New Isotopic Dates for Strata in the Late Triassic Chinle Formation, Petrified Forest National Park

I'm thrilled that this is finally out, along with the recent Irmis et al. paper. This new paper by Ramazani et all provides high-precision isotopic dates for a variety of stratigraphicl levels of the Chinle Formation in Petrified Forest National Park. Furthermore it provides the first dates for the Mesa Redondo and Owl Rock Members which essentially contrain the entire park section. The Mesa Redondo date is particularly important because it comes close to representing the base of the Chinle Formation in Arizona (see stratigraphic discussion in Irmis et al., 2011). The stratigraphic sequence and scheme used for this study is based upon independent work conducted by Dr. David Fastovsky of the University of Rhode Island and some of his students, as well as work done by a group from Baylor University.  Thus it doesn't match the current park stratigraphic nomenclature introduced by Martz and Parker (2010) and Parker and Martz (2011). Still the two are close and if anyone has any questions where these new dates fit into our scheme please contact me.

Some important points made by the paper:

- Chinle deposition in Petrified Forest ranged from about 225 - 208 million years ago or a duration of 17 million years.

- There is not enough stratal thickness in the Sonsela Member to account for the elapsed amount of time, so there must be many hiatuses in this unit.

- Despite the purported presence of numnerous hiatuses in the Sonsela none of them are large-scale, and there is little evidence for a sizable regional unconformity (i.e., the TR-4). [Note that this is also supported by other stratigraphic and biostratigraphic data].

- These authors argue that the basal Chinle date demonstrates overlap between the Chinle and Ischigualasto formations, suggesting that the rise of early dinosaurs was not diachronous (but see Irmis et al., 2011).

Ramezani, J., Hoke, G. D., Fastovsky, D. E., Bowring, S. A., Therrien, F., Dworkin, S. I., Atchley, S. C., and L. C. Nordt. 2011. High-precision U-Pb zircon geochronology of the Late Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA): Temporal constraints on the early evolution of dinosaurs. Geological Society of America Bulletin. First published online August 19, 2011, doi: 10.1130/B30433.1 

Abstract - The Triassic successions of the Colorado Plateau preserve an important record of vertebrate evolution and climate change, but correlations to a global Triassic framework are hampered by a lack of geochronological control. Tuffaceous sandstones and siltstones were collected from the Upper Triassic Chinle Formation exposed in the Petrified Forest National Park, Arizona, USA, within a refined stratigraphic context of 31 detailed measured sections. U-Pb analyses by the isotope dilution–thermal ionization mass spectrometry (ID-TIMS) method constrain maximum depositional ages for nine tuffaceous beds and provide new insights into the depositional history of the Chinle fluvial system. The base of the Blue Mesa Member of the Chinle Formation is placed at ca. 225 Ma, and the top of the Petrified Forest Member is placed at 208 Ma or younger, bracketing an ∼280-m-thick section that spans nearly the entire Norian Stage of the Late Triassic. Estimated sediment accumulation rates throughout the section reflect extensive hiatuses and/or sediment removal by channel erosion. The new geochronology for the Chinle Formation underscores the potential pitfalls of correlation of fluvial units based solely on lithostratigraphic criteria. A mid-Norian age (ca. 219–213 Ma) for the distinctive Sonsela conglomeratic sandstone bed constrains the Adamanian-Revueltian land vertebrate faunachron boundary. Our new data permit a significant time overlap between the lower Chinle sequence and the dinosauromorph-rich Ischigualasto Formation of northwestern Argentina. Near-contemporaneity of the trans-American deposits and their faunal similarities imply that early dinosaur evolution occurred rapidly across the Americas.

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