Late Triassic North American Paleodrainage Networks and Sediment Dispersal of the Chinle Formation: A Quantitative Approach Utilizing Detrital Zircons

Abstract

The Upper Triassic Chinle Formation is a succession of fluvial strata deposited in a backarc setting across the present-day Colorado Plateau of the southwestern United States. This study uses detrital-zircon (DZ) U-Pb data to examine provenance and geochronology in order to better understand Chinle deposition, its relationship to external forcing mechanisms, and its role in Late Triassic source-to-sink sediment dispersal. The dataset presented in this study includes ~6800 individual U-Pb ages from 24 samples of Chinle sandstone across the Colorado Plateau: 17 of these samples are newly collected, whereas the remaining samples were initially analyzed by Dickinson and Gehrels (2008) and upgraded to larger numbers of analyses for this study. DZ samples collected from the base through top of the Chinle Formation produced a series of maximum depositional ages that suggest a ca. 27 Myr (ca. 203 to 230 Ma) window of deposition, longer than inferred in previous studies that used biostratigraphic and geochronological data. The basal Chinle, defined as the Shinarump and Temple Mountain members, rests unconformably on older Triassic bedrock, and represents the period of ca. 215 to 230 Ma: the high relief on the basal Chinle unconformity and the considerable period of time represented by basal Chinle units suggest a series of long-lived (ca. 15 Myrs), degradational, and terraced mixed bedrock-alluvial valley systems. DZ U-Pb populations indicate at least three distinct paleovalley systems transported sediment in a generally southeast to northwest direction during basal Chinle deposition, which supports the multiple paleovalley model proposed by previous studies. These paleovalleys are the: A) Cottonwood Paleovalley, which is dominated by Mesoproterozoic Mid-Continent and Paleoproterozoic Yavapai-Mazatzal populations, that reflects headwaters with source terrains from the Ancestral Rocky Mountains and recycled Proterozoic basement in New Mexico; B) Painted Desert Paleovalley, which is dominated by Mesoproterozoic Grenville and Mid- Continent populations, with minor contributions of Appalachian, Peri-Gondwanan, and Yavapai- Mazatzal populations, that reflects headwaters with source terrains predominantly from the Appalachian-Ouachita Cordillera, as well as from recycled basement rocks in New Mexico and Texas; and C) Vermillion Cliffs Paleovalley, which is dominated by a Mesoproterozoic Mid- Continent population, with minor contributions of Grenville and Yavapai-Mazatzal populations, that reflects headwaters with source terrains predominantly from the Mogollon Highlands, as well as from recycled basement rocks in Arizona. DZ U-Pb data from stratigraphically higher members of the Chinle Formation illustrate evolution of the sediment-routing system through time. In a general sense, upper units of the Chinle represent aggradational and degradational cycles within previously established paleovalleys. Moreover, there is a significant increase in Cambrian U-Pb ages, which indicates more significant input from the Amarillo-Wichita uplift. Additionally, upper Chinle samples have more pronounced Appalachian and Peri-Gondwanan populations than basal Chinle samples, but generally contain smaller Mid-Continent and Yavapai-Mazatzal populations. These changes suggest paleodrainage reorganization with increasing contribution from the Appalachian-Ouachita Cordillera and decreasing contributions from the Mogollon Highlands and Ancestral Rockies. Statistical similarities in source terrains for all but one sample collected from stratigraphically higher members suggest only one paleovalley existed during upper Chinle deposition, which evolved from the underlying Painted Desert Paleovalley.