High-precision U-Pb zircon geochronology of the late triassic chinle formation, Petrififed Forest National Park (Arizona, USA): Temporal constraints on the early evolution of dinosaurs

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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 dinosauromorphrich 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. © 2011 Geological Society of America.

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Bulletin of the Geological Society of America