Paleobotany of the Upper Triassic Los Rastros Formation, Ischigualasto–Villa Unión Basin, La Rioja, Argentina

Abstract

The Los Rastros Formation was deposited in a deltaic-lacustrine environment during the Late Triassic. Plant remains from the Gualo area were systematically collected and compared with other localities from the Los Rastros Formation concerning the sedimentary evolution of the basin. Subsequently, the Los Rastros paleoflora was contrasted with other Carnian (Upper Triassic) units from Gondwana in a paleobiogeographic context using statistical methodologies: Raup-Crick similarity index and hierarchical cluster analysis. The systematic study revealed 14 taxa for the Los Rastros Formation at the Gualo area. Plant vegetative remains of Equisetales (Neocalamites sp.), Osmundaceae (Cladophlebis sp. cf. C. mesozoica), Umkomasiales (Dicroidium odontopteroides, Johnstonia stelzeriana, Xylopteris argentina, X. elongata, and Zuberia feistmantelii), Ginkgoales (Baiera pontifolia, Sphenobaiera sp. cf. S. argentinae), Voltziales (Heidiphyllum elongatum), Coniferales (Rissikia media), and gymnospermous reproductive structures (Cordaicarpus sp., Samaropsis sp. and strobilus sp. A). The stratigraphic distribution of taxa throughout the Los Rastros Formation in the Gualo section is variable. Diminution of taxa recorded observed across the section could be explained by the isolation of the paleolake product of the activity of a secondary fault. The plant taxa registered at the different localities of the Los Rastros Formation could be related to the location of each locality in the basin and the different depositional conditions during the infill history of the basin at each locality. Comparison among Los Rastros Formation plant records with other paleofloras from Gondwana by statistical analyses indicates differences with respect to a previous paleobiogeographic proposal.