Late Triassic climate reconstructions derived from sandstones, petrified woods and paleosols of the Chinle Formation at Petrified Forest National Park, Arizona.
Access changed 8/24/22
The Late Triassic Chinle Formation of the Petrified Forest National Park (PEFO) provides a well-preserved Late Triassic archive of climate information, and the sandstone, petrified wood and paleosol of Chinle Formation were studied by using sedimentologic, geochemical, stratigraphic and petrographic criteria. Fluvially derived tuffaceous Chinle sandstones from PEFO provides new insight into the depositional history and evolution of palaeoclimate during Chinle deposition, and this study has investigated the relationship between climate and meteoric diagenesis of sandstones as a guide for constraining climate change in western equatorial Pangea during the Late Triassic. Based on a well-constrained stratigraphic and geochronological framework of sandstones, the diverse petrological characteristics of wide range of textural attributes and shallow burial diagenetic features are indicative of the evolving Late Triassic climate. The meteoric diagenetic features of Chinle sandstones indicate a progression from wet to dry conditions during the Late Triassic. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, along with stable carbon and hydrogen isotopic (δ13CW and δDW) analyses, have been applied to study organic matter (OM) of the Chinle Formation petrified wood at the PEFO. Solid state 13C NMR results reveal that OM composition of wood samples was dominated by altered lignin structures. Stable hydrogen isotopic analysis suggests that the decomposition of OM is the controlling factor of δDW. In contrast, δ13CW and mean annual precipitation anomaly (MAPA) analysis indicates that the variation of stable carbon isotopes documented by petrified wood agrees well with the timing of the documented climatic trend in both continental and marine records. Pedogenic clay mineral δD and δ18O records in addition with mineralogic data from paleosol samples have been used to characterize the paleotemperature evolution at PEFO. Most clay samples are dominated by mixtures of smectite and kaolinite though some samples consist of trace amounts of illite. Estimated paleotemperatures indicates clay mineral crystallization temperatures ranging from 20.0 °C to 56.1 °C. In particular, the stratigraphic trend of calculated temperatures suggests that mean annual temperatures (MAT) may have been up to 7.9 °C warmer than those of after the mid-Norian in the Late Triassic.