SEDIMENT DISPERSAL IN TRIASSIC FLUVIAL SYSTEMS OF SOUTHWEST LAURENTIA: INSIGHTS FROM U-PB AGES OF DETRITAL ZIRCONS

U-Pb ages of detrital zircons (DZ) in 20 samples of Triassic fluvial sandstone (Moenkopi, Chinle, Dockum) on the Colorado Plateau and NM-TX High Plains provide insight into Triassic sediment dispersal in southwest Laurentia (~100 individual grain ages per sample from LA-ICP-MS with a beam diameter of 35 microns; average of 90 reliable grain ages per sample after rejection of ages with >20% discordance or >10% uncertainty). U-Pb age populations fall into four groups: (1) Lowermost Chinle-Dockum (n=3) from the NE fringe of exposures (TX Santa Rosa, NM Tecolotito, UT Gartra): 35%-70% Cambrian grains (540-500 Ma; peaks 525-515 Ma) derived from the Amarillo-Wichita uplift or its Paleozoic cover, but no <300 Ma grains, with Gartra of NE UT evidence for Late Triassic sediment transport across eroded roots of Paleozoic ARM uplifts. (2) Lower to Middle Triassic Moenkopi (n=2), derived from the SE, and lower Chinle (Carnian) Shinarump (n=3) from paleovalleys trending SSE to NNW (AZ-UT): dominated by Proterozoic grains (30%-60%; 1785-1400 Ma) derived from Yavapai-Mazatzal basement (intruded by anorogenic plutons) and Permian-Triassic grains (20%-40%) derived from both Cordilleran (250-220 Ma) and East Mexico (295-260 Ma) magmatic arcs; similar DZ in upper Chinle (Norian) Sonsela also derived from the south. (3) Lower Chinle (Carnian) Shinarump (n=3) and correlative Santa Rosa (NM; n=1) along the Chinle-Dockum trunk paleoriver (course ESE to WNW across Colorado Plateau): heterogeneous DZ including Grenville (40%-50%; 1290-1000 Ma) and Neoproterozoic-Paleozoic (15%-20%; 640-320 Ma) grains recycled from the Ouachita orogen, with arc-derived grains sparse (<10%); closely matching DZ downstream in a Chinle outlier at Currie NV within the Great Basin. (4) Upper Chinle (Norian) of the Cottonwood paleovalley (n=4), subparallel to the lower Chinle paleoriver but farther northeast: also heterogeneous DZ with even more Neoproterozic-Paleozoic grains (30%-40%; 690-310 Ma) but Grenville (20%-30%) still dominant over Yavapai-Mazatzal (10%-15%); broadly similar DZ in correlative upper Chinle-Dockum upstream on the High Plains (n=3), though with more arc-derived grains (275-210 Ma; 15%-25%). Our study shows the utility of detrital zircon ages for helping to delineate paleodrainage patterns in ancient fluvial systems.