Orbital pacing of redox cycles suggested by red bed color of the Late Triassic Chinle Formation, Arizona

Late Triassic records of the orbital pacing of climate are well documented from the stratigraphy of lake basins and marine facies. However, fewer studies have focused on detecting orbital climate signals preserved by fluvial depositional environments, home to terrestrial life. The sedimentary Chinle Formation of the Colorado Plateau (southwestern USA) is a succession of Late Triassic largely red beds that preserves numerous vertebrate fossils, including evidence of the Adamanian–Revueltian tetrapod faunal transition. Floodplain mudstones showing pedogenic features alternate on various thickness scales with channel sandstones. We assessed the cyclostratigraphy of red bed color for a ~250-m-thick interval of the Chinle Formation dated to 209-216 Ma using a scientific drill core from the Petrified Forest National Park, Arizona, 1A. Diffuse reflectance spectroscopy demonstrates that red bed color in this core derives from the mineral hematite, probably formed in response to the wetting and drying of soils under monsoonal rainfall. The magnetochronology and high-precision U-Pb detrital zircon dates of the core, and the astrochronostratigraphic polarity time scale of the Newark-Hartford basins are used to provide an age model for our spectral analyses and cyclostratigraphy. From the red-green and yellow-blue time series, we identified evidence of the long eccentricity, Jupiter-Venus cycle (405 kyr), longer-period grand eccentricity cycles including the Mars-Earth cycle, and possibly the Mars-Earth inclination cycle. There are also hints at higher frequency cycles. Although the relative amount of 405 kyr power is a fraction of the total variability, there is significant coherence between the Newark Basin depth rank record and the Chinle color at the 405 kyr and the ~100 kyr cycles. Our findings support previous interpretations that color and hematite variations formed during the Late Triassic and are unrelated to a younger diagenetic component of the red beds. Fluvial accumulation of the Chinle sediments was not as discontinuous as other studies have suggested, allowing for a reconstruction of orbital climate changes that may have affected the development of terrestrial ecosystems in Western Equatorial Pangaea.