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.