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Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone

N.R. Riggs^,1, S.R. Ash^,2, A.P. Barth^,3, G.E. Gehrels^,4 and J.L. Wooden^,5

¹ Department of Geology, Northern Arizona University, Flagstaff, Arizona 86001, USA
² Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
³ Department of Geology, Indiana University–Purdue University, 723 West Michigan Street, SL118, Indianapolis, Indiana 46202, USA
⁴ Department of Geosciences, University of Arizona, Tucson, Arizona 86721, USA
⁵ U.S. Geological Survey–SUMAC, Stanford University, Stanford, California 94305-2220, USA

Zircons from the Black Forest Bed, Petrified Forest Member, Chinle Formation, in Petrified Forest National Park, yield ages that range from Late Triassic to Late Archean. Grains were analyzed by multigrain TIMS (thermal-ionization mass spectrometry), single-crystal TIMS, and SHRIMP (sensitive, high-resolution ion-microprobe). Multiple-grain analysis yielded a discordia trajectory with a lower intercept of 207 ± 2 Ma, which because of the nature of multiple-grain sampling of a detrital bed, is not considered conclusive. Analysis of 29 detrital-zircon grains by TIMS yielded U-Pb ages of 2706 ± 6 Ma to 206 ± 6 Ma. Eleven of these ages lie between 211 and 216 ± 6.8 Ma. Our statistical analysis of these grains indicates that the mean of the ages, 213 ± 1.7 Ma, reflects more analytical error than geologic variability in sources of the grains. Grains with ages of ca. 1400 Ma were derived from the widespread plutons of that age exposed throughout the southwestern Cordillera and central United States. Twelve grains analyzed by SHRIMP provide ²⁰⁶Pb*/²³⁸U ages from 214 ± 2 Ma to 200 ± 4 Ma. We use these data to infer that cores of inherited material were pres ent in many zircons and that single-crystal analysis provides an accurate estimation of the age of the bed. We further propose that, even if some degree of reworking has occurred, the very strong concentration of ages at ca. 213 Ma provides a maximum age for the Black Forest Bed of 213 ± 1.7 Ma. The actual age of the bed may be closer to 209 Ma.

Dating continental successions is very difficult when distinct ash beds are not clearly identified, as is the case in the Chinle Formation. Detrital zircons in the Black Forest Bed, however, are dominated by an acicular morphology with preserved delicate terminations. The shape of these crystals and their inferred environment of deposition in slow-water settings suggest that the crystals were not far removed from their site of deposition in space and likely not far in time. Plinian ash clouds derived from explosive eruptions along the early Mesozoic Cordilleran margin provided the crystals to the Chinle basin, where local conditions insured their preservation. In the case of the Black Forest Bed, the products of one major eruption may dominate the volcanic contribution to the unit.

Volcanic detritus in the Chinle Formation was derived from multiple, distinct sources. Coarse pebble- to cobble-size material may have originated in eastern California and/or western Arizona, where Triassic plutons are exposed. Fine-grained detritus, in contrast, was carried in ash clouds that derived from caldera eruptions in east-central California or western Nevada.

Key Words: isotopic dating • Chinle For mation • Triassic • single-crystal TIMS • SHRIMP

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