Oligocene Paleogeography of Arizona (35-25 Ma).

Extension and core-complex development greatly modified topography across the region. Drainage was now towards the SE into basins formed during extension. The regional strike of the Colorado Plateau is NW-SE and strike valleys developed in this trend. A major strike valley developed south of the resistant cuesta formed by the Permian Kaibab Formation in less resistant Permian redbeds and sandstone. Sediment was funneled into this system both from the Colorado Plateau (Late Paleozoic sandstone and limestone) and Transition Zone (Proterozoic igneous and metamorphic rocks and scattered Paleozoic sedimentary rocks). This marks the initial development of the Mogollon Rim, probably at a location within 10-15 km of the present Mogollon Rim. The strike valley drained towards SE Arizona. It is important to note that in order for this 1000-meter-deep strike valley to form that considerable material had to be removed to the SE and that the streams were at least partially integrated to accomplish this major change in topography and drainage direction. Volcanism shifted into regions of core complexes and northward into the Four Corners Region (Navajo Volcanics).