IMPROVED EOCENE-OLIGOCENE CHRONOLOGY FOR A KAIBAB-DOMINATED COLORADO PLATEAU LANDSCAPE, NORTHWESTERN ARIZONA

An Oligocene tuff (⁴⁰Ar/³⁹Ar age 23.97 ± .03 Ma) and an early Eocene fossil assemblage from the Hualapai and Coconino Plateaus, Arizona, provide improved constraints for the timing of early Cenozoic uplift and erosion followed by Paleogene through Miocene deposition in the western Grand Canyon region. The Oligocene tuff, with a potential source in the Aquarius Mountains (~25 km south), is near the top of the 365-meter-thick Cenozoic paleocanyon fill in Peach Springs Wash in gravel that underlies the 18.5 Ma Peach Spring Tuff. The early Eocene mollusk assemblage is from the Duff Brown Tank locality (L4371) 50 km southwest of Grand Canyon on the Howard Spring Quadrangle (1:24,000). The mollusk-bearing limestone, intercalated with arkosic gravels, disconformably overlies an eroded Moenkopi-Kaibab landscape at elevations between 1128 and 1158 m. The paleotopography indicates that the Coconino Plateau probably was eroded to a resistant Kaibab surface with isolated remnants of the Moenkopi Formation forming local divides by early Eocene time. The improved Paleogene ages confirm that the Hualapai Plateau Cenozoic section provide a nearly continuous record of Paleocene through Pliocene events. The basal arkosic Paleocene-Eocene Music Mountain Formation (Arizona “Rim gravel”) blanketed the erosionally beveled western plateau and much of northern Arizona by the end of Eocene time, but is only well preserved in sections capped by Miocene basalts. Detrital zircon data at Peach Springs show provenance peaks for rocks from ~1.7 Ga, ~1.4 Ga, ~168 Ma, and ~99 Ma terranes. No Paleogene gravels of similar provenance have been unequivocally identified immediately north of the Colorado River, due to extensive incision by Colorado River tributaries. Widespread Oligocene gravels (Buck and Doe Fm.) and Miocene volcanic rocks in a system of partially re-exhumed paleovalleys preserve a record of subsequent long-term aggradation on the Hualapai Plateau until latest Miocene or early Pliocene time, followed by erosion and integration of the adjacent Colorado River in the western Grand Canyon. Regional chronology permits a reasonable reconstruction of Cenozoic history up through Hualapai Limestone time (~6 Ma), after which the modern Colorado River rapidly became integrated across Arizona by means that are unresolved.