The type locality of the Cañadón Asfalto Formation revisited: new stratigraphic and paleoenvironmental considerations

SCASSO, ROBERTO; ESCAPA, IGNACIO; CÚNEO, N. RUBÉN; RAMEZANI, JAHANDAR; FANTASIA, ALICIA; KARL B. FÖLLMI; THIERRY ADATTE; JORGE E. SPANGENBERG; , BLAIR SCHOENE

Buenos Aires

Simposio; VII Simposio Argentino del Jurásico; 2019

The Cañadón Asfalto Creek is regarded as the type locality of the Cañadón Asfalto Formation and shows a well-exposed, thick and complex Jurassic succession along the Chubut river. In spite of several authors worked on it, the section hasn`t been thoroughly studied. New studies allow dividing the Cañadón Asfalto Formation in four members in the type locality and U-Pb geochronology provides additional constraints on the temporal framework. Several dated tuffs suggest that deposition of the Cañadón Asfalto Formation started and ended within the Toarcian. The rocks were accumulated in rift-related, varied lacustrine environments which were intermittently subjected to volcanic input, mainly represented by andesitic tuffs and basandesitic lavas, intercalated with shales, limestones, and sandstones. This filling is strikingly similar to that currently taking place in the African Great Rift Valley. Steep slopes produced by episodic tectonism and volcanism produced numerous lowangle internal discontinuities representing short non-deposition or erosion lapses. Wedgeshaped sedimentary bodies reveal marked lateral changes and rapid, localized sedimentation in small depocenters and explain the contrasting thicknesses on surface sections and exploration wells. Episodic sedimentation was characterized by the alternation of epiclastic, calcareous and volcaniclastic deposition at different temporal scales, from short term lake level/seasonal variations to long term, basinwide, paleogeographic changes. Paleobotanical and palynological data suggest a warm, highly seasonal (dry and wet cycles) climate, consistent with a winter-wet biome. The flora is dominated by lycophytes, equisetaleans, ferns, conifers, cycadaleans and seed ferns. Abundant charcoal particles suggest frequent forest fires. Shallow water, coastal stromatolites provided large amounts of algal remains to the deeper parts of the lake. However clastic and volcaniclastic pulses promoted delta formation and gave way to frequent hyperpicnial and turbidity flows in deeper parts of the lake. Geochemistry of black shales reveal marked changes from lacustrine to terrestrial organic matter supplied to the lakes. HI and  13Corg values suggests that the type and preservation of organic matter determined the carbon-isotope ratios. High Mo and V values coincide with high TOC and HI values, suggesting that elevated primary productivity and/or oxygen-depleted bottom water were favourable to organic matter preservation. Our research provides an outlook of the paleoenvironmental conditions prevailing on land during the late Toarcian in the proximity of the Karoo-Ferrar and Chon Aike large igneous provinces, and also on how volcanism exerted a major control on the depositional conditions.