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 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.

However, the Trujillo-Cooper Canyon sequence's sediments are derived from the Ouachita orogenic belts of the Marathon Uplift.

This region is structurally separated from the Dockum Group exposures to the south by the Sierra Grande arch.