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Geochronology Advances in geochronological science
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 07 Oct 2019

Submitted as: research article | 07 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Geochronology (GChron).

LA-ICPMS U-Pb geochronology of detrital zircon grains from the Coconino, Moenkopi, and Chinle Formations in the Petrified Forest National Park (Arizona)

George Gehrels1, Dominique Giesler1, Paul Olsen2, Dennis Kent3, Adam Marsh4, William Parker4, Cornelia Rasmussen5, Roland Mundil5, Randy Irmis6, John Geissman7, and Christopher Lepre3 George Gehrels et al.
  • 1Department of Geosciences, University of Arizona, Tucson AZ 85721, USA
  • 2Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA
  • 3Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA
  • 4Petrified Forest National Park, Petrified Forest, AZ 86028, USA
  • 5Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley CA 94709, USA
  • 6Natural History Museum of Utah and Department of Geology & Geophysics, University of Utah, Salt Lake City, UT 84108, USA
  • 7Department of Geosciences, University of Texas at Dallas, Richardson, TX 75080, USA

Abstract. U-Pb geochronology was conducted by Laser Ablation-Inductively Couple Plasma Mass Spectrometry (LA-ICPMS) on detrital zircon grains from twenty-nine samples from the Coconino Sandstone, Moenkopi Formation, and Chinle Formation. These samples were recovered from ∼520 m of drill core that was acquired during the Colorado Plateau Coring Project (CPCP), located in Petrified Forest National Park (Arizona).

A sample from the lower Permian Coconino Sandstone yields a broad distribution of Proterozoic and Paleozoic ages that are consistent with derivation from the Appalachian and Ouachita orogens, with little input from local basement or Ancestral Rocky Mountain sources. Four samples from the Holbrook Member of the Moenkopi Formation yield a different set of Precambrian and Paleozoic age groups, indicating derivation from the Ouachita orogen, the East Mexico Arc, and the Permo-Triassic arc built along the Cordilleran margin.

Twenty-three samples from the Chinle Formation contain variable proportions of Proterozoic and Paleozoic zircon grains, but are dominated by Late Triassic grains. LA-ICPMS ages of these grains belong to five main groups that correspond to the Mesa Redondo Member, Blue Mesa Member and lower part of the Sonsela Member, upper part of the Sonsela Member, middle part of the Petrified Forest Member, and upper part of the Petrified Forest Member. The ages of pre-Triassic grains also correspond to these chronostratigraphic units, and are interpreted to reflect varying contributions from the Appalachian orogen to the east, Ouachita orogen to the southeast, Precambrian basement exposed in the Ancestral Mogollon Highlands to the south, East Mexico arc, and Permian-Triassic arc built along the southern Cordilleran margin. Triassic grains in each chronostratigraphic 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 new magnetostratigraphic data provides new insights into the depositional history of the Chinle Formation, as well as methods utilized to determine depositional ages of fluvial strata. For parts of the Chinle Formation that are dominated by fine-grained clastic strata (e.g. mudstone and siltstone), such as the Blue Mesa Member and Petrified Forest Member, all three chronometers agree (to within ∼1 m.y.), and robust depositional chronologies have been determined. In contrast, for stratigraphic intervals dominated by coarse-grained clastic strata (e.g., sandstone), such as most of 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-defined chronostratigraphic age model for the Chinle Formation, and to evaluate possible connections among fundamental Late Triassic biotic and climatic changes, a red siliceous horizon encountered in the CPCP core, and the ∼215.5 Ma Manicouagan impact.

George Gehrels et al.
Interactive discussion
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Status: final response (author comments only)
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George Gehrels et al.
George Gehrels et al.
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Short summary
U-Pb ages of zircon crystals are used to determine the provenance and depositional age of strata of the Triassic Chinle and Moenkopi Formations and the Permian Coconino Sandstone of northern Arizona. Primary source regions include the Ouachita orogen, local Precambrian basement rocks, and Permian-Triassic magmatic arcs to the south and west. Ages from fine-grained strata provide reliable depositional ages, whereas ages from sandstones are compromised by zircon grains recycled from older strata.
U-Pb ages of zircon crystals are used to determine the provenance and depositional age of strata...