Freshwater green algae and fungi from Upper Triassic strata of the Cuyana Basin, central-western Argentina: indicators of palaeoenvironment and petroleum source potential

Abstract

Freshwater green alga microfossils and dispersed fungal spores were recovered from among rich terrestrial palynofloras of the upper Potrerillos Formation, at the Quebrada del Durazno locality in the Cacheuta depocentre of the Cuyana Basin, Mendoza Province, central-western Argentina. Sedimentological data suggest that the Upper Triassic Potrerillos Formation comprises deltaic plain deposits with significant development of black carbonaceous shales and coal facies formed in semi-permanent swamps and/or ponds with intermittent flooding that progressed into the lacustrine Cacheuta Formation. These sub-environments enabled the development of autochthonous freshwater organic-walled green microalgae. Here, we present the first description of developmental stages for the colonial alga Botryococcus braunii (Chlorophyta, Trebouxiophyceae) from the Potrerillos Formation. The fossil algae have economic importance because they are the major organic constituents of oil shales in the Cuyana Basin. We further document (1) diverse zygospores of zygnematacean green algae (Charophyta) representing the genera Ovoidites and Schizosporis; (2) prasinophyte algae (Cymatiosphaera), sphaeromorph acritarchs (incertae sedis) and fungal representatives assigned to Portalites. A new species of Ovoidites is described, and a newly emended description and affinity are given for Greinervillites undulatus. Comparisons with modern freshwater algal habitats and sedimentology suggest that the zygnematacean zygospores indicate shallow stagnant water bodies, while abundant colonial green Botryococcaceae algae represent ponded water in paludal or lacustrine environments. The terrestrial vegetation and autochthonous freshwater green algae likely developed under warm temperate, humid to sub-humid, strongly seasonal megamonsoonal climatic conditions.