The Dockum is a

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...

(approximately late

Carnian The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followed by t ...

through

Rhaetian The Rhaetian is the latest age of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest age ...

, or 223–200 Ma) geologic group found primarily on the

Llano Estacado The Llano Estacado (), sometimes translated into English as the Staked Plains, is a region in the Southwestern United States that encompasses parts of eastern New Mexico and northwestern Texas. One of the largest mesas or tablelands on the North A ...

of western Texas and

eastern New Mexico Eastern New Mexico is a physiographic subregion within the U.S. state of New Mexico. The region is sometimes called the "High Plains," or "Eastern Plains (of New Mexico)," and was historically referred to as part of the "Great American Desert". The ...

with minor exposures in southwestern

Kansas Kansas () is a state in the Midwestern United States. Its capital is Topeka, and its largest city is Wichita. Kansas is a landlocked state bordered by Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the ...

, eastern

Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...

, and

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 ...

panhandle. The Dockum reaches a maximum thickness of slightly over 650 m but is usually much thinner. The Dockum rests on an

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 ...

over the

Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic Ep ...

(242–234 Ma) aged

Anton Chico Formation The Anton Chico Formation is a geologic Formation (geology), formation exposed in New Mexico that was deposited in the Anisian Age (geology), Age of the middle Triassic Period (geology), Period. Description The formation consists mostly of very ...

. The Dockum and

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 Ne ...

were deposited roughly at the same time and share many of the same

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

s and

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...

. They appear to have very similar

paleoenviroment Paleoecology (also spelled palaeoecology) is the study of interactions between organisms and/or interactions between organisms and their environments across geologic timescales. As a discipline, paleoecology interacts with, depends on and informs ...

s. The two units are approximately separated by the

Rio Grande The Rio Grande ( and ), known in Mexico as the Río Bravo del Norte or simply the Río Bravo, is one of the principal rivers (along with the Colorado River) in the southwestern United States and in northern Mexico. The length of the Rio G ...

in central New Mexico. This has led to controversy over the stratigraphic nomenclature for the Chinle and Dockum.

History of investigation

There is no designated type locality for this formation. The Dockum was named by William Fletcher Cummins for a good exposure in

vicinity Surroundings are the area around a given physical or geographical point or place. The exact definition depends on the field. Surroundings can also be used in geography (when it is more precisely known as vicinity, or vicinage) and mathematics, ...

of town of Dockum in

Dickens County, Texas Dickens County is a county located in the U.S. state of Texas. As of the 2020 census, its population was 1,770. Its county seat is Dickens. The county was created in 1876 and later organized in 1891. Both the county and its seat are named for J ...

, in 1890. Lucas included the Dockum Group within the Chinle when he raised the Chinle to group status,Lucas, S.G., 1993. The Chinle Group: revised stratigraphy and biochronology of Upper Triassic Nonmarine strata in the western United States. Museum of Northern Arizona Bulletin, v. 59, p. 27–50. but this has not been widely accepted. Dockum was named before the Chinle, and if Lucas is correct, his "Chinle Group" should be named the Dockum Group due to stratigraphic nomenclature rules.

Stratigraphy

Lehman (1994) advocated a simplified stratigraphy of up to five

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 ...

s. The basal unit is the

Santa Rosa Sandstone The Santa Rosa Formation is a geologic formation exposed in New Mexico that was deposited in the Carnian Age of the late Triassic Period. Description The formation consists mostly of white to brown sandstone and mudstone with some interbedded ...

, a braided stream channel-related

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

. The Santa Rosa is overlain by the

Tecovas Formation The Tecovas Formation is a geological formation in Texas.Weishampel, David B; et al. (2004). "Dinosaur distribution (Late Triassic, North America)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berk ...

(and its New Mexican equivalent, the Garita Creek Formation), which is dominated by overbank (distal

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

) deposits with lenses of channel-deposits. Minor lacustrine deposits also occur. The Trujillo Sandstone, channel-deposits, and

Cooper Canyon Formation The Cooper Canyon Formation is a geological formation of Norian age in Texas and New Mexico.Weishampel, David B; et al. (2004).Dinosaur distribution (Late Triassic, North America)" In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (e ...

(also known as the Bull Canyon Formation), overbank deposits with minor channel and lacustrine deposits, are separated from the Santa Rosa-Tecovas by an unconformity. In eastern New Mexico, the Redonda Formation overlies the Cooper Canyon Formation. The Redonda has gradational eastward transition into the upper Cooper Canyon Formation. The Santa Rosa-Tecovas sequence has sediments made up of clasts derived from the north, northeast, and east of the Dockum, very similar to clasts found in the lower Chinle. However, the Trujillo-Cooper Canyon sequence's sediments are derived from the Ouachita orogenic belts of the

Marathon Uplift The Marathon Uplift is a Paleogene-age domal uplift, approximately in diameter, in southwest Texas. The Marathon Basin was created by erosion of Cretaceous and younger strata from the crest of the uplift.McBride, E.F. and Hayward, O.T., 1988''Ge ...

. The Dockum Group in extreme northeastern New Mexico is divided four formations. These are, in ascending stratigraphic order, the Baldy Hill Formation, a mudstone with coarse-grained sandstone lenses; the Travesser Formation, a reddish-brown siltstone and sandstone with some conglomerate lenses; the

Sloan Canyon Formation The Sloan Canyon Formation is a late Triassic geologic formation exposed in northeastern New Mexico. Fossil theropod tracks have been reported from the formation. Description The Sloan Canyon Formation consists of variegated reddish-brown, purpl ...

, a red to pale green mudstone with sandstone lenses; and the Sheep Pen Sandstone, a light brown, thinly bedded sandstone. This region is structurally separated from the Dockum Group exposures to the south by the Sierra Grande arch.

Dockum and Chinle correlation

Correlations and ages based on

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 fossils ...

Land Vertebrate Faunachron Land vertebrate faunachrons (LVFs) are biochronological units used to correlate and date terrestrial sediments and fossils based on their tetrapod faunas. First formulated on a global scale by Spencer G. Lucas in 1998, LVFs are primarily used withi ...

s. The faunachrons are based on first and last appearances of

phytosaur Phytosaurs (Φυτόσαυροι in greek) are an extinct group of large, mostly semiaquatic Late Triassic archosauriform reptiles. Phytosaurs belong to the order Phytosauria. Phytosauria and Phytosauridae are often considered to be equivalent g ...

s. Simplified Chinle

stratigraphy Stratigraphy is a branch of geology concerned with the study of rock (geology), rock layers (Stratum, strata) and layering (stratification). It is primarily used in the study of sedimentary rock, sedimentary and layered volcanic rocks. Stratigrap ...

based on Litwin.

References

{{reflist

External links

GEOLEX database entry for Dockum
USGS

Geologic groups of Texas Geologic groups of New Mexico Triassic System of North America Upper Triassic Series

History of investigation

Stratigraphy

Dockum and Chinle correlation

See also

References

External links