The Jackfork Sandstone, also referred to as the Jackfork Group, is 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 ...

associated with the Ouachita Fold and Thrust Belt exposed in western

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 southeastern

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 ...

. It is named for Jackfork Mountain in Pittsburg and

Pushmataha Pushmataha (c. 1764 – December 24, 1824; also spelled Pooshawattaha, Pooshamallaha, or Poosha Matthaw), the "Indian General", was one of the three regional chiefs of the major divisions of the Choctaw in the 19th century. Many historians cons ...

counties, Oklahoma. The Jackfork Sandstone is a thin- to massive-bedded, fine- to coarse-grained, brown, tan, or gray quartzitic sandstone with subordinate brown, silty sandstone and dark gray shale. It outcrops from

Pulaski County, Arkansas Pulaski County is located in the U.S. state of Arkansas with a population of 399,125, making it the most populous county in Arkansas. The county is included in the Little Rock–North Little Rock– Conway metropolitan area. Its county seat is L ...

in the east to

Atoka County, Oklahoma Atoka County is a county located in the U.S. state of Oklahoma. As of the 2010 census, the population was 14,007. Its county seat is Atoka. The county was formed before statehood from Choctaw Lands, and its name honors a Choctaw Chief named A ...

in the west, a distance of over 200 miles. It is highly weather-resistant, resulting in a continuous chain of prominent ridges, including Rich Mountain, the second highest natural point in the

Ouachita Mountains The Ouachita Mountains (), simply referred to as the Ouachitas, are a mountain range in western Arkansas and southeastern Oklahoma. They are formed by a thick succession of highly deformed Paleozoic strata constituting the Ouachita Fold and Thru ...

Paleoflora

* '' Aphlebia'' : ''A. parksii'' * '' Archaeocalamites'' : ''A. stanleyensis'' * '' Bothrodendron'' * ''

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. inopinatus'' : ''C. menae'' : ''C. miseri'' * ''

Lepidodendron ''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 Isoetes, quillworts and Lycopodiopsida, lyco ...

'' : ''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'' : ''N. antecedens'' * '' Rhabdocarpos'' : ''R. costatulus'' * '' Rhynchogonium'' : ''R. choctavense'' * ''

Sigillaria ''Sigillaria'' is a genus of extinct, spore-bearing, arborescent (tree-like) plants. It was a lycopodiophyte, and is related to the lycopsids, or club-mosses, but even more closely to quillworts, as was its associate ''Lepidodendron''. Fossil ...

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

References

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