Community replacement of neritic carbonate organisms during the late Valanginian platform demise: A new record from the Provence Platform

Abstract

The Valanginian is marked by a major platform demise inducing a hiatus in the northern Tethyan neritic carbonate record from the top of the lower Valanginian to the lower Hauterivian. New biostratigraphic and chemostratigraphic data from the Ollioules section (Provence Platform, southern France) are presented here, demonstrating that a large part of the upper Valanginian is preserved in an inner platform environment. The thick, upper Valanginian, aggrading carbonate succession is observed in an aborted rift domain, implying relatively low subsidence. In this context, a relatively long-term sea-level rise was required to sustain a keep-up style of carbonate production. Like the Apulian Platform, the remarkable preservation of the Provence Platform may have been favored by its remoteness from terrigenous source areas, as suggested by the low clastic inputs and low P-accumulation rates. Two main biotic community replacements are observed in Ollioules. The first saw the development of abundant microbialites and algae at the onset of the late Valanginian. A Tubiphytes concentration occurred during the coolest climatic conditions and the transition towards arid conditions, whereas the subsequent Lithocodium–Bacinella and orbitolinids assemblages developed under low nutrient conditions during a warmer interval. Both assemblages may have been triggered by increased alkalinity. The second community replacement saw the installation of coral- and rudist-dominated communities during the latest Valanginian to early Hauterivian. They indicate a change to oligotrophic, open marine conditions. Six medium-scale sequences have been defined in Ollioules, indicating short-term transgressive–regressive trends superimposed on a long-term transgression. Low nutrient inputs and relatively low subsidence in an aggradational context may explain the survival of the isolated Provence Carbonate Platform during a time of widespread drowning episodes and platform demise in the northern Tethyan domain.

Introduction

The Early Cretaceous recorded several carbonate production crises and perturbations of the global carbon cycle as evidenced by excursions in the carbon isotope compositions of carbonates and organic matter. The earliest of these crises occurred during the Valanginian period. A positive carbon excursion of about 2‰ measured in bulk rock carbonates is recorded worldwide (the Weissert event; Hennig et al., 1999, Erba et al., 2004, Duchamp-Alphonse et al., 2007). This event is recorded in the Vocontian Basin from the upper part of the Busnardoites campylotoxus to the Neocomites peregrinus ammonite Zones (Gréselle et al., 2011). It coincided with a cooling event recorded by both fish tooth and belemnite δ¹⁸O and belemnite Mg/Ca values (Van de Schootbrugge et al., 2000, Pucéat et al., 2003, McArthur et al., 2007, Barbarin et al., 2012). This positive excursion may be associated with increased organic carbon burial either in the oceanic domain (Erba et al., 2004) or on land (Westermann et al., 2010). In some areas, the perturbation of the carbon cycle was mirrored by a neritic carbonate crisis that occurred from the end of the B. campylotoxus Zone (early Valanginian) to the late Valanginian or early Hauterivian (Arnaud-Vanneau and Arnaud, 1990, Weissert et al., 1998, Vilas et al., 2003, Föllmi et al., 2007, Gréselle and Pittet, 2010). In the northern Tethyan domain, this crisis entailed the demise of the carbonate platforms and major changes in carbonate producer communities. The platform crisis was triggered either by drastic flooding, as attested by the deposition of glauconite and phosphate-rich levels (Weissert et al., 1998), or by successive high-amplitude sea-level falls followed by intense flooding at the very end of the Valanginian (Gréselle and Pittet, 2010). Upwelling of nutrient-enriched, deep-water masses could also have contributed to platform demise (Föllmi et al., 1994, Weissert et al., 1998). Such high trophic conditions are mirrored by the progressive replacement of a photozoan by a heterozoan neritic community that formed close to the Berriasian/Valanginian boundary and persisted up to the acme of the platform crisis (Weissert et al., 1998, Föllmi et al., 2007). This community replacement reflects a change from oligotrophic to meso-eutrophic conditions and is associated with widespread phosphatic hardgrounds capping platform limestones. Weissert et al. (1998) suggested that an additional sea-level rise combined with the eutrophication triggered the platform demise.

In contrast to the extensively studied pre-crisis neritic assemblages, little is known about the changes in neritic carbonate producers during the crisis period and the subsequent recovery. This is because of the occurrence of a major hiatus extending from the end of the early Valanginian (end of B. campylotoxus ammonite Zone) to the early Hauterivian (Acanthodiscus radiatus ammonite Zone) on platforms worldwide (Weissert et al., 1998, Föllmi et al., 2007). In some areas, such as the southern part of the north-western Tethyan margin, the interruption was limited to the end of the early Valanginian and carbonate production resumed in the late Valanginian (Vilas et al., 2003).

The purpose of this study is to explore the replacement of biotic assemblages during the Valanginian platform crisis and the subsequent recovery and to unravel their connections with the paleoenvironmental changes that occurred during this interval. The Valanginian Ollioules section (Provence Platform) includes over 180 m of poorly dated, lower Valanginian–lower Hauterivian sediments (Masse, 1993, Virgone, 1997, Skelton and Masse, 1998). An accurate chronostratigraphic scheme is provided here for this section based on foraminifer associations and new carbon isotope data. Once ascertained, the main biotic assemblages and their paleoenvironmental interpretation will be used to propose a scenario for the regional response of platform paleoecosystems to global environmental perturbations. This exceptional Valanginian record provides new data about carbonate producers, questions the modalities of the carbonate production crisis on platforms and allows comparison with other geographical areas.