Geologic units in Pennington county, South Dakota

Additional scientific data in this geographic area

Pierre Shale (Upper Cretaceous) at surface, covers 23 % 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).

White River Group (Oligocene to Eocene) at surface, covers 15 % 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).

Terrace Deposits (Quaternary) at surface, covers 9 % 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).

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

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

Fox Hills Sandstone (Upper Cretaceous) at surface, covers 7 % 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).

Eolian Deposits (Quaternary) at surface, covers 6 % 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).

Madison Group (Mississippian to Devonian) at surface, covers 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).

Metamorphosed Shale (Early Proterozoic) at surface, covers 4 % of this area

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

Minnelusa Formation (Permian to Pennsylvanian) at surface, covers 4 % 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).

Metagraywacke (Early Proterozoic) 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).

Metamorphosed Tuffaceous Shale (Early Proterozoic) at surface, covers 2 % of this area

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

Metamorphosed Black Shale (Early Proterozoic) at surface, covers 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).

Whitewood Limestone, Winnipeg Formation, and Deadwood Formation (Ordovician to Cambrian) at surface, covers 1.0 % 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).

Metagraywacke Unit 3 (Early Proterozoic) at surface, covers 0.9 % of this area

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

Metamorphosed Conglomerate (Early Proterozoic) at surface, covers 0.7 % 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).

Inyan Kara Group (Early Cretaceous) at surface, covers 0.6 % 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).

Spearfish Formation (Triassic to Permian) at surface, covers 0.6 % of this area

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

Metagraywacke Unit 1 (Early Proterozoic) at surface, covers 0.6 % of this area

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

Metaquartzite (Early Proterozoic) at surface, covers 0.5 % of this area

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

Metamorphosed Siltstone (Early Proterozoic) at surface, covers 0.5 % 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).

Carlile Shale (Upper Cretaceous) at surface, covers 0.5 % 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).

Minnekahta Limestone and Opeche Shale (Permian) at surface, covers 0.4 % 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).

Belle Fourche Shale (Late Cretaceous) at surface, covers 0.4 % 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).

Harney Peak Granite (Early Proterozoic) at surface, covers 0.4 % 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.

Gravel Deposits (Tertiary) at surface, covers 0.4 % 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).

Morrison Formation, Unkpapa Sandstone, Sundance Formation, and Gypsum Springs Formation (Late Jurassic to Middle Jusassic) at surface, covers 0.4 % 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).

Metabasalt (Early Proterozoic) at surface, covers 0.4 % 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).

Mowry Shale, Newcastle Sandstone, and Skull Creek Shale (Early Cretaceous) at surface, covers 0.4 % 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).

Metamorphosed Carbonaceous Shale (Early Proterozoic) at surface, covers 0.3 % of this area

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

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).

Niobrara Formation (Upper Cretaceous) at surface, covers 0.3 % 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).

Metabasalt (Early Proterozoic) at surface, covers 0.3 % of this area

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

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

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

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.

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 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).

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).

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).

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.

Arikaree Group (Oligocene to Eocene) at surface, covers < 0.1 % 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).

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).

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).

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).

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

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.

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.

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).

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.