This is a new study of the palynology of the Chinle Formation of Petrified Forest National Park that fills in a previous sampling gap caused by reevaluation of the local stratigraphy. The study finds three pollen assembalge biozones that correspond to the two zones previously recovered by Litwin et al. (1991). Two main points of the paper are: 1) these faunal zones no longer pertain to the Carnian/Norian boundary as recent dating of the Chinle Formation in conjunction with a recalibration of the Late Triassic timescale demonstrates that the Chinle Formation is Norian-Rhaetian in age (Irmis et al., 2011; Ramezani et al., 2011); 2) the floral turnover roughly corresponds stratigraphically with the faunal turnover precisely located in the park by Parker and Martz (2011). Potential causal mechanisms remain under investigation.
Abstract - Recent paleontological investigations and lithostratigraphic revisions reveal a marked biotic turnover zone within the continental deposits of the Sonsela member of the Chinle Formation (Late Triassic, Norian) at Petrified Forest National Park, USA. Within the Sonsela member we found three pollen assemblage biozones: Zone II (90.5–94 m above the Mesa Redondo member) contains a relatively diverse palynological assemblage, with a mix of pteridosperms, voltzialean and some Mesozoic conifers. Following a 2.3 m hiatus, Zone IIIa (96–97.5 m) is characterized by a decrease in pteridosperms and Mesozoic conifers and a drop in voltzialean conifer diversity. The alleged voltzialean conifer pollen Klausipollenites gouldii was dominant in this part of the assemblage and a significant rise in spores and cycad pollen was also evident. In Zone IIIb (97.5–98.5 m) diversity increases and several taxa, which were absent in Zone IIIa reappear, although K. gouldii remained the most abundant taxon. The transition between the palynological assemblages Zones II and IIIa coincide approximately (within a ~2.5 m interval) with a documented faunal turnover. The floristic assemblages suggest that the climate of the south-western United States during the Norian was most likely semi-arid and highly seasonal, despite being located at tropical latitudes, with aridification occurring towards the end-Triassic as the continent drifted northwards and global volcanism increased. The gymnosperm dominated palynofloral assemblage as opposed to the fern- and horsetail-dominated macrofossil record of the Sonsela member of the Chinle Formation, conforms to a semi-arid upland environment alternated by riparian, swampy lowland.
Irmis, R. B., Mundil, R., Martz, J. W., and W. G. Parker. 2011. High-precision U-Pb zircon ages from the Chinle Formation of New Mexico, USA: Implications for Late Triassic vertebrate biostratigraphy and the rise of dinosaurs. Earth and Planetary Science Letters 309:258-267.
Litwin, R. J., Traverse, A., and S.R. Ash. 1991. Preliminary palynological zonation of the Chinle Formation, southwestern
and its correlation to the Newark Supergroup (eastern U.S.A. ). Review of Palaeobotany and Palynology 68:269-287. U.S.A.
Parker, W. G., and J. W. Martz. 2011. Constraining the stratigraphic position of the Late Triassic (Norian) Adamanian Revueltian faunal transition in the Chinle Formation of Petrified Forest National Park, Arizona. Earth and Environmental Transactions of the Royal Society of Edinburgh 101:231-260.
Ramezani, J., Hoke, G. D., Fastovsky, D.E., Bowring, S. A., Therrien, F., Dworkin, S. I., Atchley, S. C. and L. C. Nordt. 2011. High precision U-Pb zircon geochronology of the Late Triassic Chinle Formation, Petrified Forest National Park (Arizona): Temporal constraints on the early evolution of dinosaurs. Geological Society of America Bulletin 123:2142-2159.