The Dockum and Chinle Formation were deposited roughly at the same time and share many of the same vertebrates and plantfossils. 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
^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.
^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.
^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. CiteSeerX10.1.1.667.3513.
^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.
^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.
