Palaeoenvironments and palaeoclimate of the uppermost Cuyo Group and lowermost Lotena Group at Quebrada Álvarez, Picún Leufú Sub-basin, Patagonia, Argentina: a preliminary study based on palynology

The palynological (palynostratigraphical/palynofacies) analysis of samples from the Middle Jurassic outcrop at the Quebrada alvarez section located in the Neuquen Basin, Patagonia, Argentina, allowed us to begin to evaluate the palaeoecological significance of the recovered palynofloras and their importance in the evolution of these ecosystems, during the transition between the uppermost Cuyo Group and the lowermost Lotena Group. The Lajas Formation, characterised by the palynofacies type A, could be interpreted as a tide-modified delta front environment. The high abundance and diversity of the palynomorphs identified in the studied samples, contribute to improve the palynological knowledge of the Challaco Formation (palynofacies type B - F). Considering the palaeoecological requirements of the recognised plant families, relatively humid and warm climate conditions could be inferred during the deposition of this unit. Also, the 'seasonally dry (winterwet)' biome was interpreted for the first time in the Neuquen Basin. Based on selected key taxa a late Bathonian-early Callovian age is proposed for the Challaco Formation at the Quebrada alvarez section. The palynological material recorded in this unit suggests the development of a lacustrine environment with cycles of relative contraction and expansion of the water body due to fluctuations between relatively dry and wet conditions. The dominance of phytoclasts, mainly opaque particles, and freshwater algae (Botryococcus) in the Bosque Petrificado Formation (palynofacies type G - I) could suggest the development of a freshwater to brackish lacustrine environment. Based on the dominance of the same group of palynomorphs (chlorophyte algae) in the Challaco and Bosque Petrificado formations, similar palaeoenvironmental conditions would be inferred for these units. From a lithofacies analysis point of view, the Challaco and Bosque Petrificado formations show isopic facies associations in the study locality.

Automated summary

The palynological analysis of samples from the Quebrada Alvarez section in the Neuquen Basin, Patagonia, Argentina, provides insight into the palaeoecological significance of palynofloras and their role in the evolution of Middle Jurassic ecosystems in the region. The results suggest the presence of a tide-modified delta front environment in the Lajas Formation, a relatively humid and warm climate during the deposition of the Challaco Formation, and the development of a lacustrine environment with fluctuating water levels in the Bosque Petrificado Formation. Lithofacies analysis indicates similar facies associations between the Challaco and Bosque Petrificado formations in the study locality.