CLIMATE CONDITIONS IN THE CONTINENTAL WESTERN GONDWANA MARGIN DURING MIDDLE-UPPER JURASSIC

The thick volcano-sedimentary filling of the Cañadón Asfalto basin represents one of the most important records of continental Jurassic in South America and occurs in three transtensional sub-basins with asymmetric half... [ view full abstract ]

The thick volcano-sedimentary filling of the Cañadón Asfalto basin represents one of the most important records of continental Jurassic in South America and occurs in three transtensional sub-basins with asymmetric half grabens, where carbonate lacustrine systems of the Cañadon Asfalto Formation developed. This paper analyses the multicycle carbonate events as indicators of climate variations in the Cerro Cóndor sub-basin (Chubut province, Patagonia, Argentina), The lower member (Las Chacritas Mb), maximum 180m thick, is characterized by cycles of massive and stromatolite limestones, shales, tuffs, tuffites, sandstones and conglomerates interrupted by basalt flows, dykes and sills. Facies analysis defined a uniform cycle pattern related to the paloegeography, architecture and presence of basalts in this sub-basin. Carbonate cycles represent environments of: nearshore (packstone/bioclastic wackestone), mudflat (mudstones, microbialitic mudstones and storm episodies) and palustrine (paleosols and evaporites). The deepest lacustrine deposits: Asfalto, Las Chacritas, Los Loros creeks and Pichiñanes range are affected by basalt flows. These sections show three carbonate facies cycles. The Cañadon Asfalto type locality is the only one including a complete cycle starting from offshore environment shales. The other localities show incomplete cycles; in Las Chacritas and Los Loros the cycles correspond to nearshore, mudflat and palustrine environments, and in Pichiñanes range the cycles are composed of mudflat and palustrine. Two incomplete cycles (mudflat and palustrine) were identified in Miyanao, Lahuinco, Carrizal and Calcareo creeks. Evaporate levels in Carrizal creek indicate pan-lake desiccation. Only one cycle (mudflat and palustrine) is recorded in Sierra La Manea locality. The paleontological record of the Las Chacritas Mb includes terrestrial and freshwater palynomorph, conchostracans from perennial and stable lacustrine environments (Euestheria volkheimeri, “Lioestheria” sp. A, C, “L.” patagoniensis, ?E. taschi, and Eosestheriidae), bivalves (cf. Diplodon, Sphaeridae? And Corbiculidae?), ostracods (Darwinuloidea, Penthesilenula magna fauna). δ13C y δ18O values present a positive correlation (r ≥ 0.64) in agreement with the paleolake hydrological history characterized by restricted conditions with sporadic water discharges. The volcanic events would have released CO2 emissions contributing to the regional warming; additionally the basalt flows would have formed a local, low-altitude, toxic haze of SO2 favourable to evaporite levels formation. Muticyclic carbonate sequences and fossil records correspond to warm temperature and the climate belt which includes subtropical seasonal dry during Middle-Upper Jurassic in the western Gondwana margin. Expansion and contraction episodes of water bodies in the Cañadón Asfalto basin, located under 30ºS paleolatitude would be related to climate seasonality.