The Stanley Shale, or Stanley Group, is a Mississippian stratigraphic unit in the Ouachita Mountains of

Arkansas Arkansas ( ) is a landlocked state in the South Central United States. It is bordered by Missouri to the north, Tennessee and Mississippi to the east, Louisiana to the south, and Texas and Oklahoma to the west. Its name is from the Osage ...

and

Oklahoma Oklahoma (; Choctaw language, Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a U.S. state, state in the South Central United States, South Central region of the United States, bordered by Texas on the south and west, Kansas on the nor ...

. First described in Arkansas in 1892, this unit was not named until 1902 by J.A. Taff in his study of the Ouachita Mountains of Oklahoma. Taff assigned the town of

Stanley Stanley may refer to: Arts and entertainment Film and television * ''Stanley'' (1972 film), an American horror film * ''Stanley'' (1984 film), an Australian comedy * ''Stanley'' (1999 film), an animated short * ''Stanley'' (1956 TV series) ...

Pushmataha County Pushmataha County is a county in the southeastern part of the U.S. state of Oklahoma. As of the 2010 census, the population was 11,572. Its county seat is Antlers. The county was created at statehood from part of the former territory of the C ...

, Oklahoma as the type locality, but did not designate a stratotype. After introduction into Arkansas in 1909 by Albert Homer Purdue, the unit was redefined in 1918, when the formation known as the Fork Mountain Slate was abandoned and partially combined into the Stanley Shale. As of 2017, a reference section for the Stanley Shale has yet to be designated. The Stanley Shale is recognized as a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

with two sub-units in Arkansas, the Hatton Tuff Lentil and the Hot Springs Sandstone Member, however, several others have been proposed. These include the Chickasaw Creek Member, Chickasaw Creek Tuff, Gap Ridge Sandstone Member, Moyers Member, Parker Hill Sandstone Member, Polk County Ash Bed, and Tenmile Creek Member. In 1963, it was first proposed to raise the rank of the Stanley Shale Formation to Group status in Arkansas, with proposed sub-units consisting of the Chickasaw Creek Formation, Moyers Formation (with the Gap Ridge Sandstone and Parker Hill Sandstone Members), and Ten Mile Creek Formation. However, the proposal was not accepted and the unit remained as a formation. In Oklahoma, this unit is recognized as a group called the Stanley Group composed of three formations: the Chickasaw Creek Shale Formation, the Moyers Formation, and the Tenmile Creek Formation. The Hatton Tuff Lentil, recognized in Arkansas, is also in Oklahoma as a unit of the Tenmile Creek Formation. Several informal members have been noted including the Smithville chert lentil, the Faith chert member, and the Chickasaw Creek tuff among others Mining in the Stanley Shale is limited to cinnabar,

barite Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...

, and

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

. Cinnabar is primarily used for mercury, however, production in Arkansas essentially ended by the mid-20th century. Barite mining, mostly useful for oil and gas

drilling fluid In geotechnical engineering, drilling fluid, also called drilling mud, is used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also us ...

, is ongoing.

Paleoflora

* '' Adiantites'' : ''A. stanleyanus'' * '' Alloiopteris'' : ''A. arkansana'' * ''

Aphlebia Aphlebiae are the imperfect or irregular leaf endings commonly found on ferns and fossils of ferns from the Carboniferous Period, but seem to have disappeared by the beginning of the Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, ...

'' * '' Archaeocalamites'' : ''A. coralloides'' : ''A. stanleyensis'' * '' Calymmatotheca'' * ''

Calamites ''Calamites'' is a genus of extinct arborescent (tree-like) horsetails to which the modern horsetails (genus ''Equisetum'') are closely related. Unlike their herbaceous modern cousins, these plants were medium-sized trees, growing to heights o ...

'' : ''C. menae'' : ''C. miseri'' * '' Lepidodendron'' : ''L. subclypeatum'' * ''

Lepidostrobus ''Lepidodendron'' is an extinct genus of primitive vascular plants belonging to the family Lepidodendraceae, part of a group of Lycopodiopsida known as scale trees or arborescent lycophytes, related to quillworts and lycopsids (club mosses). T ...

'' : ''L. peniculus'' * ''

Neuropteris ''Neuropteris'' is an extinct seed fern that existed in the Carboniferous period, known only from fossils. Major species include ''Neuropteris loschi''. See also *Coal forest *''Macroneuropteris ''Macroneuropteris'' is a genus of Carbonifer ...

'' : ''N. elrodi'' * '' Palmatopteris'' : ''P. subgeniculata'' * '' Rhabdocarpos'' : ''R. secalicus'' * '' Rhodea'' : ''R. goepperti'' * '' Sphenophyllum'' : ''S. arkansanum'' * '' Sphenopteridium'' : ''S. dawsoni'' * ''

Sphenopteris ''Sphenopteris'' is a genus of seed ferns containing the foliage of various extinct plants, ranging from the Devonian to Late Cretaceous. One species, ''S. höninghausi'', was transferred to the genus ''Crossotheca'' in 1911. Biology The frond ...

'' * '' Stigmaria'' * '' Trigonocarpum'' : ''T. gillhami'' : ''T. vallisjohanni'' * ''

Wardia ''Wardia'' is a monotypic genus of mosses in the subclass Dicranidae; it contains only the species ''Wardia hygrometrica'', "an aquatic moss endemic to the Western Cape province of South Africa."Hedderson, Terry A., Cymon J. Cox, & J. George Gib ...

'' : ''W. suspecta''

References

Carboniferous Arkansas Carboniferous geology of Oklahoma {{US-geologic-formation-stub