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. 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. 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.
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History of investigation
There is no designated type locality for this formation. The Dockum was named by Cummins for a good exposure in vicinity of town of Dockum in Dickens County, Texas, in 1890. Lucas included the Dockum Group within the Chinle when he raised the Chinle to group status. Most researchers still maintain the Chinle is a formation and separate from the Dockum Group. It should be pointed out that the 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. 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/Chinle correlation
Correlations and ages based on Spencer G. Lucas' Land Vertebrate Faunachrons. The faunachrons are based on first and last appearances of phytosaurs. Simplified Chinle stratigraphy based on Litwin.