Paleogene rim gravel of Arizona: Age and significance of the Music Mountain Formation

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 clarifi ed 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 West Water Formation (Hualapai Plateau, Arizona, USA) as a calcrete-paleosol sequence, and its implications for the Paleogene-Neogene evolution of the southwestern Colorado Plateau

“…Hill et al (2016) obtained a U-Pb date for the Long Point limestone of 64 ± 2 Ma at Duff Brown Tank, Coconino Plateau, which is reportedly age-equivalent to the interbedded Music Mountain Formation at this site. This absolute date is older than the early Eocene age assigned to the Long Point limestone by Young and Hartman (2014) on the basis of charophyte, ostracod, and viviparid gastropod fossils.…”

“…During the earliest phase of uplift of the Hualapai Plateau, deep (~1200-1600 m) paleocanyons became incised into what is now the plateau surface, to be later partially filled with as much as 450 m of deposits (Young, 1982(Young, , 2001bWernicke, 2011;Flowers and Farley, 2012). The oldest of these canyon deposits is called the Music Mountain Formation, a series of arkosic gravels and sandstones that were derived from the Mogollon Highlands/Kingman Uplift to the south and southwest (Young and McKee, 1978;Young, 2001bYoung, , 2007Young and Hartman, 2014). Reportedly interbedded with the Music Mountain Formation is the Hindu Fanglomerate, which formed as debris flows, avalanche deposits, and mass-wasted colluviums off the edge of infilling paleocanyons Fig. 2).…”

“…2). A complete description of the stratigraphy of this area is provided by Young and Crow (2014) and Young and Hartman (2014).…”

“…The two units are now considered to be of slightly different ages: Young and Hartman (2014, their Plateau are still lacking. The age of the Buck and Doe Formation is Oligocene, based on a 24.12 ± 0.04 Ma date for an ash bed at the very top of the formation Young and Crow, 2014;Young and Hartman, 2014); however, the age of the lower part of the Buck and Doe Formation is not known, since a disconformity exists between it and the top of the West Water Formation (Fig. 2).…”

“…2). The Peach Springs Tuff, immediately above the Buck and Doe Formation, has been dated at 18.78 ± 0.02 Ma near its source at Silver Creek Caldera ~50 km to the southwest (Ferguson et al, 2013); one of the Miocene basalts above the tuff has been dated to 18.13 ± 0.04 Ma (Dominguez et al, 2012;Young and Hartman, 2014) (Fig. 2).…”