Chinle Formation

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). == 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 from Chinle Formation exposures at Petrified Forest National Park 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. one of the oldest widespread units in the badlands of the Painted Desert area. In western New Mexico (particularly the Zuni Mountains area), 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. In Petrified Forest National Park (PEFO) 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 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 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. The stratigraphic nomenclature used in southern Utah is also utilized in Monument Valley, where the coarse-grained lower members of the Chinle form a caprock for many famous buttes which characterize the valley. The Monitor Butte Member overlies the Shinarump and Temple Mountain members in southeast Utah and Monument Valley. This unit comprises drab and generally fine-grained sediments, equivalent to the Blue Mesa Member and Bluewater Creek Formation found further south. They are complex heterolithic units, representing variously braided-river facies, lacustrine, and overbank deposits. == Chronology ==

Tetrapod biostratigraphy The Chinle Formation is entirely Late Triassic in age. Tetrapod biostratigraphy for the Chinle was first developed based on phytosaurs and aetosaurs, which in 1998 were combined into global biozones in Spencer G. Lucas's Land Vertebrate Faunachrons system. Simplified stratigraphy based on Litwin. Note that age inferences devised by Lucas do not necessarily align with other chronological methods used in the Chinle Formation. Other works on Chinle biostratigraphy, such as Martz & Parker (2017), are better integrated with magnetostratigraphy and radiometric dating, and are considered more accurate. Radiometric dating Since 2011, widespread radiometric dating has helped to refine precise age data for part of the Chinle Formation, particularly in areas with a more complete stratigraphic record such as Petrified Forest National Park (PEFO). Volcanism further southwest along the Cordilleran magmatic arc supplies zircon crystals to the Chinle system, allowing for U-Pb dating of layers which host zircon grains. Eroded sediments from the Ancestral Rocky Mountains, Ouachita Mountains and Mogollon Highlands also supply older reworked zircon to the basin. Chinle radiometric dating is complicated by lithological quirks of zircon deposition. Taken at face value, U-Pb dates from coarse-grained layers are often several million years older than expected based on magnetostratigraphy, while mud-dominated layers are generally more accurate despite a lower sample size. This is likely because sandy rivers receive a higher proportion of recycled zircon grains from distant eroded rocks, while muddy plains are supplied with fresh zircon-rich ash from contemporary volcanic eruptions. While zircons from sandstone-rich layers are less useful for inferring direct depositional ages, they can be very useful for inferring sediment sources: each igneous or metamorphic sediment source has its own set of old (usually Precambrian) zircon ages, which can be traced in Triassic sediments. or ~228 Ma (2013), though these may be influenced by recycled grains. At PEFO, U-Pb estimates from the Blue Mesa Member include 223 Ma (2011), At Six Mile Canyon near Fort Wingate, New Mexico, the base of the Blue Mesa Member (or its local equivalent) is defined by a distinct sandstone bed, which has been dated to 221–219 Ma (2009) The underlying Bluewater Creek Formation has also been dated to 221–219 Ma (2014), suggesting that it overlaps in time with the Arizonan Blue Mesa Member and possibly part of the Sonsela Member. The first Chinle U-Pb age data to be published referred to the Black Forest Bed, a sandstone layer near the top of the Petrified Forest Member in PEFO. U-Pb estimates for this layer include ~213 Ma (2003 maximum), ~211 Ma (2009), and ~210 Ma (2011, 2020). A presumably older exposure of the Petrified Forest Member, the Hayden Quarry at Ghost Ranch, is dated to 212 Ma (2011). A similar age was found for the middle part of the member in PEFO. The end of the Petrified Forest Member was probably close to 208 Ma, meaning that overlying strata is presumably latest Norian-Rhaetian in age. ==Places found==

of Canyonlands N.P., with members of the Chinle FormationGeologic Province: ==See also==