Revisiting the upper Triassic Portezuelo Bayo Formation: Sedimentological and paleontological perspectives from Mendoza, Argentina

Abstract

This study focuses on the Portezuelo Bayo Formation (Upper Triassic, Cuyana Basin) through a multidisciplinary approach that includes: petrographic analysis of volcanic and pyroclastic rocks at its base, sedimentological characterization of facies associations and depositional subenvironments, preliminary paleoclimatic reconstruction based on clay mineral assemblages, and documentation of new macrofossil plant remains and palynomorphs. Volcanic deposits from the underlying Agua de la Zorra Formation indicate both explosive and effusive volcanic activity, including pyroclastic flows, ash fall deposits, and basaltic lava flows, which likely contributed to the filling of the Agua de la Zorra paleolake. The Portezuelo Bayo Formation is interpreted as a fluvial system with well-developed floodplains and single-to multi-story channel deposits. Clay mineral analysis suggests that deposition occurred under seasonal climatic conditions, alternating between warm, humid phases and more arid seasons. The paleobotanical assemblage includes in situ logs, impressions, and compressions of leaves, fronds, and stems, along with opaque phytoclasts and sparse palynomorphs dominated by fern spores and conifer pollen. This vegetation is indicative of a forest ecosystem dominated by Umkomasiales and conifers, with an understory of ferns and monotypic horsetail (Equisetales) communities in associated areas. This integrated study provides valuable insights into the depositional, climatic, and ecological conditions of the Portezuelo Bayo Formation, contributing to a more comprehensive understanding of Triassic paleoenvironmental dynamics within the Cuyana Basin.