Landscape development of central and northernArizona can no longer be ascribed mainly to events of Miocene and Pliocene age. New information on the age and distribution of older Cenozoic deposits has led to the recognition of a regional Cretaceous-Paleocene(?) surface of erosion that conforms to major elements of the present topography and to the recognition that a formerly thick deposit of gravel accumulated on this regional surface of erosion. These relations cast new light on the history of evolution of the landscape and indicate a much greater age for the main landscape elements and a more complicated and prolonged history of erosion and deposition than has been previously supposed. The timing of events postulated for development of drainage on the Colorado Plateau can now be compared and partly reconciled with events recognized in the adjacent closely related Mountain Region (Transition Zone) of central Arizona. As a consequence of Late Cretaceous-Paleocene (Laramide) compression, central and northern Arizona underwent at least 1200 m of uplift, documented by paleochannels cut into erosionally truncated Paleozoic strata on the Hualapai Plateau of the southwestern Colorado Plateau. During this time, a highly irregular erosion surface was developed on Proterozoic rocks across the Transition Zone south of the Mogollon Rim, the scarp of the Mogollon Rim was eroded to its present height (600-900 m), and an extensive stripped surface was developed on resistant upper Paleozoic strata north of the rim. Deposition of several hundred meters of Paleocene-Eocene "Rim gravels" derived from highlands south and west of the region followed, covering much of the Cretaceous-Paleocene erosion surface. Nearly complete burial of the rim is suggested by the distribution of remnants of the Rim gravels across the erosional scarps and on high plateau areas north of the rim. A second increment of uplift, apparently occurring in late Eocene time and apparently recorded by a series of fission track cooling ages from the Marble and Grand canyons, is inferred to have been responsible for ending deposition of the Rim gravels, for initiating differential uplift of contemporaneous deposits (Canaan Peak and Claron formations) to their positions in the high plateaus of central Utah, and for causing the drainage reorganization required to explain the extensive removal of Rim gravels from much of the region. A southerly flowing ancestral Verde River related to the drainage reorganization removed much of the older gravel cover from the Transition Zone of central Arizona, resulting in a younger regional erosion surface having 600-900 m of relief, a surface closely approximating the Cretaceous-Paleocene erosion surface. Late Oligocene and early Miocene rocks locally rest unconformably on remnants of Rim gravels in the Transition Zone, indicating that the second episode of regional erosion had been completed by late Oligocene time. North of the Mogollon Rim, a west flowing(?) ancestral Colorado River is inferred to have become established on the Ri...

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Specularite, apparently formed and in part altered to red hematite before deposition, carries much of the stable magnetization in sandstone, siltstone, and claystone of the Moenkopi Formation (Lower Triassic) at Gray Mountain, northern Arizona. High‐temperature exsolution textures seen in polished section indicate that some, if not much, of the specularite is detrital. Progressive thermal and chemical demagnetization analyses indicate that the most stable magnetization resides in specularite. Last, a systematic depositional inclination ‘error’ is seen in both normally and reversely polarized sandstone. Part of its cause was the deposition along fine bedding laminations of subtabular grains of specularite carrying a strong direction of magnetization in the plane of tabularity. Magnetic inclinations are significantly shallower in sandstone than in siltstone, and the amount and sense of inclination error are controlled in detail by the dip of the horizontal and cross laminations. The foregoing leads to the conclusion that the stable magnetization is a detrital magnetization acquired during the course of deposition and that the Moenkopi Formation thus is suitable for magnetostratigraphy. Neither the formation of specularite nor the acquisition of the stable magnetization appears to be the consequence of prolonged and complicated postdepositional chemical alteration, a process invoked by some workers for formation and magnetization of ancient red beds. Because specularite is common in other ancient red bed sequences, their stable magnetizations also may be partly or largely detrital.

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