DETRITAL ZIRCONS IN UPPER TRIASSIC STRATA OF THE UPPER CHINLE AND DOCKUM GROUPS, NEW MEXICO AND TEXAS

Detrital zircons were sampled from Upper Triassic fluvial sandstones in the Trujillo Formation of the upper Chinle Group in northeast New Mexico and the Trujillo and Cooper Canyon Formations of the upper Dockum Group in northwest Texas. The Trujillo and Cooper Canyon Formations of the Dockum Group were collected in the headwaters of the Red and Brazos Rivers near the eastern escarpment of the Llano Estacado, and the Trujillo Formation of the Chinle Group was collected west of Conchas Reservoir near Tucumcari. From the three samples, U-Pb ages of 276 detrital zircon grains were determined by laser ablation ICPMS using a beam diameter of 35 microns. Analytical data were filtered to exclude zircon grains with >20% age discordance or poor precision, leaving 244 reliably dated grains. The zircon grains range dominantly from 200 Ma to 2000 Ma in age, with a few Archean grains present in the Trujillo samples. Northwest-directed paleocurrent trends in the upper Dockum Group of west Texas imply a provenance lying to the southeast, and a prominent age spike of 220-280 Ma in all three samples probably reflects derivation of detritus from granitic rocks of the Permian-Triassic magmatic arc in eastern Mexico. Older grains in the age range of 380-700 Ma represent Paleozoic and Pan-African detritus possibly derived from reworking of Appalachian-derived detritus in strata exposed within the Marathon segment of the Ouachita orogenic belt in southwest Texas and adjacent Mexico or from the unexposed core zone of the Ouachita orogen. A prominent age spike of mid-Paleozoic grains (~400 Ma) in the Cooper Canyon Formation, which is younger than the Trujillo Formation, probably signifies erosional unroofing of the core of the Ouachita system over time. Nearly a third of the detrital zircons in each of the three samples were derived ultimately from bedrock sources of Grenvillian age (900-1250 Ma) along the flank of the Appalachian-Ouachita orogen or in eastern Mexico, but were possibly reworked from Ouachita sedimentary assemblages. The Dockum Trujillo sample also displays a prominent mid-Proterozoic age spike (1250-1500 Ma) suggesting ultimate derivation from anorogenic granite of southwestern Laurentia, with or without recycling through Paleozoic strata.