolomitic limestone. Englewood Limestone - Pink slabby dolomitic limestone.

Gravel, pediment, and fan deposits (Pleistocene-Holocene) at surface, covers < 0.1 % of this area

Mostly locally derived clasts. Includes some glacial deposits along east flank of Wind River Range. Locally includes some Tertiary gravels.

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

Minnekahta Limestone - Gray slabby hard limestone. Locally is a member of the Goose Egg Formation. Opeche Shale - Red soft sandy shale. Locally is a member of the Goose Egg Formation.