Latest Oligocene to Early Miocene Paleogeography of Arizona (23-18 Ma).
Agradational conditions developed and widespread braided stream deposits accumulated in the SE-trending strike valley below the Mogollon Rim and in its trbutaries. Both locally derived and more distant detritus were deposited. These deposits lack abundant basalt cobbles but do contain latite clasts of the 22+ Ma Sullivan Buttes Latite and related deposits. Along the margin of the Colorado Plateau, these deposits rest mainly on Permian Redbeds. To the southwest, they overlie older Paleozoic and Precambrian rocks. In general, the sub-tertiary unconformity overlies older rocks to the SW. Volcanism continued in the core complexes but also migrated into the SW parts of central Arizona (eg. Sullivan Buttes Latite). Probably during this time the lines of cliffs on the Colorado Plateau were established. Both the southern and western margins were topographically above much of the Basin and Range and Transition Zone.