INSTITUTO DE GEOCRONOLOGIA Y GEOLOGIA ISOTOPICA
Unidad Ejecutora - UE
congresos y reuniones científicas
CONTINENTAL AQUATIC ENVIRONMENTS OF THE JURASSIC IN EXTRAANDEAN PATAGONIA
CABALERI, N. , VOLKHEIMER, W., ARMELLA, C., GALLEGO, O., SILVA NIETO, D. , PAEZ, M AND CAGNONI M.
Congreso; XII Reunión Argentina de Sedimentologia; 2008
Asociación Argentina de Sedimentología
Some key aspects of the environmental dynamics of the lacustrine systems of the Cañadon Asfalto Basin (Jurassic of Patagonia) are presented. In the region of the middle Chubut River valley the lacustrine sedimentation began in the early Middle Jurassic in a context of semigraben filling, related with rifting as an expression of extensional tectonics during the rupture of western Pangaea. The environmental dynamics of these central Patagonian depositional centers are reflected in repeated changes of biofacies and sedimentary facies. During the Middle Jurassic, after each of the repeated basaltic and pyroclastic events, low diversity pioneer biotas installed, while during the earliest Cretaceous, in a context of the diminishing environmental stress and higher geodiversity, a higher diversity of the biota could develop. The case study of the Cerro Condor area shows 1) a deep lacustrine facies association at north, less affected by climatic variations, and eulittoral to marginal conditions, when the basin was filled, with development of microbial mats (stromatolites), conchostracans (Euestheria taschi, Lioestheria sp. and Eosestheriidae), mud cracks and even association with a pan lake represented by deposits of limestone, dolomitic limestone and gypsum levels. 2) More to the south (Cañadòn Lahuincò) water bodies were shallower, rich in conchostracans (Euestheria volkheimeri, Lioestheria patagoniensis and Eosestheriidae). They also include pelecypods (cf. Diplodon), ostracods (Darwinuloidea) and abundant coccal green algae (Botryococcus). In areas of influx of running water, non-planctonic green algae of Spirogyra-type are frequent (Ovoidites spp.). The palustrine environment of the southern facies is well represented in Cañadòn Asfalto and Cañadòn Lahuincò. Generalized warm paleotemperatures during the whole time considered in this study are indicated by the high frequency and constant presence of Classopollis, the pollen of the thermophile Cheirolepidiaceae, a group of Mesozoic conifers which inhabited areas with well drained soils near the water bodies. Warm climatic conditions are also indicated by the high frequency and the considerable thickness of lacustrine biohermal limestones. Seasonal climatic conditions are reflected by the constant migration of the marginal zone of the paleolakes, exemplified by the case study of Cañadòn Las Chacritas.