Vegetation dynamics of Cerro de Las Cabras Formation (Middle Triassic) from palynological assemblages and the Eco-Guild model: New paleoenvironmental and paleoclimatic interpretations

Abstract

The Cerro de Las Cabras Formation provides a great opportunity for advancing the understanding of Middle Triassic paleoclimates and terrestrial ecosystems. Previous palynological studies conducted at the type locality suggest it developed in a humid and warm environment, however, other lines of evidence would indicate rather semiarid 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 characterized by ruderal plants living in ephemeral to intermittent fluvial streams, and a lacustrine zonation which is subdivided into a stress-tolerant plant community related to standing water bodies of alkaline-saline conditions, and a plant association linked to small freshwater bodies in the surrounding floodplain area. Around these wetland areas were mesophytic forests of gymnosperms. On an individual scale, this whole ecosystem developed in a highly stressed environment but without major disturbance episodes. 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 would be coincident 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.