The palynology of the Sonsela member (Late Triassic, Norian) at Petrified Forest National Park, Arizona, USA

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.