{{Short description|Geologic group of western Texas and eastern New Mexico}} {{Infobox rockunit | name = Dockum Group | image = Steamboat Butte New Mexico.jpg | caption = Steamboat Butte, showing Dockum Group red beds at its base | type = Group | age = late Carnian-Rhaetian<br>~{{fossilrange|223|200|earliest=223|latest=200}} | period = late Triassic | prilithology = Sandstone | otherlithology = Siltstone, Mudstone | namedfor = Town of Dockum, Texas | namedby = William Fletcher Cummins | year_ts = 1890 | region = | country = United States | coordinates = {{coord|35.356|N|102.910|W|display=it}} | unitof = | subunits = See text | underlies = Exeter Sandstone | overlies = Anton Chico Formation | thickness = | extent = | area = | map = | map_caption = }}

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.<ref name="dunay-fisher-1979">{{cite journal |last1=Dunay |first1=R.E. |last2=Fisher |first2=M.J. |title=Palynology of the Dockum group (Upper Triassic), Texas, U.S.A. |journal=Review of Palaeobotany and Palynology |date=June 1979 |volume=28 |issue=1 |pages=61–92 |doi=10.1016/0034-6667(79)90025-3}}</ref> The Dockum reaches a maximum thickness of slightly over 650&nbsp;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 paleoenvironments.<ref>{{cite journal |last1=Cornet |first1=B. |year=1993 |title=Applications and limitations of palynology in age, climatic, and paleoenviromental analyses of Triassic sequences in North America |journal=New Mexico Museum of Natural History Bulletin |volume=3 |pages=75–93 |url=https://books.google.com/books?id=qbP9CQAAQBAJ&dq=Applications+and+limitations+of+palynology+in+age%2C+climatic%2C+and+paleoenviromental+analyses+of+Triassic+sequences+in+North+America&pg=PA75 |accessdate=1 September 2020}}</ref> 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.<ref name="lehman-1994">{{cite journal |last1=Lehman |first1=T.M. |year=1994 |title=The saga of the Dockum Group and the case of the Texas/New Mexico boundary fault |journal=New Mexico Bureau of Mines & Mineral Resources Bulletin |volume=150 |pages=37–51 |url=https://geoinfo.nmt.edu/publications/monographs/bulletins/downloads/150/B150.pdf |accessdate=1 September 2020}}</ref>

==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.<ref> {{cite report | last = Cummins | first = W. F. | date = 1892 | title = 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. | editor1-first = E. T. | editor1-last = Dumble | url = https://archive.org/details/firstfourthannua03geol/page/129/mode/1up | work = Third annual report of the Geological Survey of Texas 1891 }}</ref> Lucas included the Dockum Group within the Chinle when he raised the Chinle to group status,<ref>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.&nbsp;27–50.</ref> but this has not been widely accepted.<ref name="lehman-1994"/><ref name="cather-etal=2013">{{cite journal |last1=Cather |first1=S.M. |last2=Zeiger |first2=Kate E. |last3=Mack |first3=Greg H. |last4=Kelley |first4=Shari A. |title=Toward standardization of Phanerozoic stratigraphic nomenclature in New Mexico |journal=New Mexico Geological Society Spring Meeting |year=2013 |page=12 |citeseerx=10.1.1.667.3513 }}</ref> 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.<ref name="lehman-1994"/>

==Stratigraphy== Lehman (1994) advocated a simplified stratigraphy of up to five geologic formations.<ref name="lehman-1994"/> According to his system, the basal unit is the Santa Rosa Sandstone, a braided stream channel-related facies.<ref name="FinchEtal1988">{{cite journal |last1=Finch |first1=W.I. |last2=Lupe |first2=Robert |last3=Ash |first3=S.R. |date=1988 |title=Principal reference section for the Santa Rosa formation of Middle and Late Triassic age, Guadalupe County, New Mexico |journal=U.S. Geological Survey Bulletin |volume=1804 |doi=10.3133/b1804|doi-access=free }}</ref> 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,<ref name="LucasHuntHuber1990">{{cite journal |last1=Lucas |first1=S.G. |last2=Hunt |first2=A.P. |last3=Huber |first3=P. |year=1990 |title=Triassic stratigraphy in the Sangre de Cristo Mountains, New Mexico |url=https://nmgs.nmt.edu/publications/guidebooks/downloads/41/41_p0305_p0318.pdf |journal=New Mexico Geological Society Field Conference Guidebook |volume=41 |pages=305–318 |access-date=12 November 2021}}</ref> 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.<ref name="baldwin-muehlberger-1959">{{cite journal |last1=Baldwin |first1=Brewster |last2=Muehlberger |first2=W.R. |year=1959 |title=Geologic studies of Union County, New Mexico |journal=New Mexico Bureau of Mines and Mineral Resources Bulletin |volume=63 |number=2 |url=https://geoinfo.nmt.edu/publications/monographs/bulletins/downloads/63/Bulletin63.pdf |accessdate=1 September 2020 |archive-date=30 August 2017 |archive-url=https://web.archive.org/web/20170830172016/http://geoinfo.nmt.edu/publications/monographs/bulletins/downloads/63/Bulletin63.pdf |url-status=dead }}</ref><ref name="ziegler-etal-1019">{{cite journal |last1=Ziegler |first1=Kate E. |last2=Ramos |first2=Frank C. |last3=Zimmerer |first3=Matthew J. |title=Geology of Northeastern New Mexico, union and Colfax Counties, New Mexico: A Geologic Summary |journal=New Mexico Geological Society Field Conference Series |date=2019 |volume=70 |issue=4 |pages=47–54 |url=https://nmgs.nmt.edu/publications/guidebooks/downloads/70/70_p0047_p0054.pdf |accessdate=1 September 2020}}</ref> {| class="wikitable" |+'''Regional stratigraphic subunits of the Dockum Group''' ! colspan="2" |East-central New Mexico ! colspan="2" |Texas Panhandle !southern Garza County, TX !Howard County, TX !Dry Cimarron Valley (northeast NM, southeast CO, Oklahoma Panhandle) |- | colspan="2" rowspan="3" |Redonda Formation | colspan="4" rowspan="3" | |Sheep Pen Sandstone |- |Sloan Canyon Formation |- |Travesser Formation |- | colspan="4" |Bull Canyon Formation |upper Cooper Canyon Formation | rowspan="4" | | rowspan="2" | |- | colspan="4" |Trujillo Formation |middle Cooper Canyon Formation |- | colspan="2" |Garita Creek Formation | rowspan="4" |Tecovas Formation | rowspan="2" |Magenta beds | rowspan="3" |lower Cooper Canyon Formation | rowspan="5" |Baldy Hill Formation / Cobert Canyon Sandstone?

(correlation uncertain) |- | rowspan="4" |Santa Rosa Formation |Tres Lagunas Member |- | rowspan="2" |Los Esteros Member | rowspan="2" |Variegated beds | rowspan="2" |Colorado City Formation? (correlation uncertain) |- |Boren Ranch Sandstone |- |Tecolotito Member | colspan="4" |Camp Springs Conglomerate |}

== Dockum and Chinle correlation == Correlations and ages based on Spencer G. Lucas' Land Vertebrate Faunachrons.<ref name="lucas-1998">{{cite journal |last1=Lucas |first1=Spencer G |title=Global Triassic tetrapod biostratigraphy and biochronology |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=November 1998 |volume=143 |issue=4 |pages=347–384 |doi=10.1016/S0031-0182(98)00117-5|bibcode=1998PPP...143..347L |url=http://doc.rero.ch/record/16228/files/PAL_E3457.pdf }}</ref> The faunachrons are based on first and last appearances of phytosaurs.<ref name="lehman-chatterjee-2005">{{cite journal |last1=Lehman |first1=Thomas |last2=Chatterjee |first2=Sankar |title=Depositional setting and vertebrate biostratigraphy of the Triassic Dockum Group of Texas |journal=Journal of Earth System Science |date=June 2005 |volume=114 |issue=3 |pages=325–351 |doi=10.1007/BF02702953|bibcode=2005JESS..114..325L |doi-access=free }}</ref> Simplified Chinle stratigraphy based on Litwin.<ref name="litwin-etal-1991">{{cite journal |last1=Litwin |first1=Ronald J. |last2=Traverse |first2=Alfred |last3=Ash |first3=Sidney R. |title=Preliminary palynological zonation of the Chinle formation, southwestern U.S.A., and its correlation to the Newark supergroup (eastern U.S.A.) |journal=Review of Palaeobotany and Palynology |date=July 1991 |volume=68 |issue=3–4 |pages=269–287 |doi=10.1016/0034-6667(91)90028-2}}</ref>

{| class="wikitable" |- ! 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== {{reflist}}

==External links== *[https://ngmdb.usgs.gov/Geolex/NewUnits/unit_7944.html GEOLEX database entry for Dockum], USGS *[https://ngmdb.usgs.gov/Geolex/NewRefsmry/sumry_7944.html GEOLEX database bibliographic references for Dockum]

Category:Geologic groups of Texas Category:Geologic groups of New Mexico Category:Triassic System of North America Category:Upper Triassic Series Category:Sandstone groups of the United States Category:Siltstone groups Category:Mudstone groups