An oligocene (?) colorado plateau edge in arizona

Our emphasis is on the development history of the southern and southwestern physiographic edge of the Colorado Plateau Province in Arizona. This edge, which marks the southern termination of Permian cliff-making strata, diagonally bisects Arizona over a distance of about 500 km. This escarpment zone, frequently called the Mogollon Rim in central Arizona, long has been of geologic and popular interest. Its origin has often been linked with a major, late Cenozoic "plateau uplift" and faulting associated with the late Cenozoic Basin and Range disturbance. Our studies indicate that a pre-mid-Miocene rim history should be recognized, in general (Peirce et al., 1978), and an Oligocene (?) erosional event, in particular. Basically, a plateau edge escarpment had evolved by mid-Miocene time, prior to the onset of Basin and Range faulting and the development of the modern Grand Canyon. The location and trend of the physiographic edge of the plateau is influenced by a combination of a sequence of incompetent—competent late Paleozoic strata, and slight northeast tilting caused by at least two recognized tectonic events. These are: (1) pre-Turonian Mesozoic regional tilting, and (2) late Cretaceous—early Tertiary Laramide orogeny. The Eocene—Oligocene rim gravel was deposited by northerly-directed drainage above an early Tertiary erosion surface of regional extent. Later, in Oligocene (?) time, a generally unrecognized tectonic event induced erosion and down-cutting that outlined an ancestral Colorado Plateau physiographic margin with relief of up to 600 m. This escarpment zone formed a barrier to the regional, northerly trending drainage of Rim gravel time. Sediments of late (?) Oligocene—early Miocene age were deposited in the valleys at the base of the plateau edge. Subsequently, about 13 m.y. ago, relatively minor, discontinuous faulting, associated with the Basin and Range disturbance, was superimposed on or near the topographically low zone adjacent to the escarpment. One, the Oak Creek fault, extended into the preexistent plateau adge and influenced erosional modification. The preservation of Cretaceous marine strata along portions of the present plateau physiographic edge at elevations above 2 km is indicative of uplift relative to sea level. However, it is not yet possible to decipher the respective influences that the Laramide, mid-Tertiary, and late Cenozoic tectonic events might have had on elevations. Geographically, tectonic influence boundaries cannot yet be accurately assigned. The Colorado Plateau southern physiographic border in Arizona is primarily an Oligocene (?) erosional feature and is not a major tectonic boundary along which late Cenozoic differential "plateau uplift" is manifested.