The Spearfish Formation is a geologic formation, originally described from the Black Hills region of South Dakota, United States, but also recognised in North Dakota, Wyoming, Montana and Nebraska. It is a heterogeneous

red bed Red beds (or redbeds) are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain th ...

formation, commonly with

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

and gypsum low in the formation and sandstone and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

higher up. Other

rock types In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition, and the way in which it is formed. Rocks ...

include

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too sm ...

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** Co ...

, 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– Triassic 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.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 stromatolites, casts of

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

s, and trace fossils have been found.

References

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 Shallow marine deposits Reservoir rock formations {{US-geologic-formation-stub