Paper No. 65-8
Presentation Time: 3:35 PM
LA-ICPMS U-PB GEOCHRONOLOGY OF DETRITAL ZIRCON GRAINS FROM THE CHINLE FORMATION (COLORADO PLATEAU CORING PROJECT)
GEHRELS, George1, GIESLER, Dominique2, OLSEN, Paul3, KENT, Dennis4, MARSH, Adam D.5, PARKER, William G.5, RASMUSSEN, Cornelia6, MUNDIL, Roland7, IRMIS, Randall B.8 and GEISSMAN, J.W.9, (1)Deptartment of Geosciences, University of Arizona, Tucson, AZ 85721, (2)Department of Geosciences, University of Arizona, Tucson, AZ 85721; Dept. of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, (3)Earth and Environmental Sciences, Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-1000, (4)Earth and Planetary Sciences, Rutgers University, Wright-Rieman Labs, 610 Taylor Road, Piscataway, NJ 08854; Earth and Environmental Sciences, Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-1000, (5)Division of Science and Resource Management, Petrified Forest National Park, 1 Park Road, #2217, Petrified Forest, AZ 86028, (6)Jackson School of Geosciences, Institute for Geophysics, University of Texas at Austin, 10601 Exploration Way, Austin, TX 78758, (7)Berkeley Geochronology Center, 2455 Ridge Rd, Berkeley, CA 94709, (8)Natural History Museum of Utah and Department of Geology & Geophysics, University of Utah, 301 Wakara Way, Salt Lake City, UT 84108-1214, (9)Department of Geosciences, University of Texas at Dallas, 800 West Campbell Road, ROC 21, Richardson, TX 75080
U-Pb geochronologic analyses have been conducted by LA-ICPMS on detrital zircon grains from twenty-three samples from the Chinle Formation. These samples were recovered from ca. 520 m of drill core that was acquired during the Colorado Plateau Coring Project (CPCP), located in Petrified Forest National Park (Arizona). The samples contain variable proportions of Proterozoic and Paleozoic grains, but are dominated by Late Triassic grains. LA-ICPMS ages of the Late Triassic grains belong to four distinct groups that correspond to the Mesa Redondo Member, Blue Mesa Member and lower portion of the Sonsela Member, upper portion of the Sonsela Member, and Petrified Forest Member. The ages of pre-Triassic grains also correspond to these chronostratigraphic units, and are interpreted to reflect varying contributions from the Ouachita orogen, Precambrian basement exposed in the Ancestral Mogollon Highlands to the south, East Mexico arc, and Permo-Triassic arc built along the southern Cordilleran margin. Triassic grains in each unit also have distinct U and Th concentrations, which are interpreted to reflect temporal changes in the chemistry of arc magmatism.
Comparison of our LA-ICPMS ages with available CA-TIMS ages and magnetostratigraphic information provides new insights into the depositional history of the Chinle Formation, as well as current methods for determining depositional ages of fluvial strata. For portions of the Chinle Formation that are dominated by mudstone, such as the Blue Mesa Member and Petrified Forest Member, all three chronometers agree (to within ca. 1 m.y.), and robust depositional chronologies have been determined. In contrast, for strata dominated by sandstone, such as most of as the Sonsela Member, the three chronologic records disagree due to recycling of older zircon grains and variable dilution of syn-depositional-age grains. This results in LA-ICPMS ages that significantly pre-date deposition, and CA-TIMS ages that range between the other two chronometers. These complications challenge attempts to establish a well-constrained age model for the Chinle Formation, and to evaluate possible connections between fundamental Late Triassic biotic and climatic changes, a red silcrete horizon encountered in the CPCP core, and the ca. 215.5 Ma Manicouagan impact.
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