CHRONOLOGY OF JURASSIC FLORISTIC EVENTS IN THE CAÑADON ASFALTO BASIN, PATAGONIA, ARGENTINA

CÚNEO RUBÉN; ESCAPA IGNACIO; RAMEZANI JAHAN; SCASSO ROBERTO; ZAVATTIERI ANA AND ELGORRIAGA ANDRÉS

Padova

Simposio; European Paleobotanical Meeting EPPC 2014; 2014

Sociedad Paleobotánica Europea

One of the historical problems when dealing with terrestrial biotas is related with the lack of a reliable chronostratigraphy that allows for fine phylogenetic resolutions. This is the case in some Gondwana areas due to disconnection with nearby marine basins bearing time biomarkers. In order to tackle these constrains, an extensive geochronological analysis was carried out by means of U-Pb zircon ages. The obtrained high-resolution ages allow differentiation of at least four mainbiotic events from the Early to the Late Jurassic. The oldest event (late Sinemurian- early Pliensbachian) occurs previous to the oldest U-Pb date of 188.9 Ma. It records the first accepted presence of early cupressaceous conifers, the two main osmundaceous clades, the first clearly established marattialean ferns and diverse dipteridaceous, along with caytonials and Equisetum remains (with characters seen in the two modern subgenera). The second event occurred mostly during Pliensbachian times (188.9-178.8 Ma); some elements from the former event remain present, i.e. cupressaceous conifers, pteridosperms, same fern families plus Gleicheniaceae, and cycads; the novelty being the first appearance of basal Araucariaceae. The third event took place during the Toarcian (from 178.8 Ma) and probably until the Aalenian; it shows a change in composition of the conifers, in particular with the explosion of Cheirolepidiaceae (Classopollis maximum expression), along with Araucariacaeae and Cupressaceae and minor podocarps. Interestingly, a certain retreat in particular fern groups (e.g., Dipteridaceae) is seen, while cycads and pteridosperms remain abundant. The fourth event occurs at the early Late Jurassic (Oxfordian), around 157.4 Ma. Conifers are at its maximum diversity with the pair Araucariaceae-Cheirolepidaceae, occurring as permineralized remains. Ferns continue reduced (only recorded from palynological samples), with few equisetaleans and pteridosperms, suggesting dryer conditions that accentuated by the end of the event 1 million years later. The recorded plant events, which can be paralleled with coeval vertebrate ones, evidence a series of evolutionary stages in terms of plant phylogenies, including first records of family clades (Cupressaceae, Araucariaceae, Cheirolepidiaceae, Osmundaceae among others) that represent first appearances of modern looking plant lineages. In this context, permineralized, cutinized and compression floras from Central Patagonia introduce a novel source of information for better understanding in the evolution and paleobiology of the Jurassic vegetation.