Sedimentology (2012) 59, 156–184
doi: 10.1111/j.1365-3091.2010.01221.x
Drowning of a carbonate platform as a precursor stage of the Early Toarcian global anoxic event (Southern Provence sub-Basin, South-east France) PHILIPPE LE´ ONIDE*,, MARC FLOQUET*, CHRISTOPHE DURLET, FRANC ¸ OIS BAUDIN §, BERNARD PITTET – and CHRISTOPHE LE´ CUYER– *EA 4234 Laboratoire de Ge´ologie des Syste`mes et des Re´servoirs Carbonate´s, Universite´ de Provence, 3 place Victor Hugo, case 67, 13331 Marseille Cedex 03, France (E-mail:
[email protected]) Department of Sedimentology and Marine Geology, Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands UMR CNRS 5561 Bioge´osciences, Universite´ de Bourgogne, 6 Boulevard Gabriel, 21000 Dijon, France §UMR CNRS 7072 Tectonique, Universite´ Pierre et Marie Curie – Paris 06, 4 place Jussieu, case 117, 75252 Paris Cedex 05, France –UMR CNRS 5125 Pale´oEnvironnements et Pale´obioSphe`re, Universite´ Lyon 1, 2 rue Dubois, Baˆtiment Ge´ode, F-69622 Villeurbanne Cedex, France Associate Editor – Adrian Immenhauser ABSTRACT
The Toarcian Oceanic Anoxic Event is well-known as coinciding with a carbonate crisis, coupled with organic matter accumulation and perturbation of the carbon cycle expressed by carbon-isotope excursions. In this palaeoenvironmental setting, the present research attempts to better constrain the palaeoenvironmental conditions leading to the drowning of a carbonate platform during Late Pliensbachian to Early Toarcian times. This study is based on the integrated sedimentological, diagenetic and geochemical (stable isotopes and Rock-Eval pyrolysis) analysis of several stratigraphic successions located in the Southern Provence sub-Basin (South-east France). Eodiagenetic ferroan calcite cements below two hardground surfaces at the Pliensbachian–Toarcian transition record episodic eutrophication events. Such events were precursor stages of a global palaeoenvironmental perturbation (i.e. a global carbon cycle perturbation) that led to the demise of the carbonate platform and occurred prior to the Toarcian Oceanic Anoxic Event. Subsequently, the acme of the palaeoenvironmental deterioration that coincided with the Toarcian Oceanic Anoxic Event is recorded by a pronounced negative carbon-isotope excursion ()3&) followed by a positive one (2Æ87&), organic matter preservation and inferred high temperature of surface sea water (ca 25C) as given by oxygen-isotope values from chondrichthyan tooth enamel. Sedimentological comparisons, as well as chemostratigraphic and biostratigraphical correlations between the Early Toarcian sedimentary series of the Southern Provence and Eastern Dauphinois sub-basins, indicate that tectonic tilting occurred earlier in the Southern Provence sub-Basin; this resulted in the preservation of organic-rich deposits in the first sub-basin and depositional hiatus in the second sub-basin. Thus, local conditions, such as contrasting structural setting, could markedly modify the sedimentary signature of a global climatic event. Keywords Carbon isotope, carbonate cements, drowning, South-east France, Southern Provence carbonate platform, Toarcian Oceanic Anoxic Event. 156
2011 The Authors. Journal compilation 2011 International Association of Sedimentologists
Drowning of a carbonate platform INTRODUCTION The sedimentary history of Mesozoic carbonate systems was intermittently marked by crises during which biological carbonate production rapidly ceased (Jenkyns, 1985; Arthur et al., 1988; Jenkyns et al., 1994; Weissert et al., 1998; Price, 1999; Dera et al., 2011). In this respect, the Pliensbachian– Toarcian transition is a key interval in the evolution of Tethyan sedimentary basins: this transition is marked by carbonate crises (Dromart et al., 1996; Blomeier & Reijmer, 1999), coupled with enhanced organic matter accumulation (Jenkyns, 1988; Baudin et al., 1990) and perturbation of the carbon cycle expressed by d13C excursions (Hesselbo et al., 2000; Beerling et al., 2002). These features correspond to the Toarcian Oceanic Anoxic Event (T-OAE) sensu Jenkyns (1988). This event was coeval with climatic changes (Bailey et al., 2003; Rosales et al., 2004a; van de Schootbrugge et al., 2005), including cooling in the Late Pliensbachian followed by warming during Early to Middle Toarcian times (Suan et al., 2008b; Dera et al., 2009a). The Pliensbachian–Toarcian transition was also marked by the demise of carbonate platform production (Dromart et al., 1996; Blomeier & Reijmer, 1999) and a nannofossil biocalcification crisis (Mattioli & Pittet, 2002; Erba, 2004; Mattioli et al., 2004a,b; Tremolada et al., 2005). The interval is also characterized by a recovery of biodiversity during the latest Pliensbachian (Cecca & Macchioni, 2004) and the Early Toarcian (Hallam, 1987; Aberhan & Baumiller, 2003; van de Schootbrugge et al., 2005). The origin of these events and the cause to effect relationship are still enigmatic and strongly debated (Wignall et al., 2005; Newton et al., 2006). The sedimentary evolution of the Southern Provence sub-Basin is particularly informative as an illustration of the impact of environmental changes on carbonate production and preservation across the Pliensbachian–Toarcian transition. In the Southern Provence sub-Basin this transition is marked by: (i) a lithological change from pure carbonate production and sedimentation within a photic, shallow–marine depositional environment (Late Pliensbachian) to hemipelagic sedimentation in deeper oligophotic or non-photic marine depositional environments (Early Toarcian) (Le´onide et al., 2007); and (ii) the transition is highlighted by a major unconformity throughout the Southern Provence sub-Basin. This study focuses on: (i) the sedimentary unconformities and their related specific ferroan
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sparry calcite cements that precipitated at the top of a neritic carbonate platform prior to and during the global T-OAE; and (ii) a comparative study of the T-OAE record comparing depositional events in a shallow carbonate platform setting and sedimentary depositional processes in the adjacent deeper offshore, marl dominated basin. This study presents the results of an integrated study based on fieldwork analyses, the analysis of the cement stratigraphy and organic matter content, as well as elemental and stable-isotope geochemistry. Fifteen successions were used in this study, which also include three type sections that were selected for specific geochemical analyses: (i) Cuers, Southern Provence sub-Basin; and (ii) and (iii) La Robine and Marcoux, Eastern Dauphinois sub-Basin (Fig. 1).
METHODS A study of carbonate diagenesis is based on cement stratigraphy sensu Meyers (1991) and entails establishing the sequence of diagenetic events observed in thin sections (for example, cement, recrystallization, dissolution, boring, fracture, etc.). Samples collected from 15 outcrops (Le´onide, 2007) displaying the Pliensbachian–Toarcian transition in the Southern Provence sub-Basin and in the Eastern Dauphinois sub-Basin were used to make 154 polished thin sections. Cement stratigraphy was performed using optical microscopy on alizarin–potassium ferricyanide stained thin sections (Dickson, 1966). Observations were completed by Scanning Electron Microscopy (XL 30 ESEM; FEI/Philips, Eindhoven, The Netherlands) and by cathodoluminescence petrography with a 8200 MKII Technosyn (20 kV and 600 mA; Technosyn, Cambridge, UK) coupled to an Olympus microscope (Olympus Optical Corporation, Tokyo, Japan) and a digital MRc5 camera (Zeiss, Go¨ttingen, Germany). Cross-cutting relationships observed on thin sections from the bored hardground surfaces at the top of the Pliensbachian limestones allow the diagenetic phases recorded in these unconformity surfaces to be identified precisely. The successive diagenetic phases are represented graphically by diagenetic logs (Durlet et al., 1992) and are used to determine the diagenetic sequences of the unconformities (Durlet & Loreau, 1996; Le´onide, 2007). The diagenetic sequence of a hardground surface is the ordered list of all the diagenetic phases that occur contemporaneous with the sedimentary gap.
2011 The Authors. Journal compilation 2011 International Association of Sedimentologists, Sedimentology, 59, 156–184
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Fig. 1. (A) Palaeogeographical map of the Early Jurassic showing the location of the Dauphinois Basin (modified from Bassoulet et al., 1993). (B) Palaeogeography of South-eastern France and location of the three type sections (Cuers, La Robine and Marcoux).
It starts after the deposition of the sediments now situated just beneath the surface and ends with the truncation, the borings or the Fe-oxide crusts that precede the deposition of the next sedimentary unit (Loreau & Durlet, 1999). Micro-sampling of calcite cements was attempted using a micromill in order to analyse carbon and oxygen isotopes and to constrain the nature of fluid composition, but the fineness of the cements (
