LA-ICPMS U–Pb geochronology of detrital zircon grains from the Coconino, Moenkopi, and Chinle formations in the Petrified Forest National Park (Arizona)
Authors: Gehrels George , Giesler Dominique , Olsen Paul , Kent Dennis , Marsh Adam , Parker William , Rasmussen Cornelia , Mundil Roland , Irmis Randall , Geissman John , Lepre Christopher 
Published: September 2020

Abstract. Uranium–lead (U–Pb) geochronology was conducted by laser ablation – inductively coupled
plasma mass spectrometry (LA-ICPMS) on 7175 detrital zircon grains from
29 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. A total of 23 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 thorium (Th) concentrations, which are interpreted to reflect
temporal changes in the chemistry of arc magmatism. Comparison of our LA-ICPMS ages with available chemical abrasion thermal ionization mass spectrometry (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 Myr), 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 predate 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.