VALIDITY OF PLAYA LACUSTRINE SYSTEMS BASED ON EVAPORITE PRESENCE WITHIN THE SONSELA MEMBER OF THE CHINLE FORMATION, PETRIFIED FOREST NATIONAL PARK, ARIZONA

Late Triassic in age, approximately 225.0 – 207.8 Ma, the Chinle Formation of northeastern Arizona is comprised of the Mesa Redondo Member, Blue Mesa Member, Sonsela Member, Petrified Forest Member, Owl Rock Member, and Rock Point Member. The Chinle Formation is interpreted as a combination of lacustrine basins, deltas, playa mudflats, floodplains, swamps, small lakes and fluvial channels. These units were deposited during a climatic transition from tropical to subtropical to an increasingly hot and arid environment. Espegren (1985) suggests that the Martha’s Butte Bed within the Sonsela Member of the Chinle Formation contains interbedded mudstones that may potentially represent playa lakes. The Sonsela Member (223.0 – 211.0 Ma) is interpreted as interbedded mudstones and sandstones including traces of conglomerate and siltstone. A thorough literature review, field analysis, and laboratory analysis followed by data interpretation was conducted to further validate or refute Espegren’s proposal. Field work was conducted within Petrified Forest National Park (PEFO), on a private residential location outside of PEFO, and at an outcrop exposure along Rt. 191 in Arizona. At each Sonsela Member locality, data was collected on the sedimentology, stratigraphic context, and abundance of evaporite minerals. Collected mineral samples were taken back to the SCSU Earth Science Department where a Rigaku Miniflex II Desktop X-Ray Diffractometer was used to identify the chemical compositions. Interpretation of the XRD data indicates the presence of evaporite minerals, dominantly Calcite with Magnesium/Magnesian (Dolomite), Gypsum, and minor Barite, suggesting there is evidence to support the presence of playa lakes during the Late Triassic. This also further validates previous paleoenvironmental interpretations of extended periods of aridity during deposition of the Sonsela Member.