NEW ID-TIMS U-PB AGES FOR CHINLE GROUP STRATA (UPPER TRIASSIC) IN NEW MEXICO AND ARIZONA, CORRELATION TO THE NEWARK SUPERGROUP, AND IMPLICATIONS FOR THE “LONG NORIAN”

We report new air-abraded ID-TIMS U-Pb analyses of euhedral detrital zircons from previously-dated beds that constrain the pace of deposition and faunal change in Upper Triassic Chinle Group strata long interpreted as spanning the Carnian-Norian boundary. A volcanic-rich litharenite at the base of the Blue Mesa Member of the Petrified Forest Formation in Sixmile Canyon, New Mexico, yields an age of 220.9±0.6 Ma, and reanalysis of the Black Forest Bed of the Petrified Forest Formation in Arizona yields an age of 211±0.7 Ma. Preliminary analysis of euhedral detrital zircons from the Sixmile Canyon bed by LA-ICP-MS also suggests a maximum depositional age of 219.6±3.1 Ma, less precise but in agreement with the ID-TIMS data. Correlation of the Sixmile Canyon bed to the Petrified Forest National Park (PFNP) indicates that a ~150-m thick lithostratigraphic and magnetostratigraphic section from the PFNP represents no more than 9-11 Myr (average sedimentation rate ~0.022mm/yr). Comparison to the astronomically-calibrated Late Triassic polarity timescale indicates that the Black Forest Bed likely corresponds to the normal chron E16, and the next predominantly normal interval below this is in the Sonsela Member and probably corresponds to chron E15, with the largely reversed interval in the Blue Mesa Member likely representing chrons E11-E13, with E14 apparently absent at the Tr-4 unconformity. Palynostratigraphic constraints supported by vertebrate and conchostracan biostratigraphy assign a Carnian age to the Blue Mesa Member, which is consistent with the ~218 Ma Carnian-Norian boundary in the Newark Supergroup, not a “long Norian” extending to ~228 Ma as recently proposed. These ages indicate that dinosaurs were present, albeit rare, in the Upper Triassic of North America prior to ~221 Ma and were still not a dominant component of the ecosystem by ~211 Ma. Tetrapod faunal turnover during this interval is not catastrophic and instead appears to represent “normal” background origination and extinction events. There is no evidence that the Manicouagan impact event at 215.5 Ma had a significant effect on nonmarine tetrapod evolution in western North America.