U-PB AGES OF DETRITAL ZIRCONS IN RELATION TO PALEOGEOGRAPHY: TRIASSIC PALEODRAINAGE NETWORKS AND SEDIMENT DISPERSAL ACROSS SOUTHWEST LAURENTIA

Integration of detrital zircon studies with data on biostratigraphy, isopachs, facies, and paleocurrents can improve reconstructions of paleogeography for ancient fluvial systems. U-Pb ages for 1808 individual detrital zircon grains in 20 samples of Triassic fluvial sandstone on the Colorado Plateau and High Plains, U.S.A., help constrain paleodrainage paths and patterns of sediment dispersal across southwest Laurentia. The dominant paleoflow was from southeast to northwest from sources as distant as the Ouachita orogen, and segments of Mesoamerica beyond it, to the Cordilleran margin in the Great Basin. Detrital zircons in Middle Triassic Moenkopi strata deposited along the distal eastern flank of the Sonoma foreland basin were derived from varied sources to the south and southeast, as were similar populations of detrital zircons in Upper Triassic Chinle strata exposed along the southern margin of the Colorado Plateau. Basal Chinle (-Dockum) strata of the High Plains to the east contain detrital zircon populations dominated by Cambrian grains derived from the nearby Amarillo-Wichita uplift, as do Gartra strata of northeast Utah to which the Cambrian grains were transported across eroded roots of Ancestral Rocky Mountains uplifts along upstream reaches of the Eagle paleoriver of northwest Colorado. The centeal locus of the Chinle-Dockum fluvial system lay along the tectonic furrow of an elongate backarc basin formed by dynamic subsidence inland from the Cordilleran magmatic arc. A lower Chinle-Dockum trunk paleoriver and the upper Chinle-Dockum Cottonwood paleovalley document longitudinal paleoflow parallel to the axis of the basin. Detrital zircon populations show that headwaters of both paleodrainages; tapped the Ouachita orogen, the Permian-Triassic East Mexico arc, and associated rock assemblages of southwest Laurentia and adjacent Mesoamerica. Sediment sources in the Mesozoic Cordilleran arc became increasingly more prominent during Late Triassic time.