DETERMINING FLOODPLAIN PLANT DISTRIBUTIONS AND POPULATIONS USING PALEOPEDOLOGY AND FOSSIL ROOT TRACES: UPPER TRIASSIC SONSELA MEMBER OF THE CHINLE FORMATION AT PETRIFIED FOREST NATIONAL PARK, ARIZONA

The Upper Triassic Sonsela Member of the Chinle Formation is an alluvial succession containing interbedded sandstone and pedogenically modified mudstone. Despite preservation of silicified logs within channel sandstone beds, the Sonsela plant ecosystem is less understood than other intervals due to decreased preservation of nonconifer plant taxa. Sonsela paleosols and rhizoliths are evaluated using macromorphology, micromorphology, and geochemistry to determine the spatial distribution of paleosol characteristics and plant sizes and densities across the study area. Three pedotypes identified within the Sonsela are classified as Inceptisols and Vertisols that exhibit fining of matrix textures (from clayey siltstone to claystone) and reduced drainage with distance from the paleochannel. Overall, Sonsela paleosols are immature, suggesting that the Sonsela fluvial system experienced high rates of lateral migration and cannibalization of overbank sediments in a low-subsidence regime. Rhizohalos within the Sonsela Member are likely diagenetic and commonly include silicified roots (silica root petrifactions). Silicified roots provide information on root size and density that is not commonly afforded by other rhizolith types. Diagenetic rhizohalo diameters may be controlled by paleosol matrix textures within the Sonsela Member. Rhizolith characteristics suggest that channel-proximal paleosols contained only small-stature plants while distal floodplain paleosols may have hosted both small-stature and arborescent plants. Paleosols within the Sonsela Member do not contain rhizoliths whose size or abundance are reflective of a dense coniferous forest. Floodplain plants were commonly small of stature and immature, unable to evolve into more mature communities due to high rates of floodplain cannibalization during fluvial migration.