The Berriasian–Valanginian boundary in the Mediterranean Province of the Tethyan Realm: Ammonite and calcareous nannofossil biostratigraphy of the Vergol section (Montbrun-les-Bains, SE France), candidate for the Valanginian GSSP

Abstract

The Vergol locality (Montbrun-les-Bains, South-East France), a candidate section for the Valanginian GSSP (Global Stratotype Section and Point), was studied in detail for ammonites and calcareous nannofossils. A bed-by-bed sampling and a quantitative approach were carried out on this expanded succession presenting a continuous record of calcareous-marl alternations covering the uppermost Berriasian and lowermost Valanginian. The ammonite distribution observed allows a precise characterization of the Berriasian–Valanginian boundary with the recognition of the Tirnovella alpillensis (latest zone of the Berriasian) and “Thurmanniceras” pertransiens (earliest zone of the Valanginian) standard zones. The base of the Valanginian is here defined by the First Appearance Datum of “T.” pertransiens; this boundary can be well-characterized by ammonite assemblages and by using various calcareous nannofossil bioevents, like the First Occurrence of Calcicalathina oblongata. A tidy comparison with previous works on the Vergol section is made in order to integrate data on calpionellids. A discussion on the choice of the primary marker (ammonites versus calpionellids) for the Valanginian GSSP is proposed.