Late Toarcian continental palaeoenvironmental conditions: An example from the Cañadón Asfalto Formation in southern Argentina

Highlights

• Early Jurassic (late Toarcian) lacustrine environment from central Patagonia.

• New U–Pb ages from Jurassic continental su.ccessions.

• Chon Aike silicic volcanism exerted a control on the depositional conditions.

• The development of oxygen-deficient conditions favoured organic matter preservation.

• Organic carbon-isotope signal is linked to the type/composition of organic matter.

Abstract

The Cañadón Asfalto continental Basin preserves terrestrial deposits interbedded with volcanic rocks, providing a unique opportunity for palaeoenvironmental reconstructions of the Jurassic continental realm. This study presents a sedimentological, mineralogical, and geochemical analysis of three lacustrine successions in the Cerro Cóndor area of the Cañadón Asfalto Basin, Argentina. New CA ID-TIMS U–Pb data from tuffaceous deposits indicate a late Toarcian age (179.481 ± 0.059, 179.41 ± 0.13, and 177.27 ± 0.40 Ma), suggesting that the palaeolakes developed contemporaneously to the Chon Aike volcanic activity. The sedimentary successions are composed of carbonate, organic matter-rich mudstone — with up to 8 wt% total organic carbon (TOC) content — sandstone and conglomerate, all with an important contribution of volcanic and volcanogenic material. The clay mineral assemblage dominated by corrensite (chlorite-vermiculite mixed layers), smectite and vermiculite is likely related to hydrothermal activity and alteration of volcanic material, rather than to change in weathering processes and climate. Organic matter preserved in the sedimentary successions has a lacustrine and terrestrial/reworked origin. Changes in the source of carbon dioxide (CO₂) and in the type and/or degree of preservation of the organic matter had a major impact on the δ¹³C_org values. This further exemplifies the necessity to evaluate the influence of the depositional environment before interpretation of the organic carbon isotope records. The combined high phosphorus and TOC contents suggest that episodes of increased nutrient availability into the basin enhanced lacustrine primary productivity, which favoured the development of oxygen-depleted conditions bottom waters and ultimately organic matter preservation and burial. High C_org/P_tot ratios indicate that phosphorus was likely released back into the water column in a positive feedback loop, further sustaining primary productivity. This study provides an important clue to understand the palaeoenvironmental conditions prevailing on land during the late Toarcian and on how volcanism exerted a control on the depositional conditions.

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.