Depositional style and tectonic implications of the Mogollon Rim Formation (Eocene), east-central Arizona
Andre R. Potochnik
The Mogollon Rim formation is an alluvial braidplain deposit that blanketed part of the southern Colorado Plateau and Transition Zone province in east-central Arizona during the Eocene. These coarse elastics were shed northeastward off the flanks of the actively rising Laramide Mogollon highland onto a locally channeled but regionally flat erosion surface that bevels increasingly older early Teritiary(?) through Precambrian rocks toward the southwest. Evidence for contemporaneous uplift is recorded by facies relationships across several northwest-trending intrabasinal faults that cross the Transition Zone. Regional transport of sediment was generally eastward toward the Baca basin in New Mexico. At least three large trunk streams from the Mogollon highland distributed discrete lithologic suites of clasts across a broad alluvial braidplain. The source area extended west and southwest beyond the present Tonto basin and Globe-Miami mining district. Deposition began with the widespread distribution of a coarse boulder conglomerate across the moderately channeled bedrock surface. As bedrock lows were filled, clast size diminished, and arkosic sand dominated the rivers. The alluvial plain was deposited in a semiarid climate, but the source area was sufficiently humid to support large-volume rivers. Paleocurrents suggest a 31° northward rotation of paleoslope following deposition of the basal conglomerate. Increasingly tuffaceous sandstones in the upper member signal incipient mid-Tertiary magmatism in latest Eocene time. The basal contact serves as a datum for post-depositional structural lowering of the Transition Zone. The previously active northwest-trending faults in the area were later reactivated with the opposite sense of movement, causing the Mogollon Rim formation in the Transition Zone to be downfaulted and downwarped at least 760 m to the south.
- Potochnik, Andre R., 1989, Depositional style and tectonic implications of the Mogollon Rim Formation (Eocene), east-central Arizona, in: Southeastern Colorado Plateau, Anderson, Orin J.; Lucas, Spencer G.; Love, David W.; Cather, Steven M., New Mexico Geological Society, Guidebook, 40th Field Conference, pp. 107-118.