U-Pb zircon geochronology and depositional age models for the Upper Triassic Chinle Formation (Petrified Forest National Park, Arizona, USA): Implications for Late Triassic paleoecological and paleoenvironmental change Cornelia Rasmussen1,2,3,†, Roland Mundil4, Randall B. Irmis1,2, Dominique Geisler5, George E. Gehrels5, Paul E. Olsen6, Dennis V. Kent6,7, Christopher Lepre6,7, Sean T. Kinney6, John W. Geissman8,9, and William G. Parker10 1 Department of Geology & Geophysics, University of Utah, Salt Lake City, Utah 84112-0102, USA 2 Natural History Museum of Utah, University of Utah, Salt Lake City, Utah 84108-1214, USA 3 Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78758, USA 4 Berkeley Geochronology Center, Berkeley, California 94709, USA 5 Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 6 Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA 7 Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA 8 Department of Geosciences, University of Texas at Dallas, Richardson, Texas 75080, USA 9 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, 87131-0001, USA 10Division of Science and Resource Management, Petrified Forest National Park, Petrified Forest, Arizona 86028, USA ABSTRACT zircon ages and magnetostratigraphy. (<1 Ma difference) the true time of depo- From 13 horizons of volcanic detritus-rich sition throughout the Sonsela Member. The Upper Triassic Chinle Formation siltstone and sandstone, we screened up to This model suggests a significant decrease is a critical non-marine archive of low- ∼300 zircon crystals per sample using laser in average sediment accumulation rate in paleolatitude biotic and environmental ablation–inductively coupled plasma–mass the mid-Sonsela Member. Hence, the biotic change in southwestern North America. spectrometry and subsequently analyzed turnover preserved in the mid-Sonsela The well-studied and highly fossiliferous up to 19 crystals of the youngest age popu- Member at PFNP is also middle Norian Chinle strata at Petrified Forest National lation using the chemical abrasion–isotope in age, but may, at least partially, be an Park (PFNP), Arizona, preserve a biotic dilution–thermal ionization mass (CA-ID- artifact of a condensed section. Model 2 turnover event recorded by vertebrate and TIMS) spectrometry method. These data following the magnetostratigraphic-based palynomorph fossils, which has been alter- provide new maximum depositional ages age model for the CPCP core 1A suggests natively hypothesized to coincide with tec- for the top of the Moenkopi Formation instead that the ages from the lower and tonically driven climate change or with the (ca. 241 Ma), the lower Blue Mesa Mem- middle Sonsela Member are inherited pop- Manicouagan impact event at ca. 215.5 Ma. ber (ca. 222 Ma), and the lower (ca. 218 to ulations of zircon crystals that are 1–3 Ma Previous outcrop-based geochronologic age 217 Ma) and upper (ca. 213.5 Ma) Sonsela older than the true depositional age of the constraints are difficult to put in an accu- Member. The maximum depositional ages strata. This results in a model in which no rate stratigraphic framework because lat- obtained for the upper Chinle Formation sudden decrease in sediment accumulation eral facies changes and discontinuous out- fall well within previously proposed age rate is necessary and implies that the base crops allow for multiple interpretations. A constraints, whereas the maximum depo- of the Sonsela Member is no older than ca. major goal of the Colorado Plateau Coring sitional ages for the lower Chinle Forma- 216 Ma. Independent of these alternatives, Project (CPCP) was to retrieve a continu- tion are relatively younger than previously both age models agree that none of the pre- ous record in unambiguous superposition proposed ages from outcrop; however, served Chinle Formation in PFNP is Car- designed to remedy this situation. We sam- core to outcrop stratigraphic correlations nian (>227 Ma) in age, and hence the biotic pled the 520-m-long core 1A of the CPCP to remain uncertain. By correlating our new turnover event cannot be correlated to the develop an accurate age model in unques- ages with the magnetostratigraphy of the Carnian–Norian boundary but is rather a tionable superposition by combining U-Pb core, two feasible age model solutions can mid-Norian event. Our age models demon- be proposed. Model 1 assumes that the strate the powers, but also the challenges, youngest, coherent U-Pb age clusters of of integrating detrital CA-ID-TIMS ages Cornelia Rasmussen http://orcid.org/0000- 0001-9979-8224 each sample are representative of the maxi- with magnetostratigraphic data to properly †[email protected]. mum depositional ages and are close to interpret complex sedimentary sequences. GSA Bulletin; Month/Month 2020; 0; p. 1–20; https://doi.org/10.1130/B35485.1; 5 figures; 1 table; 1 supplemental file. For permission to copy, contact [email protected] 1 © 2020 Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/doi/10.1130/B35485.1/5095463/b35485.pdf by Rutgers University Libraries user on 20 July 2020 Rasmussen et al. INTRODUCTION lithostratigraphic correlations even over short events. For example, a biotic turnover in paly- distances (e.g., Parker and Martz, 2010; Irmis nomorph (Reichgelt et al., 2013; Baranyi et al., The Triassic Period represents a critical time et al., 2011; Ramezani et al., 2011; Atchley et al., 2018) and vertebrate taxa (Parker and Martz, in Earth history that experienced major envi- 2013). Exemplifying these difficulties, some 2010) occurs in outcrop in the middle Sonsela ronmental changes, and is embedded between precise U-Pb ages are available for the intensely Member between 213.1 Ma and 218.0 Ma, two mass extinction events (e.g., Payne et al., studied Upper Triassic Chinle Formation and based on outcrop correlation of dated horizons 2004; Mundil et al., 2004; Irmis and Whiteside, its equivalents in western North America (Irmis in Ramezani et al. (2011) and the biostrati- 2010; Preto et al., 2010; Schoene et al., 2010). et al., 2011; Ramezani et al., 2011, 2014; Atch- graphic record (e.g., Parker and Martz, 2010). Key Triassic Earth system events include the ley et al., 2013), yet these outcrop samples are Three main hypotheses have been suggested as onset of the break up and northward drift of spread over hundreds of square kilometers, so the potential cause for the biotic turnover: (1) the supercontinent Pangea (Dietz and Holden, placing them into a single unambiguous superpo- increasing aridification driven by either north- 1970; Robinson, 1973; Olsen, 1997; Olsen and sition remains difficult despite excellent outcrop. ward drift of Pangaea or tectonic changes along Kent, 2000), frequent carbon cycle excursions, To date, the only comprehensive timescale for the western margin of the North American Cor- and major changes in pCO2 levels (e.g., Payne the Late Triassic has been the Newark– Hart- dillera (Atchley et al., 2013; Lindström et al., et al., 2004; Berner, 2006; Whiteside et al., 2010; ford Astrochronostratigraphic Polarity Time 2016); (2) ecosystem disruption caused by Hönisch et al., 2012; Schaller et al., 2015; Foster Scale (N–H APTS) (Kent et al., 2017), which the relatively large (crater diameter ∼100 km) et al., 2017). During the Late Triassic, sudden is calibrated by orbitally paced lacustrine cycles Manicouagan impact event in Quebec, Canada paleoenvironmental events include reoccurring but has direct geochronologic control for only (e.g., Grieve, 2006) at ca. 215.5 Ma (Ramezani flood basalt volcanism such as the Wrangellia a ∼600 k.y. window around the Triassic–Juras- et al., 2005) (Mundil and Irmis, 2008; Olsen Large Igneous Province (Greene et al., 2010, and sic boundary (Blackburn et al., 2013). Though et al., 2010; Parker and Martz, 2010); and (3) references within) and the Central Atlantic Mag- the Astrochronostratigraphic Polarity Time partially an artifact of the stratigraphic record matic Province (Whiteside et al., 2010; Schoene Scale (N–H APTS) has been incorporated into (for example due to one or more hiatuses), mak- et al., 2010; Blackburn et al., 2013), as well as at various time scale compilations (e.g., Ogg, 2012; ing a gradual biotic turnover appear sudden and least three hypervelocity impact events, the larg- Walker et al., 2018), until very recently (Kent coordinated due to a condensed section (Heck- est of which is the Manicouagan impact struc- et al., 2018; Olsen et al., 2019) there has been ert and Lucas, 1996). Similarly, although recent ture in Canada (Ramezani et al., 2005; Grieve, no direct tests of its validity. geochronology suggests the fossil-bearing levels 2006; Schmieder et al., 2010, 2014; Cohen The Chinle Formation exposed at Petrified of the Chinle Formation are wholly Norian in et al., 2017). At the same time, the Late Triassic Forest National Park (PFNP) in northern Ari- age (Irmis et al., 2011; Ramezani et al., 2011, witnessed a number of important evolutionary zona, USA, represents an ideal study area to 2014), some authors maintain that the lower events, such as the origin and early diversifica- resolve these issues for two reasons: first, the sed- Chinle is Carnian in age (e.g., Lucas and Tan- tion of dinosaurs, lizards, mammaliaforms, and imentary strata contain a vast amount of volcanic ner, 2018)