The proposal of a GSSP for the Berriasian Stage (Cretaceous System): Part 2

In part 1 of this work we discussed the possibilities for the selection of a GSSP for the Berriasian Stage of the Cretaceous System, based on prevailing practical methods for correlation in that J/K interval, traditional usage and the consensus over the best boundary markers that had developed in the last forty years. This consensus has developed further, based on the results of multidisciplinary studies on numerous sites over the last decade. Here in Part 2 we give an account of the application of those results by the Berriasian Working Group (ISCS), and present the stratigraphic evidence that justifies the selection of the locality of Tré Maroua (Hautes-Alpes, SE France) as the proposed GSSP. We describe a 45 m-thick section in the Calcaires Blancs vocontiens – that part of the formation covering the calpionellid Chitinoidella, Remanei. Intermedia, Colomi, Alpina, Ferasini, Elliptica and Simplex biozones. The stratigraphic data collected here has been compiled as part of a wider comparative study of complementary Vocontian Basin sites (with localities at Charens, St Bertrand, Belvedere and Le Chouet). Evidence from Tré Maroua thus sits in this substantial regional biostratigraphic and magnetostratigraphic context. For the purposes of the GSSP definition, here we particularly concentrate on the unbroken sequence and biotic markers in the interval immediately below the boundary, the Colomi Subzone (covering circa 675,000 years), and immediately above, the Alpina Subzone (covering circa 725,000 years). Particularly significant fossil datums identified in the Tré Maroua profile are the primary basal Berriasian marker, the base of the Alpina Subzone (a widespread event marked by dominance of small Calpionella alpina, with rare Crassicollaria parvula and Tintinopsella carpathica): the base of the Berriasian Stage is placed at the base of bed 14, which coincides with the base of the Alpina Subzone. Secondary markers bracketing the base of the Calpionella Zone are the FOs of the calcareous nannofossil species Nannoconus wintereri, close below the boundary, and the FO of Nannoconus steinmannii minor, close above. The Tithonian/Berriasian boundary level occurs within M19n.2n, in common with many documented sites, and is just below the distinctive reversed magnetic subzone M19n.1r (the so-called Brodno reversal). We present data which is congruent with magnetostratigraphic and biostratigraphic data from other key localities in France and in wider regions (Le Chouet, Saint Bertrand, Puerto Escaño, Rio Argos, Bosso, Brodno, Kurovice, Theodosia…), and thus the characteristics and datums identified at Tré Maroua are key for correlation and, in general, they typify the J/K boundary interval in Tethys and connected seas