Provenance Investigation of the Early Tertiary “Rim Gravels”: Implications for Cenozoic Evolution of the Colorado Plateau
Friday, December 7th, 2012
The early Tertiary “Rim gravels” were deposited by northerly and easterly flowing fluvial systems across the boundary of the southwestern Colorado Plateau (Mogollon Rim) and the Transition Zone of central Arizona and western New Mexico. The gravels occupy a topographically high position along and near the Mogollon Rim such that the source regions of the deposits are topographically lower now in the Transition Zone and Basin and Range Province. The former highland source region of the gravels has been termed the “Mogollon Highlands” which as envisioned trended generally parallel to the present Mogollon Rim along the southern margin of the Colorado Plateau. Late Laramide uplift of the ancestral Mogollon highlands shed coarse fluvial deposits of the Rim gravels northeastwards onto the southern edge of the Colorado Plateau. To the south and west of Mogollon highlands Eocene fluvial clasts were being shed to the Pacific coast from potentially the same highland source regions as the Rim gravels. Precise depositional ages of the Rim gravel deposits are difficult to determine but the entire belt mostly predates the voluminous Oligocene western Cordilleran flareup of volcanic activity. Fossils in fresh water limestone in the western part of the belt yield early Eocene ages while intercalated ash beds in a section ~225 km to the southwest yield late Eocene age of ~35 Ma suggesting that the Rim gravels may have been deposited over a considerable interval of time.
The Rim gravel deposits are considered to be broadly correlative deposits but have been mapped as distinct formations which include from west to east the Music Mountain, Naco, Mogollon Rim, Eagar, and Baca Formations. This project presents the results of a provenance study of the Rim Gravels focused on well-rounded durable rhyolitic volcanic clasts that comprise a relatively small but widespread component portion of the gravels. Three Music Mountain Formation clasts yield middle Jurassic zircon U-Pb ages (160.4±4 Ma, 161.3±3 Ma, and 163.0±3 Ma) suggesting derivation from the middle Jurassic arc that traverses southern Arizona and a possible correlation with San Diego Poway-type clasts. Volcanic clasts from Mogollon Rim and Baca Formations yield Proterozoic zircon U-Pb ages mostly from 1705 to 1738 Ma, while the Eagar Formation yielded ages of 1434 Ma and 1657 Ma from two clasts. The age and chemistry of these clasts match potential source rocks in the Tonto Basin Supergroup within the Transition Zone of Arizona. Results of this study suggest that the Rim gravels may subdivided into at least two distinct depositional systems; an earlier Eocene north flowing system represented by the Music Mountain Formation that tapped Mogollon Highland regions as far south as southern Arizona, and a late Eocene system represented by the Mogollon Rim, Eagar and Baca Formations that tapped more localized region of the central Arizona Transition Zone.