Dockum Group

Dockum Group
Stratigraphic range: late Carnian-Rhaetian
~223–200 Ma

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Dockum Group; Stratigraphic range: late Carnian-Rhaetian; ~223–200 Ma PreꞒ Ꞓ O S D C P T J K Pg N
	Steamboat Butte, showing Dockum Group red beds at its base
Type	Group
Sub-units	See text
Underlies	Exeter Sandstone
Overlies	Anton Chico Formation
Lithology
Primary	Sandstone
Other	Siltstone, Mudstone
Location
Coordinates	35.356°N 102.910°W
Country	United States
Type section
Named for	Town of Dockum, Texas
Named by	William Fletcher Cummins
Year defined	1890

The Dockum is a Late Triassic (approximately late Carnian through Rhaetian, or 223–200 Ma) geologic group found primarily on the Llano Estacado of western Texas and eastern New Mexico with minor exposures in southwestern Kansas, eastern Colorado, and Oklahoma panhandle.[1] The Dockum reaches a maximum thickness of slightly over 650 m but is usually much thinner. The Dockum rests on an unconformity over the Anisian (242–234 Ma) aged Anton Chico Formation. The Dockum and Chinle Formation were deposited roughly at the same time and share many of the same vertebrates and plant fossils. They appear to have very similar paleoenviroments.[2] The two units are approximately separated by the Rio Grande in central New Mexico. The Chinle and Dockum are thought by some to be two separate units, deposited in separate depositional basins.

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 of town of Dockum in Dickens County, Texas, in 1890.[3] Lucas included the Dockum Group within the Chinle when he raised the Chinle to group status.[4] Most researchers still maintain the Chinle is a formation and separate from the Dockum Group. 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 formations.[5] The basal unit is the Santa Rosa Sandstone, a braided stream channel-related facies. The Santa Rosa is overlain by the Tecovas Formation, which is dominated by overbank (distal floodplain) deposits with lenses of channel-deposits. Minor lacustrine deposits are also occurring. The Trujillo Sandstone, channel-deposits, and Cooper 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.

Dockum and Chinle correlation

Correlations and ages based on Spencer G. Lucas' Land Vertebrate Faunachrons.[6] The faunachrons are based on first and last appearances of phytosaurs.[7] Simplified Chinle stratigraphy based on Litwin.[8]

Sub-stage	Faunachron	Chinle	Dockum
late Norian-Rhaetian	Apachean	Rock Point/Church Rock	Redonda
early to middle Norian	Revueltian	Owl Rock, Upper Petrified Forest	Cooper Canyon
upper late Carnian	Adamanian	Upper Petrified Forest, Moss Back, Monitor Butte	Trujillo, Tecovas
lower late Carnian	Otischalkian	Shinarump, Temple Mountain	Santa Rosa

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References

Dunaway, R.E., and Fisher, M.J., 1979. Palynology of the Dockum Group (Upper Triassic), Texas, U.S.A. Review of Paleobotany and Palynology, v. 28, p. 61–92.
Cornet, B., 1993. Applications and limitations of palynology in age, climatic, and paleoenviromental analyses of Triassic sequences in North America. New Mexico Museum of Natural History Bulletin 3:75–93.
Cummins, W.F. 1892. Report on the geography, topography, and geology of the Llano Estacado or Staked Plains with notes on the geology of the country west of the Plains. In: Dumble, E.T. (ed), Third annual report of the Geological Survey of Texas 1891. Austin: Henry Hutchings, State Printer.
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.
Lehman, T. M. 1994. The saga of the Dockum Group and the case of the Texas/New Mexico boundary fault. New Mexico Bureau of Mines & Mineral Resources Bulletin, v. 150, p. 37-51.
Lucas, S.G., 1998. Global Triassic tetrapod biostratigraphy and biochronology. Palaeogeography, Palaeoclimatology, and Palaeoecology, v. 143, p. 347–384.
Lehman, T.M., and Chatterjee, S., 2005. Depositional setting and vertebrate biostratigraphy of the Triassic Dockum Group of Texas. Journal of Earth System Science, v. 114, p. 325-351.
Litwin, R.J., Traverse, A., and Ash, S.R., 1991. Preliminary palynological zonation of the Chinle Formation, southwestern U.S.A., and its correlation to the Newark Supergroup (eastern U.S.A.). Review of Paleobotany and Palynology, v. 77, p. 269–287.

External links

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[1] Dunaway, R.E., and Fisher, M.J., 1979. Palynology of the Dockum Group (Upper Triassic), Texas, U.S.A. Review of Paleobotany and Palynology, v. 28, p. 61–92.

[2] Cornet, B., 1993. Applications and limitations of palynology in age, climatic, and paleoenviromental analyses of Triassic sequences in North America. New Mexico Museum of Natural History Bulletin 3:75–93.

[3] Cummins, W.F. 1892. Report on the geography, topography, and geology of the Llano Estacado or Staked Plains with notes on the geology of the country west of the Plains. In: Dumble, E.T. (ed), Third annual report of the Geological Survey of Texas 1891. Austin: Henry Hutchings, State Printer.

[4] 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.

[5] Lehman, T. M. 1994. The saga of the Dockum Group and the case of the Texas/New Mexico boundary fault. New Mexico Bureau of Mines & Mineral Resources Bulletin, v. 150, p. 37-51.

[6] Lucas, S.G., 1998. Global Triassic tetrapod biostratigraphy and biochronology. Palaeogeography, Palaeoclimatology, and Palaeoecology, v. 143, p. 347–384.

[7] Lehman, T.M., and Chatterjee, S., 2005. Depositional setting and vertebrate biostratigraphy of the Triassic Dockum Group of Texas. Journal of Earth System Science, v. 114, p. 325-351.

[8] Litwin, R.J., Traverse, A., and Ash, S.R., 1991. Preliminary palynological zonation of the Chinle Formation, southwestern U.S.A., and its correlation to the Newark Supergroup (eastern U.S.A.). Review of Paleobotany and Palynology, v. 77, p. 269–287.