The Spearfish Formation is a geologic
formation, originally described from the
Black Hills region of
South Dakota
South Dakota (; Sioux language, Sioux: , ) is a U.S. state, state in the West North Central states, North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Dakota people, Dakota Sioux ...
,
United States
The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, but also recognised in
North Dakota
North Dakota ( ) is a U.S. state in the Upper Midwest, named after the indigenous Dakota people, Dakota and Sioux peoples. It is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north and by the U.S. states of Minneso ...
,
Wyoming
Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...
,
Montana
Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...
and
Nebraska
Nebraska ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Ka ...
.
It is a heterogeneous
red bed formation, commonly with
siltstone and
gypsum low in the formation and
sandstone
Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
and
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
higher up. Other
rock types include
claystone,
conglomerate,
dolomite, and
oil shale.
[Sabel, J. M. 1984. Sedimentology and depositional history of the Permo-Triassic Spearfish Formation, southwestern Black Hills, South Dakota. Pages 295-307 in Goolsby, J. and D. Morton, editors. The Permian and Pennsylvanian geology of Wyoming. Wyoming Geological Association, Casper, WY. Guidebook 35.] It is typically regarded as
Permian
The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
–
Triassic
The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
in age,
[ although its original description included Jurassic rocks.
The Spearfish Formation is interpreted as representing shallow marine to coastal terrestrial deposition, like the modern ]Persian Gulf
The Persian Gulf, sometimes called the Arabian Gulf, is a Mediterranean seas, mediterranean sea in West Asia. The body of water is an extension of the Arabian Sea and the larger Indian Ocean located between Iran and the Arabian Peninsula.Un ...
.[Sabel, J. M. 1983. Sedimentology of Spearfish Formation. AAPG Bulletin 67(3):543-544.] Depositional environments may have included restricted marine bodies and ephemeral lakes (gypsum), hypersaline waters (limestone), hypersaline microbial mats (oil shale), and sabkhas (dolomite). The marine waters of the shallow continental sea retreated during the deposition of the formation, reflected in a change from dominantly nearshore marine to coastal terrestrial deposition over time.[
The Spearfish Formation is sparsely fossiliferous. So far, only ]stromatolite
Stromatolites ( ) or stromatoliths () are layered Sedimentary rock, sedimentary formation of rocks, formations (microbialite) that are created mainly by Photosynthesis, photosynthetic microorganisms such as cyanobacteria, sulfate-reducing micr ...
s, casts of bivalve
Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
s, and trace fossils have been found.[
]
References
External links
Geologic formations of South Dakota
Geologic formations of North Dakota
Geologic formations of Wyoming
Geologic formations of Montana
Geologic formations of Nebraska
Triassic System of North America
Permian System of North America
Sandstone formations of the United States
Shale formations of the United States
Siltstone formations of the United States
Shallow marine deposits
Reservoir rock formations
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