Chronostratigraphic and palaeogeographic significance of an early Cambrian microfauna from the Heraultia Limestone, northern Montagne Noire, France

Abundant and diverse Small Shelly Fossils (SSFs) have been reported in Northern Montagne Noire (Southern France) since the last 20th century from early Cambrian phosphatic carbonates called Heraultia Limestone. This assemblage includes Watsonella crosbyi, a mollusc with high-potential for the definition of the base of the Cambrian Stage 2, as recently emphasized by the Cambrian Stage Subdivision Working Group of International Commission of Stratigraphy. Nevertheless, the Heraultia Limestone is traditionally considered Cambrian Stage 3 to 4 based on questionnable lithological correlations among different tectonostratigraphic units. The focus of this paper is to re-evaluate the age and the paleogeographical significance of the Heraultia Limestone in the light of recent advances in SSF systematic and biostratigraphy. Reassessed assemblage is dominated by molluscs (helcionellids, ?polyplacophors and other problematic taxa), and abundant orthothecid hyoliths, problematic tubes; few problematica are also present. Twenty-eight species and three morphotypes are described. Two species and one genus are new (Obscurania tormoi Devaere, sp. nov.; Alaconcha Devaere, gen. nov. and Alaconcha rugosa Devaere, sp. nov.), seven are reported for the first time. Among previously mentionned species, 13 are reassigned, 7 confirmed, and 9 unrecovered from the studied sampled. The global stratigraphic range of each species is established based on updated inventory of known occurrences. It argues for a Terreneuvian (Nemakit-Daldynian/Tommotian according to the Siberian stratigraphic chart) age of the microfossil assemblage. The Watsonella crosbyi–Oelandiella korobkovi Interval Zone is defined and is correlated with base of Cambrian Stage 2 (Tommotian) of Siberia, China, Mongolia, and Avalonia. The Northern Montagne Noire would accordingly witness for one of the earliest, isolated but consequent Tommotian carbonate-platform on the Western Gondwana margin. As a result, present tectonic and palaeogeographic models have to be emended, and factors that favored such isolated platforms should be further investigated. DEVAERE ET AL.: FRENCH CAMBRIAN MICROFAUNA 2 Léa Devaere. UMR 8217 Géosystèmes CNRS-Université LILLE 1, F59655 Villeneuve d’Ascq, France, lea.devaere@ed.univ-lille1.fr. Sébastien Clausen. UMR 8217 Géosystèmes CNRS-Université LILLE 1, F59655 Villeneuve d’Ascq, France, sebastien.clausen@univ-lille1.fr. Michael Steiner. Department of Earth Sciences, Freie Universität Berlin, Malteserstrasse 74-100, Haus D, Berlin, 12249, Germany, steiner@zedat.fu-berlin.de. J. Javier Álvaro. Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Spain, alvarobjj@cab.inta-csic.es. Daniel Vachard. UMR 8217 Géosystèmes CNRS-Université LILLE 1, F59655 Villeneuve d’Ascq, France, daniel.vachard@univ-lille1.fr.

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