LA FLORA TRIÁSICA DEL GRUPO EL TRANQUILO, PROVINCIA DE SANTA CRUZ (PATAGONIA). ESTRATIGRAFÍA

Abstract

The El Tranquilo Formation is formally elevated to Group status and is subdivided into the Cañadón Largo and Laguna Colorada Forrnations from the base to top respectively. The Cañadón Largo Formation (Late Ladinian-Early Carnian) is about 550 m thick, consisting dominantly of tuffaceous sandstones, minor clast-supported conglomerates and mudstones with an abundant Dicroidium flora. The facies assemblages suggest that the Cañadón Largo Formation was originated in moderate to high sinuosity fluvial channels which flowed through a broad and intensely vegetated floodplain. The overlaying Laguna Colorada Forrnation is 170 m thiek and shows a wider geographic distribution. From northeastern to southwestern it uncomfortably overlies progressively younger stratigraphic units ranging from Late Perrnian (La Juanita Formation) to Late Middle Triassic. The Laguna Colorada Formation is made up of reddish to brownish siltstones and claystones, medium to fine sandstones and subordinate thin conglomerates that have been deposited in moderate sinuosity fluvial systems. The overbank deposits also contain a Dicroidium flora as well as the hatched eggs of dinosaurs and large saurisquian remains (Prosauropoda). On the basis of both the stratigraphic relationships with the Leona intrusive of Liassic age (191 ± 10 Ma) and paleontological data the Laguna Colorada Formation is assigned to the Norian age.