New sporomorph-based paleoecological analysis of the Cerro de Las Cabras Formation (Middle Triassic, Mendoza, Argentina) by the Eco-Guild model: paleoenvironmental and paleoclimatic implications

RUFFO REY, LAUTARO JULIÁN

Ciudad Autónoma de Buenos Aires

Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleontológica Argentina

The Cerro de Las Cabras Formation provides a great opportunity for advancing the understanding of Middle Triassic paleoclimates and terrestrial ecosystems. Previous palynological studies suggest this unit developed in a humid and warm environment, however, other lines of evidence would indicate rather semi-arid conditions during its deposition. In order to test the first hypothesis, these same palynofloras are re-studied with a new methodological approach. The presented Eco-Guild model provides a conceptual framework for identifying the main abiotic factors that rule vegetation dynamics and composition. It incorporates the moisture requirement, temperature preference, adaptive strategy and body size of parent plants into the context of ecological successions for their statistical analysis. The paleoecological re-study of palynofloras led to the identification of four palynophases (Woodland “A”, “B”, Riparian Forest “A”, “B”, Palustrine, lacustrine “A”, “B”) related to two distinct plant zonations: a riverine zonation, distinguished by ruderal plants living in ephemeral to intermittent fluvial streams, characterized by Caytoniales, Peltasperms, and herbaceous conifers, and a lacustrine zonation which is subdivided into a stress-tolerant plant community (composed mainly by Cycadales, Equisetales, Selaginellales and Pleuromeiales), related to standing water bodies of alkaline-saline conditions, and a plant association linked to small freshwater bodies in the surrounding floodplain with ferns (Osmundales and Filicales), bryophytes (Sphagnaceae) and aquatic algae. Around these wetland areas were mesophytic forests of Umkomasiales and Voltziales forming the upper canopy, while Caytoniales and Filicales would have occupied the understory below. Pinaceans and Podocarpaceans likely inhabited the highlands, constituting the source area for micro-mesothermic palynotaxa. On an individual scale, this whole ecosystem developed in a highly stressed environment but under long-lasting stability conditions. However, in stratigraphic terms, it developed under highly fluctuating climatic circumstances. Furthermore, at the middle part of the studied section a significant pulse of moistness was detected, which was correlated to a major humidity episode occurred during the late Anisian–early Ladinian in central-western Argentina. These results would confirm for the first time the presence of this climatic event in the Uspallata Group (Cacheuta Sub-basin), thereby improving the knowledge regarding the paleoclimatic evolution of southwestern Gondwana during the Middle Triassic.