USGS Home
Contact USGS
Search USGS

Mineral Resources > Online Spatial Data > Geology > by state > South Dakota Geology

Geologic units in Custer county, South Dakota

[Additional scientific data in this geographic area]

Metamorphosed Black Shale (Proterozoic | Paleoproterozoic) at surface, covers 0.7 % 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).

Lithology: schist; chert

Niobrara Formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.7 % 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 160-225 ft (49-69 m).

Lithology: limestone; limestone; shale; clay or mud; sand

Metamorphosed Tuffaceous Shale (Proterozoic | Paleoproterozoic) at surface, covers 0.2 % of this area

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

Lithology: schist; phyllite

Metagraywacke Unit 2 (Proterozoic | Paleoproterozoic) at surface, covers 0.3 % of this area

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

Lithology: schist; quartzite

Spearfish Formation (Phanerozoic | Paleozoic Mesozoic | Permian Triassic) at surface, covers 6 % of this area

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

Lithology: shale; siltstone; sandstone; evaporite; limestone

Eolian Deposits (Phanerozoic | Cenozoic | Quaternary) at surface, covers 1 % 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).

Lithology: silt; sand

Madison Group (Phanerozoic | Paleozoic | Devonian Carboniferous Mississippian) at surface, covers 7 % 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).

Lithology: limestone; dolostone (dolomite); chert

Metabasalt (Proterozoic | Paleoproterozoic) at surface, covers 0.1 % of this area

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

Lithology: basalt; greenschist

Mowry Shale, Newcastle Sandstone, and Skull Creek Shale (Phanerozoic | Mesozoic | Cretaceous-Early ) at surface, covers 0.7 % 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).

Lithology: shale; siltstone; sandstone; claystone

Minnekahta Limestone and Opeche Shale (Phanerozoic | Paleozoic | Permian) at surface, covers 5 % 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).

Lithology: siltstone; sandstone; shale; limestone

Terrace Deposits (Phanerozoic | Cenozoic | Quaternary) at surface, covers 7 % 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).

Lithology: clay or mud; silt; sand; gravel

Greenhorn Formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.4 % of this area

Gray shale, mudstone, marl, calcarenite, and shaley limestone grading upward into light-gray to tan, alternating marl and thin-bedded, fossiliferous limestone. Thickness 225-315 ft (69-96 m).

Lithology: shale; mudstone; limestone

Metaconglomerate (Proterozoic | Paleoproterozoic) 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).

Lithology: schist; marble

Metagraywacke (Proterozoic | Paleoproterozoic) at surface, covers 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).

Lithology: schist; quartzite

Gravel Deposits (Phanerozoic | Cenozoic | 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).

Lithology: clay or mud; silt; sand; gravel

Pierre Shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 9 % of this area

Blue-gray to dark-gray, fissile to blocky shale with persistent beds of bentonite, black organic shale, or light-brown chalky shale. Contains minor sandstone, conglomerate, and abundant carbonate and ferruginous concretions. Thickness 1,000-2,700 ft (305-823 m).

Lithology: shale; clay or mud; sandstone; conglomerate

Morrison Formation, Unkpapa Sandstone, Sundance Formation, and Gypsum Spring Formation (Phanerozoic | Mesozoic | Jurassic-Middle Jurassic-Late) at surface, covers 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).

Lithology: sandstone; siltstone; claystone; clay or mud; shale; limestone; evaporite

Carlile Shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 1 % of this area

Dark-gray to black, silty to sandy shale with several zones of septarian, fossiliferous, carbonate concertions. Contains up to three sandstone beds near the middle of the formation and sandy calcareous marl at the base. Thickness 345-620 ft (105-189 m).

Lithology: shale; sandstone; limestone

Harney Peak Granite (Proterozoic | Paleoproterozoic) at surface, covers 2 % 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.

Lithology: granite

Metamorphosed Conglomerate (Proterozoic | Paleoproterozoic) 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).

Lithology: phyllite; schist; quartzite; iron formation

Upper Metagraywacke (Proterozoic | Paleoproterozoic) at surface, covers 3 % 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).

Lithology: schist; metasedimentary rock; gneiss

White River Group (Phanerozoic | Cenozoic | Tertiary | Eocene Oligocene) at surface, covers 12 % 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).

Lithology: claystone; siltstone; clay or mud; sandstone; mudstone; conglomerate

Iron-Formation (Proterozoic | Paleoproterozoic) 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).

Lithology: iron formation; chert; schist

Minnelusa Formation (Phanerozoic | Paleozoic | Carboniferous Pennsylvanian Permian) at surface, covers 16 % 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).

Lithology: sandstone; siltstone; shale; limestone; dolostone (dolomite); chert

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 9 % of this area

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

Lithology: clay or mud; silt; sand; gravel

Inyan Kara Group (Phanerozoic | Mesozoic | Cretaceous-Early ) at surface, covers 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).

Lithology: shale; sandstone; conglomerate; siltstone; limestone; coal

Belle Fourche Shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.7 % 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).

Lithology: shale; limestone; clay or mud

Landslide Deposits (Phanerozoic | Cenozoic | 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).

Lithology: sand; gravel

Pahasapa Limestone, Englewood Formation, Whitewood Limestone, Winnipeg Formation, and Deadwood Formation (Phanerozoic | Paleozoic | Cambrian Ordovician(?) Silurian(?) Devonian(?) Carboniferous Mississippian) at surface, covers < 0.1 % of this area

Note: see individual descriptions

Lithology: conglomerate; limestone; dolostone (dolomite); shale; siltstone

Metamorphosed Dolomite (Proterozoic | Paleoproterozoic) 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).

Lithology: marble; phyllite

Metaquartzite (Proterozoic | Paleoproterozoic) at surface, covers 7 % of this area

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

Lithology: quartzite; schist; chert

Lower Metagraywacke (Proterozoic | Paleoproterozoic) at surface, covers 2 % 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).

Lithology: schist

Metabasalt (Proterozoic | Paleoproterozoic) 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).

Lithology: amphibolite; schist; greenstone; chert; iron formation

Whitewood Limestone, Winnipeg Formation, and Deadwood Formation (Phanerozoic | Paleozoic | Cambrian Ordovician) at surface, covers 1 % 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).

Lithology: conglomerate; shale; limestone; dolostone (dolomite); siltstone

Metagabbro (Proterozoic | Paleoproterozoic) 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).

Lithology: amphibolite; schist; greenstone; serpentinite