Early and Middle Jurassic environmental perturbations in south-western Gondwana: An example from the Neuquén Basin, Argentina

Abstract

The Sierra Chacai Co and Los Molles formations in the Neuquén Basin, Argentina, are expressions of the first widespread marine transgression from the Pacific Ocean occurring in the Early to Middle Jurassic Period and provide a unique sedimentary record. Because of the presence of continuous and well-exposed outcrops, the basin has the potential to shed light on the global extent of the carbon isotope events (CIEs), as well as to test the large-scale occurrence of other environmental events so far only documented in western Tethys. The Neuquén Basin in Argentina is one of the rare places in south-western Gondwana where strata deposited during the Toarcian Oceanic Anoxic Event (T-OAE) have been documented. Here, we present the results of a multidisciplinary study integrating geochemical (δ13Corg, TOC, major and trace elements) and calcareous nannofossil data from a novel section in the southern part of Neuquén Basin. The new data set allowed the identification of the T-OAE time-equivalent besides two additional significant negative carbon isotope excursions distributed across the middle Toarcian and Toarcian-Aalenian intervals. Six calcareous nannofossil zones and several subzones were recognised, covering the Pliensbachian-Aalenian interval, allowing to place the δ13C excursions within a precise stratigraphic framework and correlate them to other records worldwide. The new geochemical and calcareous nannofossils data provide the most continuous and time-extended record for the Neuquén Basin and the SW Gondwana. Additionally, these results support the global extent of CIE of the T-OAE and the presence of other Early to Middle Jurassic isotopic excursions. These findings illustrate the utility of combining organic carbon stable isotopes and calcareous nannofossils for global age correlations and palaeoenvironmental reconstructions in the evolution of the marine community during major, long-term environmental changes.