First occurrence of charophyte algae from a Triassic Paleolake in Argentina and their paleoenvironmental context

Abstract

This paper presents the first record of charophytes from the Triassic sedimentary rocks of Argentina and the entire southern hemisphere. Algal remains were found in the Cerro Puntudo Formation dated as Anisian (243.8±1.9 Ma). The formation is part of the Cuyana Basin, which formed as consequence of the Triassic rifting of Pangaea. The paleoenvironments represented by this succession include from bottom to top: (1) a proximal to medial alluvial fan system; (2) a distal alluvial fan deposit; (3) a braided fluvial system with small channelized bodies; (4) a distal fluvial system containing rare carbonates; and (5) a carbonate-rich lacustrine system. The lacustrine facies association corresponds mainly to carbonate microbialites with three recognized facies, namely disrupted micritic limestones (Lmd), stromatolitic limestones (Ls), and oncolitic limestones (Lo). The Lo facies is rich in oncolites formed by filamentous algae in the cortex and charophyte remains (gyrogonites) in the nucleus and is interpreted as being deposited in a carbonate-rich pond environment, perhaps sourced from springs. Filamentous algae are thought to cause the precipitation of carbonates and trigger the development of micritic coatings in the ponds. Sedimentary and paleobiologic evidence suggest minor movement of the oncolites in a ponded environment where charophytes developed. The charophyte remains found in the Cerro Puntudo Formation are gyrogonites attributed to the Porocharaceae family. All the previous records for Triassic Charophyta correspond to Laurasia in the northern hemisphere, therefore, the paleolatitudinal location of this finding allows the expansion of the paleobiogeographic distribution of the group into Gondwana.