U-PB TIMS-TEA GEOCHRONOLOGY AND A NEW CHRONOSTRATIGRAPHY FOR THE CAÑADON ASFALTO BASIN, CENTRAL PATAGONIA

Abstract

The Cañadón Asfalto basin in Chubut province, central Patagonia, is characterized by fluvial and lacustrine sediments, intercalated with basaltic flows and tuffaceous beds. Samples collected from these sediments with negative δ13C and positive total organic carbon (TOC) excursions would suggest a possible connection with the Toarcian ocean anoxic event (TOAE), which occurred at ~183 Ma. This event occurred simultaneously with the eruption and emplacement of the Karoo–Ferrar large igneous province (LIP) and the Pliensbachian–Toarcian boundary (PToB) and extinction event. Previous 206Pb/238U analyses have placed the maximum deposition age for the Cañadón Asfalto at 178.77 ± 0.09 Ma. However, the samples from this study were collected from stratigraphically lower primary tuff beds interposed between the organic-rich sediments. New U-Pb TIMS-TEA analyses of zircons from these ash beds have augmented the chronostratigraphic framework for the Cañadón Asfalto. 206Pb/238U dates suggest an updated maximum deposition age for this formation of 179.48 ± 0.10 Ma. The sediments of the Cañadón Asfalto therefore likely do not contain evidence of the TOAE. Nevertheless, this formation overlies the lithologically distinct Lonco Trapial Formation, the deposition of which is now bracketed between a previous date of 188.95 ± 0.10 Ma and our date of 179.48 ± 0.10 Ma, both determined by 206Pb/238U geochronology. Our data thus support the notion that the formation of the Cañadón Asfalto took place from the mid-Toarcian to the late-Jurassic and that the PToB in central Patagonia would likely exist within the Lonco Trapial formation, if it is observable in terrestrial sediments.