Dockum Group

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.

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 / 35.356; -102.910
Country	United States
Type section
Named for	Town of Dockum, Texas
Named by	William Fletcher Cummins
Year defined	1890

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 paleoenvironments.^[2] The two units are approximately separated by the Rio Grande in central New Mexico. This has led to controversy over the stratigraphic nomenclature for the Chinle and Dockum.^[3]

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.^[4] Lucas included the Dockum Group within the Chinle when he raised the Chinle to group status,^[5] but this has not been widely accepted.^[3][6] 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.^[3]

Stratigraphy

Lehman (1994) advocated a simplified stratigraphy of up to five geologic formations.^[3] According to his system, the basal unit is the Santa Rosa Sandstone, a braided stream channel-related facies.^[7] The Santa Rosa is overlain by the Tecovas Formation (and its New Mexican equivalent, the Garita Creek Formation), which is dominated by overbank (distal floodplain) deposits with lenses of channel-deposits. Minor lacustrine deposits also occur. The Trujillo Sandstone, channel-deposits,^[8] and Cooper Canyon Formation (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 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, 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.^[9][10]

Regional stratigraphic subunits of the Dockum Group

East-central New Mexico		Texas Panhandle		southern Garza County, TX	Howard County, TX	Dry Cimarron Valley (northeast NM, southeast CO, Oklahoma Panhandle)
Redonda Formation						Sheep Pen Sandstone
						Sloan Canyon Formation
						Travesser Formation
Bull Canyon Formation				upper Cooper Canyon Formation
Trujillo Formation				middle Cooper Canyon Formation
Garita Creek Formation		Tecovas Formation	Magenta beds	lower Cooper Canyon Formation		Baldy Hill Formation / Cobert Canyon Sandstone? (correlation uncertain)
Santa Rosa Formation	Tres Lagunas Member
	Los Esteros Member		Variegated beds		Colorado City Formation? (correlation uncertain)
				Boren Ranch Sandstone
	Tecolotito Member	Camp Springs Conglomerate

Dockum and Chinle correlation

Correlations and ages based on Spencer G. Lucas' Land Vertebrate Faunachrons.^[11] The faunachrons are based on first and last appearances of phytosaurs.^[12] Simplified Chinle stratigraphy based on Litwin.^[13]

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

See also

• List of dinosaur-bearing rock formations
• Chinle Formation
• Triassic land vertebrate faunachrons

References

1. Dunay, R.E.; Fisher, M.J. (June 1979). "Palynology of the Dockum group (Upper Triassic), Texas, U.S.A.". Review of Palaeobotany and Palynology. 28 (1): 61–92. doi:10.1016/0034-6667(79)90025-3.
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. Retrieved 1 September 2020.
3. Lehman, T.M. (1994). "The saga of the Dockum Group and the case of the Texas/New Mexico boundary fault" (PDF). New Mexico Bureau of Mines & Mineral Resources Bulletin. 150: 37–51. Retrieved 1 September 2020.
4. Cummins, W. F. (1892). Dumble, E. T. (ed.). 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. Third annual report of the Geological Survey of Texas 1891 (Report).
5. 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.
6. Cather, S.M.; Zeiger, Kate E.; Mack, Greg H.; Kelley, Shari A. (2013). "Toward standardization of Phanerozoic stratigraphic nomenclature in New Mexico". New Mexico Geological Society Spring Meeting: 12. CiteSeerX 10.1.1.667.3513.
7. Finch, W.I.; Lupe, Robert; Ash, S.R. (1988). "Principal reference section for the Santa Rosa formation of Middle and Late Triassic age, Guadalupe County, New Mexico". U.S. Geological Survey Bulletin. 1804. doi:10.3133/b1804.
8. Lucas, S.G.; Hunt, A.P.; Huber, P. (1990). "Triassic stratigraphy in the Sangre de Cristo Mountains, New Mexico" (PDF). New Mexico Geological Society Field Conference Guidebook. 41: 305–318. Retrieved 12 November 2021.
9. Baldwin, Brewster; Muehlberger, W.R. (1959). "Geologic studies of Union County, New Mexico" (PDF). New Mexico Bureau of Mines and Mineral Resources Bulletin. 63 (2). Archived from the original (PDF) on 30 August 2017. Retrieved 1 September 2020.
10. Ziegler, Kate E.; Ramos, Frank C.; Zimmerer, Matthew J. (2019). "Geology of Northeastern New Mexico, union and Colfax Counties, New Mexico: A Geologic Summary" (PDF). New Mexico Geological Society Field Conference Series. 70 (4): 47–54. Retrieved 1 September 2020.
11. Lucas, Spencer G (November 1998). "Global Triassic tetrapod biostratigraphy and biochronology" (PDF). Palaeogeography, Palaeoclimatology, Palaeoecology. 143 (4): 347–384. Bibcode:1998PPP...143..347L. doi:10.1016/S0031-0182(98)00117-5.
12. Lehman, Thomas; Chatterjee, Sankar (June 2005). "Depositional setting and vertebrate biostratigraphy of the Triassic Dockum Group of Texas". Journal of Earth System Science. 114 (3): 325–351. Bibcode:2005JESS..114..325L. doi:10.1007/BF02702953.
13. Litwin, Ronald J.; Traverse, Alfred; Ash, Sidney R. (July 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 Palaeobotany and Palynology. 68 (3–4): 269–287. doi:10.1016/0034-6667(91)90028-2.

External links

• GEOLEX database entry for Dockum, USGS
• GEOLEX database bibliographic references for Dockum