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.