The Bluff Formation is a

geological 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 expo ...

found in the

Four Corners The Four Corners is a region of the Southwestern United States consisting of the southwestern corner of Colorado, southeastern corner of Utah, northeastern corner of Arizona, and northwestern corner of New Mexico. The Four Corners area ...

area. It was deposited in the

late Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the ...

Period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

Description

The Bluff Formation consists of massive wind-deposited ( eolian)

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

beds at its type location at

Bluff, Utah Bluff is a town in San Juan County, Utah, United States. The population was 320 at the 2000 census. Bluff incorporated in 2018. History Under the direction of John Taylor, Silas S. Smith and Danish settler Jens Nielson led about 230 Mormons o ...

. These cap the cliffs north of town and are thick. Further to the southeast, the sandstone beds lack high-angle cross beds, being dominated instead by horizontal bed forms, and are overlain by mixed 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, especia ...

beds of the Recapture

Member Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...

. The Recapture Member has been variously assigned either to the overlying

Morrison Formation The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone, ...

or to the Bluff Formation. The Recapture Member is separated from the Salt Wash Member of the Morrison Formation by the regional J-5

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

History of investigation

The formation was first named as the Bluff Sandstone by A.A. Baker, C.H. Dane, and J.B. Reeside, Jr., in 1938. They assigned it to the lower Morrison Formation. J.W. Harshbarger,

Charles Repenning Charles Albert Repenning (August 4, 1922 in Oak Park, Illinois – January 5, 2005 in Lakewood, Colorado) was an American paleontologist and zoologist noted for his work on shrews, fossil rodents, modern pinnipeds and their extinct relatives, ...

, and J.H. Irwin promoted it to formation rank in 1957, but this was not universally accepted.

Spencer G. Lucas Spencer George Lucas is an American paleontologist and stratigrapher, and curator of paleontology at the New Mexico Museum of Natural History and Science. His main areas of study are late Paleozoic, Mesozoic and early Cenozoic vertebrate fo ...

advocated both for formation rank and for assigning the Recapture Member to the Bluff Sandstone. Steven M. Cather agreed with promotion of the Bluff Sandstone to formation rank but advocated leaving the Recapture Member with the Morrison Formation.

References

{{Reflist Jurassic Arizona Jurassic Colorado Jurassic formations of New Mexico Jurassic geology of Utah