Stratigraphic distribution and significance of a 15 million-year record of fusain in the Upper Triassic Chinle Group, southwestern USA

• Stratigraphic distribution of fusain in Chinle Group spans 15 million years.
• Mainly attributed to conifers
• Wildfire occurrence not rare during Late Triassic
• Supports models of robust Late Triassic pO2

We document the occurrence of Upper Triassic fusain in northern Arizona, southern Utah and northern New Mexico in latest Carnian(?) to Norian-age alluvial strata of the Monitor Butte Formation and the Sonsela and Painted Desert members of the Petrified Forest Formation (Chinle Group). The fusain, identified by standard techniques of macroscopic and microscopic morphology, and resistance to chemical oxidation, is present at multiple stratigraphic horizons at several locations that are approximately correlative, although separated by several hundred km. The morphology of this fossil charcoal includes large, partially charred silicified to coalified logs, completely charred smaller limbs, charcoal fragments reworked in fluvial deposits, and particulate fusain disseminated in sediments. The apparent rarity of fusain in Triassic strata previously was cited as evidence for relatively low levels (compared to modern) of atmospheric oxygen and the consequent infrequent occurrence of wildfire during this time interval. However, our findings demonstrate a significantly greater abundance of fusain in Upper Triassic strata than previously acknowledged. Therefore, we suggest that wildfire was not an unusual occurrence during the Late Triassic. This conclusion supports recent experimental studies and geochemical models that indicate atmospheric oxygen levels at or above modern levels during the Late Triassic.