Three successive phases of platform demise during the early Aptian and their association with the oceanic anoxic Selli episode (Ardèche, France)

https://doi.org/10.1016/j.palaeo.2014.11.008Get rights and content

Highlights

  • Precise documentation and age model for lower Aptian platform succession in Ardèche

  • Progressive demise of the platform in three steps near and during the OAE1a

  • At least two of the three drowning steps preceded by platform emersion

  • Rapid and high-amplitude sea-level change affected platform evolution.

  • The drowning phases started before the onset of the Selli anoxic episode.

Abstract

A stratigraphic and depositional model, constrained by biostratigraphy, geochemistry, total phosphorus contents, and bulk-rock mineralogy, is proposed for lower Aptian sediments from the Languedoc platform in Ardèche, SE France. The upper lower Aptian is documented by the Chabert Formation (upper Deshayesites forbesi Zone to upper Dufrenoyia dufrenoyi Subzone), deposited on a discontinuity surface on top of the Urgonian platform, recording a first emersion phase and consecutive drowning event. The Chabert Formation starts with the marly Violette Member, which passes into crinoidal limestone of the Rocherenard Member. The top of this member is associated with a second discontinuity, recording a further drowning phase, which is followed by the deposition of the glauconitic and partly phosphatic Picourel Member (upper Deshayesites grandis to upper Dufrenoyia dufrenoyi Subzone). A third erosive phase is documented by a phosphatic conglomerate (upper Dufrenoyia furcata Zone), which represents a lag deposit derived from underlying sediments. The formation of this conglomerate was associated with a substantial emersion phase. This emersion was followed by a drowning event reworking the phosphatic conglomerate into the base of the upper Aptian black marls (Frayol Formation). The carbon-isotope record shows a negative excursion which coincides with the onset of the early Aptian oceanic anoxic Selli episode (OAE 1a) in the middle/upper part of the Deshayesites forbesi Zone. Emersion phases were an important factor implied in the formation of the sequence boundaries, which were transformed into drowning unconformities during subsequent phases of significant transgressions. These phases were associated with the installation of higher trophic levels, transforming or impeding carbonate production. The first drowning phase preceded the onset of the Selli episode, suggesting that rapid sea-level change and associated environmental change were already an important element of the early Aptian before the major phase of environmental change during the Selli episode.

Introduction

During the late Barremian and earliest Aptian (late Early Cretaceous), the northern Tethyan inner shelf was occupied by a large photozoan carbonate factory, which led to the formation of the so-called Urgonian platform. Profound palaeoceanographic and climatic change occurred during the early Aptian, including rapid eustatic variations (Arnaud-Vanneau, 1980, Arnaud and Arnaud-Vanneau, 1989, Arnaud and Arnaud-Vanneau, 1990, Haq, 2014), coastal upwelling and the appearance of a marginal geostrophic current (Delamette, 1986, Delamette, 1989), rise in atmospheric CO2 (Shaffer et al., 2009), increase in continental biogeochemical weathering and corresponding siliciclastic (Weissert, 1990) and phosphorus output (Föllmi et al., 1994, Föllmi et al., 2006), rise in organic-carbon burial (Arthur et al., 1985, Weissert and Lini, 1991) and positive excursions in the δ13C record (Weissert, 1989). These changes coincided with: (I) the disappearance of the carbonate platforms in three successive steps and; (II) the installation of a dense succession of global anoxic episodes, prevailing during the Aptian.

The detailed pattern of progressive platform demise and the exact ages of the main phases during the early Aptian are still poorly understood for several reasons. Firstly, the sea-level record preserved in drowning unconformities is often complex and multiphased, and not always optimally interpretable due to condensation and erosion processes (e.g., Godet, 2013). An important and not always well-established component is the effect of emersion phases prior to drowning. These emersions were the consequence of sea-level fluctuations for which their correlation is not in all cases established due to the lack of age-diagnostic fossils. Bottom currents, shifting onto the shelf during transgressions, were equally responsible for sediment-reworking processes including erosion and winnowing, creating large variations in sediment-accumulation rates and multiple unconformities.

A second point is the lack of detailed studies on the heterozoan sediments covering the Urgonian Formation (Chabert Formation in Languedoc, Upper Orbitolina beds in Vercors and Chartreuse, Grünten Member in the Helvetic Alps) widely known from the northern Tethyan margin. These sediments are currently often confounded with other formations, partly of other ages, because of the lack of characteristic fossils or disagreements on ammonite and foraminifera determinations. The study of these sediments is of special interest, in that they do not only document the environmental changes and successive drowning events to which the Urgonian platform was submitted during the early Aptian, but also because they document the evolution of the shallow-water platform during the Selli episode, the first major oceanic anoxic episode (OAE 1 a) of the Early Cretaceous (Arthur et al., 1990, Föllmi and Gainon, 2008, Föllmi, 2012).

A detailed study of the sedimentology, sequence stratigraphy, biostratigraphy and geochemistry of the Chabert Formation from the former Languedoc platform, Ardèche, SE France (Fig. 1a), allows us to (I) propose a detailed model for the evolution of the lower Aptian platform sediments attached to the southern margin of the European continent; (II) better understand the palaeoecological setting, and; (III) precisely date this formation including its different drowning unconformities and to demonstrate their synchronicity with comparable successions in other regions.

Section snippets

Geological and palaeogeographical settings

The Ardèche plateau is located on the southeastern edge of the Massif Central (Fig. 1b). The latter is separated by the Dauphiné Alps from the Rhodanian graben, which forms the base of the Rhône Valley. The Cretaceous rocks of the Ardèche plateau are separated from the crystalline rocks of the Massif Central by the Cevennes fault system passing through Largentière, Aubenas, Privas and La Voulte to the northwest, and by the Alès graben to the south. Currently, the entire sedimentary succession

Methods

Aptian sediments which outcrop on the Languedoc platform were investigated by traditional fieldwork consisting of cartography, detailed logging, macroscopical sedimentological and palaeontological descriptions, and sampling of key sections. The sections were analysed for their microfacies (thin sections), sequence stratigraphy, sedimentology, and ammonite biostratigraphy. A particular aspect is the documentation of depositional geometries, discontinuity surfaces and their large-scale

Lithology and microfacies

Six sections were logged and sampled on a centimetre to metre scale for geochemical, mineralogical and microfacies analyses. Macrofauna including oysters, other bivalves, gastropods, serpulids, sponges, echinoderms, nautilids and ammonites were collected bed-by-bed for palaeoecological reconstructions and biostratigraphic age control. Sections are described here from more proximal to more distal parts of the platform along a transect across the Languedoc platform, and illustrated in Fig. 4,

Fauna and depositional environment

The microfacies and facies show a fluctuating evolution throughout the sections studied here, with a general trend from an inner-platform to a pelagic setting in two steps. The lithologies and fauna trace three main depositional environments and fauna associations, which are separated by three discontinuities (D1 to D3) and related drowning events.

The first lithology is represented by the Urgonian Formation, which includes the products of a photozoan, oligotrophic carbonate factory dominated by

Conclusions

The lower Aptian succession from the Ardèche area is well exposed and of great interest for the study of the late early Aptian platform drowning episodes near and during the oceanic anoxic Selli episode. Sections comprising the Selli episode, are relatively thick, and rich in ammonites.

The demise of the oligotrophic Urgonian platform is associated with an emersion phase followed by a drowning event during the D. forbesi Zone. This event is followed by the installation of a transgressive

Acknowledgements

We are grateful to François Gischig and Pierre Desjaques (University of Geneva), who carefully prepared the thin sections. We thank Bernard Clavel and Robert Busnardo for their initial assistance. We thank Yves Dutour (Aix-en-Provence Museum) for checking some ammonite identifications. We gratefully acknowledge the Swiss National Science Foundation and the University of Lausanne for their financial support, and the reviewers Hubert Arnaud and Josep Moreno-Bedmar for their detailed and very

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