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

Abstract

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 206 Pb*/ 238 U ages from 214 ؎ 2 Ma to 200 ؎ 4 Ma. We use these data to infer that cores of inherited material were present in many zircons and that single-crystal analysis provides an accurate estimation of

The origin and early radiation of dinosaurs

“…1), consistent with the Newark Astrochronological Polarity Timescale from eastern North America (Muttoni et al, 2004). The Norian is very poorly dated: there is only one published precise radioisotopic age (Riggs et al, 2003), and it is from terrestrial strata that cannot be directly correlated to the marine biostratigraphic events that define stage boundaries. Calibration of magnetostratigraphic records using palynomorph assemblages (e.g., Kent and Olsen, 1999;Muttoni et al, 2004) and magnetostratigraphy from a key marine section (Muttoni et al, 2010) indicate an age of 208-209 Ma for the Norian-Rhaetian boundary.…”

“…Although the most recent estimates indicate that the Late Triassic Epoch is over 30 million years long (e.g., Muttoni et al, 2004;Furin et al, 2006), there are only four published precise radioisotopic ages Riggs et al, 2003;Furin et al, 2006;Schaltegger et al, 2008) for this time period. The base of the Late Triassic is poorly dated: there are no precise radioisotopic ages from near the Ladinian-Carnian boundary and there is no published magnetostratigraphic record that crosses the boundary.…”

“…The Chinle Formation of the Colorado Plateau in western North America is traditionally considered late CarnianNorian in age (e.g., Litwin et al, 1991;Lucas, 1998), but new U-Pb radioisotopic age constraints indicate that even the oldest fossiliferous strata are Norian in age (Riggs et al, 2003;Irmis and Mundil, 2008;Mundil et al, 2008). Footprint assemblages from the Newark Supergroup of eastern North America (e.g., Olsen et al, 2002) are tied to a high-resolution magnetostratigraphic record that is calibrated using palynomorph biostratigraphy (Kent and Olsen, 1999).…”

The Late Triassic pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs

“…Equivalent strata within a square kilometre of the quarry contain the remains of unionid bivalves, lungfish teeth and associated remains of the metoposaurid Apachesaurus, the diapsid Vancleavea (sensu Hunt et al 2002), a fragmentary phytosaur, the aetosaur Typothorax coccinarum, the 'rauisuchian' Postosuchus, a chatterjeeid, Chindesaurus, a coelophysoid theropod and additional material of Revueltosaurus. An isotopic date for the Black Forest Bed (Riggs et al 2003) provides an age of 209 Myr, thus supplying a relative minimum age (Norian) for the quarry.…”

Wood anatomy of Late Triassic trees in Petrified Forest National Park, Arizona, USA, in relation to Araucarioxylon arizonicum Knowlton, 1889

“…Logs also occur above the Sonsela in the Wolverine Petrified Forest (southeastern Utah) and in the upper Chinle within the Black Forest Bed (localized to the northern part of PEFO). Although PEFO is undoubtedly the best location in southwestern USA to readily find exposed petrified logs within the Chinle, even there the logs are found as fairly discrete deposits separated by a number of lithological units representing many millions of years of deposition extending from Late Carnian into Norian (Riggs et al 2003, Parker 2006, Woody 2006. Detailed studies aimed at substantiating the implicit assumption of persistently homologous wood anatomy among all the region's large diameter petrified logs across this time span have only begun (Savidge 2006, Savidge & Ash 2006).…”