Journal of Sedimentary Research, 2013, v. 83, 1147–1161 Research Article DOI: 10.2110/jsr.2013.89 A LINKAGE AMONG PANGEAN TECTONISM, CYCLIC ALLUVIATION, CLIMATE CHANGE, AND BIOLOGIC TURNOVER IN THE LATE TRIASSIC: THE RECORD FROM THE CHINLE FORMATION, SOUTHWESTERN UNITED STATES STACY C. ATCHLEY, 1 LEE C. NORDT, 1 STEPHEN I. DWORKIN, 1 JAHANDAR RAMEZANI, 2 WILLIAM G. PARKER, 3 SIDNEY R. ASH, 4 AND SAMUEL A. BOWRING 2 1 Department of Geology, Baylor University, 101 Bagby Avenue, Waco, Texas 76706, U.S.A. 2 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, U.S.A. 3 Division of Resource Management, Petrified Forest National Park, Box 2217, Petrified Forest, Arizona 86028, U.S.A. 4 Department of Earth and Planetary Sciences, Northrop Hall, University of New Mexico, Albuquerque, New Mexico, 87131, U.S.A. e-mail: stacy_atchley@baylor.edu ABSTRACT: High-precision geochronology provides unprecedented insights into the depositional history of the Upper Triassic Chinle Formation of the Colorado Plateau, as well as its paleoenvironmental and paleobiological records. The Chinle succession exposed in the Petrified Forest National Park (PEFO) and vicinity, Arizona, includes two large-scale alluvial composite sequences. Although each composite sequence fines upward, the upper composite sequence is more dominated by coarser-grained deposits. Petrographic analysis of sandstone lithic content indicates an upward decrease in the proportion of volcanic rock fragments in each composite sequence. Paleocurrent indicators in the lower composite sequence suggest a variable paleoflow direction, whereas northward paleoflow dominated the upper composite sequence. The change in paleoflow appears to coincide with a reorganization of alluvial depositional processes and associated source terranes, and precedes a rapid acceleration in basin subsidence. Climate proxy records from paleosol geochemistry indicate a gradual shift from humid to dry conditions across the transition between the lower and upper composite sequences and the Adamanian–Revueltian biotic turnover. Composite-sequence depositional reorganization, climatic shift and biologic turnover, in turn, appear to coincide with episodes of magmatism recorded in Triassic granitoid plutons presently exposed in southern California. Taken collectively, these observations suggest that the Late Triassic depositional, climatic, and ecologic history at PEFO may be related to emergence of the incipient Cordilleran magmatic arc along the convergent western margin of Pangea. A new U-Pb date for the lower part of the Chinle Formation suggests that most or all of the formation was deposited in the Norian Stage. INTRODUCTION The Upper Triassic Chinle Formation of the southwestern United States archives a remarkable record of terrestrial ecosystems that existed across the western portion of equatorial Pangea. Among the thickest and most easily accessible outcrop exposures occur at Petrified Forest National Park (PEFO) in northeastern Arizona. Although the Chinle Formation at PEFO has been extensively studied (see summary in Parker and Martz 2011), a reliable lithostratigraphic framework and high- precision geochronology have only recently become available (Martz and Parker 2010; Ramezani et al. 2011; Martz et al. 2012). It has long been recognized that two distinct vertebrate faunal assemblages occur in the Chinle Formation of Arizona, the Adamanian and the Revueltian (e.g., Heckert and Lucas 2002); however, the exact stratigraphic position of the turnover was ambiguous (Parker 2006). The recent lithostratigraphic revisions of the Chinle Formation at PEFO by Martz and Parker (2010) allow the precise placement of the Adamanian–Revueltian (A-R) boundary in the lower portion of the Lots Wife beds of the Sonsela Member (Parker and Martz 2011). Furthermore, a distinctive floral change also occurs at the same stratigraphic position (Reichgelt et al. 2013). These new data, along with the newly established geochronologic framework (Ramezani et al. 2011), allow the determination of possible causal events related to this biotic turnover. In this paper, we supplement the geochronologic framework of Ramezani et al. (2011) with two new U-Pb zircon ages from key stratigraphic intervals. The resulting age model and associated strati- graphic and paleoclimatic record is the most refined ever established for the Chinle Formation. From this record, the history of cyclic environmental change and corresponding biotic turnover is temporally correlated to the Late Triassic tectonic history and physiographic evolution of western Pangea. As such, this paper sheds light on the previously undetected balance between life, paleogeography, and tectonism in the Late Triassic of the southwestern United States. PALEOGEOGRAPHIC SETTING The Chinle Formation was deposited in fluvial and lacustrine environments in a large backarc basin that extended across what is presently the Colorado Plateau of the southwestern United States (Stewart et al. 1972; Blakey and Gubitosa 1983, 1984; Dubiel et al. 1991; Dubiel 1994; Dubiel and Hasiotis 2011; Cleveland et al. 2007) Published Online: January 2014 Copyright E 2013, SEPM (Society for Sedimentary Geology) 1527-1404/13/083-1147/$03.00