Abstract

The early Tertiary exotic rim gravel scattered across the Colorado Plateau in Arizona (USA) provides the only widespread evidence concerning the nature of the regional Paleogene drainage system that preceded the emergence of the modern Colorado River. The term “rim gravel” includes a wide range of Laramide (herein ca. 85–40 Ma) and younger reworked quartzite-dominated, arkosic sediments with diverse origins and ages that have been clarified only recently. The parent arkoses, with subordinate gravel lenses, contain volcanic clasts with ages ranging from Late Jurassic to early Eocene. The Laramide-age arkoses were shed off uplifted Precambrian terranes to the south and west; they are separated from their reworked derivatives and younger sediments by a disconformity that is best preserved in the structurally isolated paleocanyons of the Hualapai Plateau. Younger generations of reworked gravels continued to evolve during and following the widespread eruption of basalts of late Oligocene to Pleistocene age. Basaltic clasts in the reworked gravels attest to their much younger ages. Combined paleontologic, stratigraphic, paleomagnetic, K-Ar, U-Th/He, and zircon studies of the parent arkoses indicate that the tectonic framework corresponds to the Laramide orogeny, the concurrent regional erosion of the Colorado Plateau margin, and ensuing widespread deposition. The overall time frame coincides with the similar tectonic history recorded in the early Tertiary strata of southern Utah. Comparable but unrelated quartzite-dominated gravels, reworked southward from the Utah source rocks, are widespread on strath terraces north of the Colorado River. The Utah-derived gravels are products of the much younger Neogene incision of the modern Grand Canyon drainage system. The name Music Mountain Formation redefines the Laramide-age suite of the Arizona rim gravel parent sediments, best preserved on the Hualapai and Coconino Plateaus of northern Arizona, and distinguishes them from younger reworked gravels occurring both north and south of the Colorado River. A lacustrine mollusk assemblage collected from thin limestones intertonguing with Music Mountain arkose constrains the age of one Coconino Plateau exposure to early Eocene, whereas the association with the Laramide orogeny indicates that a Late Cretaceous to middle Eocene time frame comprises the broader geologic setting.

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