U-PB ID-TIMS GEOCHRONOLOGY OF THE LATE TRIASSIC CHINLE FORMATION, PETRIFIED FOREST NATIONAL PARK, ARIZONA

The Late Triassic Chinle Formation (Group) is a continental fluvial and lacustrine succession that was deposited in a back-arc basin on the western margin of Pangaea. Broad exposures throughout southwestern US preserve a series of paleo-valley fills with well-developed fluvial channels, floodplains, and thick successions of stacked paleosols. Sedimentation in the Chinle was influenced by volcanic activity, presumably from the cordilleran arc to the west and south. The Chinle is central to both the biotic and geological events that shaped the North American craton. It records the era when dinosaurs and their near-relatives became ascendant over non-dinosaurian amniotes and is arguably the best-exposed terrestrial record of the time antecedent to the Triassic-Jurassic extinction.

Precise correlation of Chinle strata to a global Triassic stratigraphic framework has been problematic due to absence of calibrated biostratigraphy and reliance on sparse paleomagnetic and/or palynological data. Although the input of volcanic debris into the Chinle basin has been known for decades, high-precision zircon geochronology has been hampered by the reworked nature of tuffaceous rocks and incorporation of old grains. U-Pb analyses by in situ techniques (SHRIMP and LA-ICPMS) have been useful in understanding the age distribution and provenance of zircon in the Chinle, but lack the precision to reliably constrain depositional ages.

We present new U-Pb ID-TIMS single-zircon data from a number of stratigraphically well-constrained volcaniclastic sandstones of the Petrified Forest Member of Chinle Formation within the PFNP. High-precision U-Pb analysis reveals multiple age groups within the morphologically distinct late Triassic zircon population, the youngest of which constrain the time of deposition. Results indicate that the lower two-thirds of the Petrified Forest section was deposited within a period of ca. 10 Myr entirely within the Norian Stage of Late Triassic.