High-resolution U-Pb ages from the Chinle Formation with a Stratigraphic Revision of the Chinle and Support for the Diachronous Rise of Dinosaurs

This is the full paper providing two new radioisotopic ages for Chinle Formation strata in New Mexico. One for the purported base of the Blue Mesa Member (thus a basal Chinle age constraint) from Six Mile Canyon in New Mexico, and the other from the younger Hayden Quarry, which has produced a number of new dinosauromorphs including the type specimens of Dromomeron romeri and Tawa hallae (Irmis et al., 2007; Nesbitt et al., 2009). Although the Blue Mesa date has been widely cited these have previously only appeared in abstract form (Irmis & Mundil, 2008, 2010).

Jeff's and my contribution to this paper is mainly a revised hypothesis of stratigraphic relationships of units in the lower part of the Chinle Formation based on several years of detailed fieldwork in Arizona, Utah, and New Mexico. Our revised correlations demonstrate that the date from Six Mile Canyon is more likely from strata from the uppermost part of the Blue Mesa Member (Chinle Formation) type section in Petrified Forest National Park (PEFO), and not from the base of the unit as previously supposed. Therefore this date is from stratigraphically younger rocks than previously believed. We also argue that the lower part of the Blue Mesa Member type section at PEFO is equivalent to the Bluewater Creek Member in New Mexico and northeastern Arizona, and that the Bluewater Creek is not directly equivalent to the Mesa Redondo Member. Instead we argue that the Mesa Redondo Member is mainly a lateral equivalent of the Shinarump Member. These are novel hypotheses that we have much supporting data for and we hope to have a full manuscript detailing these revised correlations prepared in the near future.

Irmis, R. B., Mundil, R., Martz, J. W., and W. G. Parker. 2011. High-resolution U-Pb ages from the Upper Triassic Chinle Formation (New Mexico, USA) support a diachronous rise of dinosaurs. Earth and Planetary Science Letters [advance online publication]. doi:10.1016/j.epsl.2011.07.015

Abstract - Though the Late Triassic preserves major paleoenvironmental fluctuations and is key for understanding the evolution of Mesozoic and modern terrestrial ecosystems, comparisons of Late Triassic non-marine sedimentary and fossil records are difficult because global correlations lack precise radioisotopic ages, and have instead been based upon unconstrained biostratigraphic ranges of palynomorph and vertebrate fossils. The Chinle Formation in southwestern North America preserves a major Late Triassic record of paleoenvironmental and biotic change, including significant early dinosaur fossils. Previous high-resolution radioisotopic age constraints for the formation are limited to a single U-Pb zircon age from the upper third of the formation. The extraction of a geologically meaningful age is challenging from these redeposited units and preference is given to considering the youngest age of a deposit as a maximum age and closest approximation of the depositional age. Because calculating a weighted mean age (or median age) from a group of ages from such deposits is often not adequate, the precision of our two new CA-TIMS single crystal zircon U-Pb ages from the Chinle Formation of New Mexico is limited to ca 0.3% (or ± 0.7 Ma) of the youngest crystal age. Our 206Pb/238U age of ~ 218 Ma from the Blue Mesa Member in Six Mile Canyon, western New Mexico, demonstrates that strata, palynomorphs, and vertebrate fossils previously considered to be late Carnian in age are actually middle Norian in age. Our new age of ~212 Ma from the Hayden Quarry within the Petrified Forest Member at Ghost Ranch, northern New Mexico, provides the first maximum age for important vertebrate assemblages from this area that record the rise of dinosaurs, and demonstrates that basal dinosauromorphs (‘dinosaur precursors’) co-existed with dinosaurs for at least 18 Ma. These new radioisotopic data allow a new correlation of the Chinle Formation to the Late Triassic timescale, suggesting that most if not all of the lower Chinle is Norian in age. This new correlation has global implications as it allows us to make more precise comparisons with early dinosaur assemblages from the Ischigualasto Formation of Argentina, indicating that Chinle dinosaur assemblages are significantly younger than those from South America. The revised age of the Chinle Formation also demonstrates that dinosaurs were much rarer in North America at a time when they were abundant in South
America, supporting hypotheses of paleolatitudinal variation during the
rise of dinosaurs.

REFERENCES

Irmis, R., Mundil, R., 2008. New age constraints from the Chinle Formation revise global comparisons of Late Triassic vertebrate assemblages. J. Vertebr. Paleontol. 28, 95A(Suppl.).

Irmis, R.B., Mundil, R., 2010. New U–Pb zircon ages from the Chinle Formation (western US) revise understanding of Late Triassic terrestrial vertebrate evolution. Geol. Soc. Am. Abstracts Prog. 42, 393.

Irmis, R.B., Nesbitt, S.J., Padian, K., Smith, N.D., Turner, A.H., Woody, D., Downs, A., 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317, 358-361.

Nesbitt, S.J., Smith, N.D., Irmis, R.B., Turner, A.H., Downs, A., Norell, M.A., 2009. A complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs. Science 326, 1530–1533.