2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 74
Presentation Time: 8:00 AM-12:00 PM


DICKINSON, William R.1, GEHRELS, G.E.1, FOX, Jennifer D.1, STAIR, Kelley N.1, ANDERSON, Carl E.1, OJHA, Jaika1, BROWN, Richard A.1, NORTON, Michael B.1, RIGGS, Nancy R.2 and LEHMAN, Thomas M.3, (1)Department of Geosciences, University of Arizona, Tucson, AZ 85721, (2)Geology Department, Northern Arizona University, Flagstaff, AZ 86011-4099, (3)Department of Geosciences, Texas Tech University, Lubbock, TX 79409-1053, wrdickin@dakotacom.net

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.