Chinle Formation
The Chinle Formation is an Upper Triassic continental geological formation of fluvial, lacustrine, and palustrine to eolian deposits spread across the U.S. states of Nevada, Utah, northern Arizona, western New Mexico, and western Colorado. In New Mexico, it is often raised to the status of a geological group, the Chinle Group. Some authors have controversially considered the Chinle to be synonymous to the Dockum Group of eastern Colorado and New Mexico, western Texas, the Oklahoma panhandle, and southwestern Kansas. The Chinle Formation is part of the Colorado Plateau, Basin and Range, and the southern section of the Interior Plains. A probable separate depositional basin within the Chinle is found in northwestern Colorado and northeastern Utah. The southern portion of the Chinle reaches a maximum thickness of a little over. Typically, the Chinle rests unconformably on the Moenkopi Formation.
The Chinle Formation was probably mostly deposited in the Norian stage, according to a plethora of chronological techniques. It is a thick and fossiliferous formation with numerous named members throughout its area of deposition.
History of investigation
While colorful Triassic sediments of the Colorado Plateau have been investigated since the 19th century, the Chinle Formation was only formally named and described by Herbert E. Gregory in 1917. It was named for Chinle Valley in Apache County, Arizona, land which is largely within the Navajo Nation. Gregory did not designate a type locality. He split the Chinle into four subunits, labelled A to D. This did not include the underlying Shinarump Conglomerate, which he considered a separate formation.United States Geological Survey geologists and paleontologists continued to map out the Chinle Formation through the 20th century, revising the unnamed subunits of Gregory. A basic stratigraphy of the formation was developed for north-central New Mexico by Wood and Northrop, and stratigraphy in the Four Corners Region was established by the late 1950s. In 1956, Economic geologist Raymond C. Robeck identified and named the Temple Mountain member as the basal-most unit in the area of the San Rafael Swell of Utah. In 1957, John H. Stewart revised the Shinarump Conglomerate and renamed it the Shinarump member of the Chinle formation.
Study of the formation expanded northwards into northern Utah and Colorado, facilitated through papers by Forrest G. Poole and Stewart and Steve W. Sikich, who named informal local members equivalent to those of Arizona and New Mexico. The complete areal extent of the unit was mapped by R.F. Wilson and Stewart in 1967. Stewart and his colleagues created an expansive overview and revision of the formation in 1972, summarizing previous knowledge on Chinle stratigraphy.
V.C. Kelley assigned more members and revised the unit in 1972. Spencer G. Lucas and S.N. Hayden did the same thing in 1989. The Rock Point Member was assigned by R.F. Dubiel in 1989.
The Chinle was raised to group rank by Lucas in 1993, thus also raising many of the members to formation status. He also included the formations of the Dockum Group of eastern New Mexico and west Texas within the "Chinle Group". This modified nomenclature is controversial; many still retain the Chinle as a formation and separate out the Dockum Group. The Dockum was named in 1890, before the Chinle. Lucas also advocated abandoning the name Dolores Formation as a parochial synonym for the Chinle Group.
Overviews of the Chinle were created by Dubiel and others and Hintze and Axen.
Paleobiota
The Chinle Formation is fossiliferous, with a diverse array of extinct reptile, fish, and plant fossils, including early dinosaurs and the famous petrified wood of Petrified Forest National Park in Arizona.Stratigraphy
The formation members and their thicknesses are highly variable across the Chinle.| Arizona and western New Mexico | North-central New Mexico | Monument Valley and southern Utah | Colorado and northeast Utah |
| Rock Point Member | "siltstone member" | Church Rock Member | "upper member" "red siltstone member" "sandstone and conglomerate member" "ocher siltstone member" |
| Owl Rock Member | "siltstone member" ? | Owl Rock Member Kane Springs beds | "upper member" "red siltstone member" "sandstone and conglomerate member" "ocher siltstone member" |
| Petrified Forest Member sensu stricto / "Upper Petrified Forest" / Painted Desert Member | Petrified Forest Member | Petrified Forest Member Kane Springs beds | "upper member" "red siltstone member" "sandstone and conglomerate member" "ocher siltstone member" |
| Sonsela Member | Poleo Formation | Moss Back Member | "upper member" "red siltstone member" "sandstone and conglomerate member" "ocher siltstone member" |
| Blue Mesa Member / "Lower Petrified Forest" Bluewater Creek Formation | Salitral Formation | Monitor Butte Member Cameron Member | "mottled member" Gartra Member? |
| Mesa Redondo Member Shinarump Conglomerate Zuni Mountains Formation | Agua Zarca Sandstone / Shinarump Conglomerate "mottled strata" | Shinarump Conglomerate Temple Mountain Member | "mottled member" Gartra Member? |
Arizona and western New Mexico
Some of the most extensive deposits of the Chinle Formation are found in the southern Colorado Plateau, including Arizona and the western portion of New Mexico. In this region, the oldest and stratigraphically lowest portion of the Chinle is the Shinarump Conglomerate. The Shinarump includes braided-river system channel-deposit facies. The Shinarump interfingers with a finer-grained subunit, the Mesa Redondo Member, one of the oldest widespread units in the badlands of the Painted Desert area. In western New Mexico, the Mesa Redondo Member may be replaced by another sandy unit known as the Zuni Mountains Formation. Sediments from this time interval are followed by a geological unit called the Bluewater Creek Formation.Most Chinle outcrops in the Painted Desert have traditionally been placed within the following Petrified Forest Member, a segment of Triassic sediments which are so diverse and extensive that it is sometimes raised to its own formation, subdivided further, or redefined more narrowly. In its widest definition, the Petrified Forest Member is split into three sections: the muddy Lower Petrified Forest and Upper Petrified Forest, and the sandy Sonsela Sandstone bed, which separates them. The Lower "Petrified Forest Member" is generally known as the Blue Mesa Member. In Petrified Forest National Park and its vicinities, the Sonsela Sandstone is thick enough that it can be resolved into several distinct sandstone-rich layers. It is renamed as the Sonsela Member in this situation. The Sonsela Sandstone is a collection of braided-stream channel facies. The Upper "Petrified Forest Member" is sometimes called the Painted Desert Member, or simply referred to as the Petrified Forest Member in a more restricted definition of the term. The Petrified Forest is predominately overbank deposits with thin lenses of channel-deposit facies and lacustrine deposits.
The Petrified Forest Member grades into the Owl Rock Member, a marginal lacustrine to lacustrine facies possibly representing a large lake system. The Owl Rock Member is followed by the youngest and sandiest subunit of the Chinle, the Rock Point Member. The Rock Point is distinct enough that it was previously considered a unit of the Wingate Sandstone, a latest Triassic - early Jurassic aeolian formation which overlies the Chinle in many areas.
Central New Mexico
Unambiguous exposures of the Chinle Formation extend into central New Mexico, beyond the eastern edge of the Colorado Plateau. Most of these are found in the Chama Basin of north-central New Mexico, particularly several famed paleontological sites at Ghost Ranch near Abiquiu. Minor exposures also occur in the Lucero Uplift west of Albuquerque, as well as other areas along the Rio Grande Rift.File:Hayden Quarry.png|thumb|315x315px|Stratigraphic column and outcrop photos of the Hayden Quarry fossil locality at Ghost Ranch, NM, alongside a map of Chinle exposures in NM As in the Colorado Plateau, the lowest major unit in north-central New Mexico is a sandstone-rich member. This layer, the Agua Zarca Sandstone, is often synonymized with the Shinarump Conglomerate, though it may be derived from a different erosional source. It is often preceded by a very thin layer of silty mottled strata. This mottled strata is sometimes termed the Zuni Mountains Formation, though the application of this term beyond the Zuni Mountains is questionable. In the Chama Basin at least, the mottled strata is derived from the eroded and pedogenically modified surface of the Moenkopi Formation.The coarse lower unit grades into the fine-grained Salitral Formation, which is equivalent to the Blue Mesa Member and Bluewater Creek Formation. In south-central New Mexico, it may instead grade into the San Pedro Arroyo Formation, a similar heterolithic unit. Coarse sandstone returns along a sharp contact with the following Poleo Formation, an equivalent of the Sonsela Member. The Poleo Formation grades into the thick colorful sediments of the Petrified Forest Member. Authors which raise this member to a formation subdivide it into the lower Mesa Montosa Member and the upper Painted Desert Member. The Petrified Forest Member is fossiliferous in the Chama Basin, with major sites including the Hayden, Canjilon, and Snyder quarries of Ghost Ranch.
The stratigraphically highest unit in north-central New Mexico is the informally-named "siltstone member". This unit is best exposed at Ghost Ranch, where it has produced the famous Whitaker Quarry, also known as the Coelophysis quarry due to a high concentration of fossils belonging to the theropod dinosaur Coelophysis bauri. The "siltstone member" may be equivalent to the Rock Point Member, and some authors refer to it as such.