Late Toarcian continental palaeoenvironmental conditions: An example from the Cañadón Asfalto Formation in southern Argentina
Graphical abstract
Introduction
The Jurassic period was marked by repeated carbon cycle perturbations associated with global climatic and environmental changes (e.g., Hesselbo et al., 2003; Dera et al., 2010; Littler et al., 2010). The impact of these palaeoenvironmental perturbations has been extensively examined in the marine records, whereas the response on the continents is comparatively poorly understood (e.g., Hesselbo et al., 2002; Bacon et al., 2011; Schnyder et al., 2016; Xu et al., 2017). This is likely due to a paucity of well-dated and complete successions and lack of reliable correlations between continental and marine strata.
The Cañadón Asfalto Formation (Fm) of the Cañadón Asfalto Basin in Argentina (Fig. 1) exposes continental sedimentary rocks, and it is particularly well known for the presence of organic matter-rich lacustrine strata and extraordinarily well-preserved invertebrates, vertebrates and flora (e.g., Frenguelli, 1949; Bonaparte, 1979; Escapa et al., 2008; Cúneo et al., 2013). Numerous studies have focused on the Cañadón Asfalto Fm, advancing our knowledge of its palaeontology (Tasch and Volkheimer, 1970; Bonaparte, 1986; Gallego et al., 2011), palaeobotany (Escapa et al., 2008), palynology (Volkheimer et al., 2008; Olivera et al., 2015), and sedimentology and depositional conditions (Cabaleri and Armella, 1999; Cabaleri and Benavente, 2013; Figari et al., 2015). Geochemical and mineralogical studies are still scarce, limiting our understanding of palaeoenvironmental conditions. The age of the formation has been variably constrained using palaeontological data (Frenguelli, 1949; Salani et al., 2007; Volkheimer et al., 2009), sequence stratigraphy (Figari et al., 2015), and radiometric data (Cabaleri et al., 2010; Cúneo et al., 2013; Bouhier et al., 2017; Hauser et al., 2017), leading to uncertainties concerning the precise temporal framework of the Cañadón Asfalto Fm. Lateral variability of sedimentary facies and the absence of continuous exposure of the different sedimentary successions have often hampered stratigraphic correlation between sites. Recently, a new chronostratigraphy has been proposed for the Cañadón Asfalto Basin and a Toarcian age for the base of the Cañadón Asfalto Fm has been provided by high-precision U–Pb geochronologic data (Cúneo et al., 2013), hence giving the opportunity to place the continental record in a global context. In particular, it is still needed to better evaluate whether the organic matter-rich strata preserved in the Cañadón Asfalto Basin were coeval to the Toarcian Oceanic Anoxic Event (T-OAE; Jenkyns, 1985), as it was postulated by Figari et al. (2015), or to the marine faunal extinction events and environmental instabilities recorded in the middle–late Toarcian (e.g., Dera et al., 2010; Caruthers et al., 2013).
The climatic and environmental changes associated with the T-OAE are relatively well constrained and were likely triggered by the onset of Karoo-Ferrar (K–F) volcanic activity and associated injection of greenhouse gases into the atmosphere (Courtillot, 1994; Pálfy and Smith, 2000). To date, in contrast to the T-OAE, it is still not clear whether the middle–late Toarcian marine extinction events were related to major changes in carbon cycling, climate conditions, nutrient input and sea-level such as those documented for the T-OAE, but a causal link likely exists with a phase of renewed volcanic activity during the middle–late Toarcian time interval (Caruthers et al., 2013). Providing a solid temporal framework of the Cañadón Asfalto Fm appears hence crucial to reconstruct the biotic evolution on land during the Toarcian, correlate the terrestrial strata with the marine record and evaluate the potential triggers of the terrestrial faunal and floral turnovers recorded in the Cañadón Asfalto Basin (Cúneo et al., 2013).
Here we report the results of a high-resolution sedimentological, mineralogical, geochemical, and geochronological (CA-ID-TIMS U–Pb) study performed on three stratigraphically successive lacustrine successions interbedded with basaltic rocks of the Cañadón Asfalto Fm. The new dataset serves to (i) provide precise U–Pb ages of primary ash beds and tuffaceous material interbedded within the lacustrine sediments, which constrain the age of the Cañadón Asfalto Fm, (ii) characterize the organic matter content and type, (iii) highlight the effect of local processes on the carbon-isotope record, (iv) determine the palaeoenvironmental conditions during deposition of the sediments, and (v) evaluate the possible influence of volcanic activity.
Section snippets
Geological setting
The Cañadón Asfalto Basin, located in Argentina (Fig. 1), records a complex tectonic history from the Triassic up to the Late Cretaceous (Figari et al., 2015). It was an extensional (pull-apart or rift) basin related to the breakup of Gondwana and was composed of several depocenters evolving as isolated segments with complex facies distributions (Cabaleri and Benavente, 2013). These depocenters were filled in by thick volcano-sedimentary sequences since the Early Jurassic (Figari et al., 1996,
Sampling
Samples were collected throughout the measured successions for sedimentological, mineralogical and geochemical analyses. A bed-by-bed sampling was done at a vertical resolution comprised between 5 and 40 cm at Barreño, 5–100 cm at Alice Creek, and 5–200 cm at Quebrada Subsidiaria. Obvious diagenetic neoformed or recrystallized calcite phases were macroscopically recognized and avoided during the sample preparation. Rock samples were powdered using an agate mortar for macroscopically homogenous
Lithology and sedimentology
Nine representative lithofacies were recognized in the studied sections and are described in Table 1. They include: laminated mudstone to siltstone (Ml), laminated mudstone and sandstone rhythmic alternation (MSr), brecciated carbonate (Cb), stromatolitic carbonate (Cs), micritic carbonate (Cm), lenticular carbonate (Cl), volcanoclastic sandstone and conglomerate (Sv), polygenic sandstone and conglomerate (Sp), and tuffs (T).
Palaeoenvironments
At Barreño, the lower basalt is overlain by carbonate beds associated with brecciation and disruptive fabrics (Cb), which can be interpreted as pedogenic alteration features (Freytet and Plaziat, 1982; Alonso-Zarza and Wright, 2010; Cabaleri and Benavente, 2013). Laminated stromatolitic carbonates (Cs) associated with dogtooth cements may indicate formation in the vadose zone (Freytet and Verrecchia, 2002; Alonso-Zarza and Arenas, 2004). These observations suggest a palustrine environment, in a
Conclusions
The Cañadón Asfalto Fm provides a unique record of Jurassic lacustrine settings. The high-resolution geochemical and chronostratigraphic study of these deposits brings new insights into our understanding of the environmental conditions in central Patagonia during the Toarcian. The new ages (179.481 ± 0.059, 179.41 ± 0.13, and 177.27 ± 0.40 Ma, internal uncertainties) from the Cerro Cóndor area place the studied successions in the late Toarcian, suggesting that they are contemporaneous with the
Author Contributions
A.F: fieldwork, investigation and conceptualization, geochemical and mineralogical analyses, data acquisition, original draft preparation. K.B·F: conceptualization, fieldwork, funding acquisition. T.A: conceptualization, Rock-Eval and mineralogical analyses, data acquisition. J.E.S: Carbon and oxygen stable isotopic analysis, data acquisition. B·S: U–Pb isotopic analysis and data acquisition. R.T.B: U–Pb isotopic analysis and data acquisition. R.A.S: Fieldwork, resources. All authors have
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the Swiss National Science Foundation (project 200021-1495461/1). We would like to thank Jean-Claude Lavanchy, Tiffany Monnier, and Laurent Nicod (Institute of Earth Sciences, UNIL) for the XRF analysis, help in the laboratory, and thin sections, respectively. Thank you to Stéphane Bodin and François-Nicolas Krencker (Department of Geoscience, Aarhus University) for their help with the cathodoluminescence microscopy. We are grateful to Ignacio Escapa for his
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