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Cenozoic paleogeography of the central Mogollon Rim–southern Colorado Plateau region, Arizona, revealed by Tertiary gravel deposits, Oligocene to Pleistocene lava flows, and incised streams

¹ 7550 North Snowbowl Road, Flagstaff, Arizona 86001, USA

Eocene to Pleistocene paleogeography of the Mogollon Rim–southern Colorado Plateau region in central Arizona is interpreted by use of Tertiary sedimentary deposits and mafic lavas that have been correlated on the bases of provenance of pebbles, stratigraphic sequence, geomorphic positions of deposits and flows, published K-Ar ages, tracing of lava flows to vents, and lava textures and structures. Five categories of Tertiary sedimentary deposits are (1) Paleocene(?) to Oligocene(?) fluvial and lacustrine deposits on the margin of the Colorado Plateau that were derived from local and distant southern sources; (2) Oligocene(?) to Miocene deposits in valleys at the base of the Mogollon Rim that were derived from local and distant southern sources; (3) Oligocene(?) to Pliocene locally derived gravel on pediments and in canyons along the rim; (4) Miocene locally derived lag gravel beneath sheet flows on the plateau; and (5) Miocene to Pleistocene locally derived channel-filling gravel on the plateau. Mafic lavas used are (1) upper Oligocene to lower Miocene basalt and minette in the Transition Zone; (2) Miocene pahoehoe sheet flows on the Colorado Plateau and draped over the Mogollon Rim; (3) middle Miocene to Pleistocene cones and flows of basalt scattered across the region; and (4) Pliocene to Pleistocene basalts of the Mormon and San Francisco volcanic fields. Basalt sheet flows in ten middle (ca. 12 Ma) and upper (ca. 6 Ma) Miocene flow fields, traced to 12 low-profile vent structures, cover nearly 30% of a 9000 km² area of the central Mogollon Rim region and depict the paleogeography at the times of eruption.

Principal conclusions are: (1) the Mogollon Rim developed near its present position after Precambrian rocks in central Arizona had been unroofed during the Paleocene to Eocene Epochs, and before late Oligocene to early Miocene mafic volcanism; (2) after erosional quiescence during the Paleogene, Triassic strata were stripped off the southern Colorado Plateau in a Miocene to Pleistocene erosional sweep from the southwest to the northeast; (3) incision of meanders on the Mogollon Slope occurred in the late Pliocene to Pleistocene Epochs as a result of integration of the Little Colorado River with the Colorado River in the Grand Canyon.

Key Words: Arizona • basalt flows • Colorado Plateau • gravel deposits • Mogollon Rim • paleogeography

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