In search of the Triassic-Jurassic boundary: Palynostratigraphy and carbon-isotope stratigraphy of the lower Dinosaur Canyon Member on the Colorado Plateau (Kanab, Utah)

The Triassic-Jurassic boundary remains an enigmatic event due to a lack of conformable, terrestrial sections. On the Colorado Plateau the Triassic-Jurassic boundary has been placed within the J-0 unconformity at the Chinle Formation-Glen Canyon Group contact. Recent tetrapod, palynofloral, and magnetostratigraphic data has suggested the J-0 unconformity is of Triassic age, and the Triassic-Jurassic boundary is preserved within the Dinosaur Canyon Member of the Moenave Formation (Glen Canyon Group). This study presents palynofloral and stable carbon-isotope data from a mudstone interval within the lower Dinosaur Canyon Member of the Moenave Formation on the Colorado Plateau at Kanab, Utah. Palynofloral analysis has yielded a high diversity palynofloral assemblage containing many bisaccate and monosaccate palynomorphs followed by a low diversity palynofloral assemblage containing greater than 90% Classopollis meyerianus. The high diversity palynofloral assemblage has yielded the Late Triassic index palynomorphs Camerosporites verrucosus, Patinasporites densus, Patinasporites toralis, and Vallasporites ignacii. It also contains the earliest Jurassic index palynomorphs Pityosporites parvisaccatus and Rugubivesiculites species 303, which according to this study are not Jurassic index palynomorphs. The high diversity palynofloral assemblage is separated from the low diversity palynofloral assemblage by a 60% species turnover (18 of 30 species), which corresponds to a rapid extinction event. The greater than 90% Classopollis meyerianus correlates to the basal Jurassic Classopollis meyerianus Palynofloral Zone of the Newark Supergroup and Europe. Thus, the Triassic-Jurassic boundary is located within conformable, terrestrial strata of the lower Dinosaur Canyon Member and not the J-0 unconformity. Stable carbon-isotope analysis has yielded one major negative isotopic excursion of -2&permil in organic carbon with the possibility of another negative excursion. Whether this excursion corresponds to the initial or main excursion, or if oxidation has overprinted the signal cannot be determined at this time. Higher resolution sampling is needed to check for other excursions and their positions, as well as sampling higher and lower than the mudstone interval. More terrestrial boundary sections, especial those far removed from the opening of the Atlantic Ocean and the flood basalts of the Central Atlantic Magmatic Province (CAMP) are needed to determine extinction event timing and the timing of evolutionary events of certain flora. This could also provide much need data and a new marine-terrestrial correlation to the Global Stratotype Section Boundary and Point (GSSP) candidate at New York Canyon, Nevada.