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 520 meters (1,710 ft). 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 (subunits) throughout its area of deposition.

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 (youngest) to D (oldest). This did not include the underlying Shinarump Conglomerate (named by G. K. Gilbert and Edwin E. Howell in 1875), 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 (1946), 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 (1964) and Steve W. Sikich (1965), 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 (1992) and Hintze and Axen (1995).