Robert Busnardo

Répartition biostratigraphique des orbitolinidés dans la biozonation à ammonites (plate-forme urgonienne du Sud-Est de la France). Partie 1 : Hauterivien supérieur - Barrémien basal.

The proposed biostratigraphic distribution of orbitolinids from the Upper Hauterivian and the lowermost Barremian of SE France is correlated with the ammonite biozonation. It is based on the detailed study of eight sections which delivered orbitolinids intercalated with or capped by levels with ammonites and, in some cases, echinids and dasycladales. Among the 24 species of orbitolinids present during that period, 3 are restricted to the Late Hauterivian: Praedictyorbitolina busnardoi, Paleodictyoconus beckerae and Valserina primitiva.

The 'Pseudocrioceras beds' at La Bédoule (SE France) and the position of the Barremian-Aptian boundary in the historical lower Aptian stratotype

Marly limestones from La Bedoule (Bouches-du-Rhone, SE France), historical type-locality of the lower Aptian, contain in their lowermost part a rich ammonite fauna belonging to the heteromorphic genus Pseudocrioceras. Up to the present this level, corresponding to the Pseudocrioceras coquandi Zone of Busnardo (1984), was considered by ammonitologists as early Aptian. However, a detailed study of newly collected faunas from both this level and immediately under- and overlying beds leads us to reconsider the age assignment of this interval. Our results show that the Pseudocrioceras are first accompanied, in the upper part of the Martelites sarasini Zone, by a heteroceratid fauna showing a Barremian signature, and then coexist briefly with the first Deshayesitidae of the Aptian Deshayesites tuarkyricus Zone. Therefore, our proposal is to place the Barremian/Aptian boundary in the historical type-locality at the FAD (First Appearance Datum) of Deshayesites, being better defined and having a wider geographic extension in the Mesogean realm.

Contribution à la datation de la croûte océanique de l'Atlantique central: Age valanginien inférieur des basaltes océaniques et âge néocomien des calcaires Maiolica de Maio (Iles du Cap Vert)

Abstract The discovery of calpionellids and radiolarians in thin sedimentary layers intercalated in the upper part of MORB tholeiitic pillow basalts allows us to date the oceanic crust of Maio Island (Capo Verde Islands) as early Valanginian instead of, as was hitherto believed Late Jurassic. This new dating fits better with reconstructions of the geological history of the Central Atlantic based on magnetic anomalies. The overlying light-coloured pelagic limestones (Maiolica facies) with radiolarians, aptychi and ammonites are Valanginian-Barremian in age. These limestones are lithologically similar to the white limestones drilled at several DSDP sites in the Central Atlantic.

Discussion of the paper by Godet et al. 2011, entitled “Reconciling strontium-isotope and K–Ar ages with biostratigraphy: the case of the Urgonian platform, Early Cretaceous of the Jura Mountains, Western Switzerland” (Swiss Journal of Geosciences, 104, 147–160)

For decades, the age of the Urgonian deposits, in Switzerland and in the southeast of France in particular, has been addressed and disputed in several articles, by authors belonging to two opposing groups of researchers. For Arnaud-Vanneau and Arnaud (1990) and a number of other workers (e.g. Adatte et al. 2005; Godet et al. 2010, 2011, 2012, and references therein), the Urgonian deposits found in the Canton Vaud, in the western part of the Swiss Jura, and the lower member of the Urgonian limestones, in the French Subalpine Chains, are Late Barremian in age. These authors rely on a certain approach to sequence stratigraphy, combined with chemostratigraphical and geochronological data and selected palaeontological data (rejecting or neglecting some fossil groups that might support alternative interpretations). For instance, Godet et al. (2012) state that ‘‘The orbitolinid biostratigraphy fide Conrad et al. (2012) is questionable’’. This is in contrast to Clavel et al. (1987) and a number of other researchers, such as the present authors (see Clavel et al. 2007; Conrad et al. 2012 and references therein) who maintain that foraminifers provide reliable biostratigraphical information, particularly in depositional environments where the classical markers such as ammonites, planktonic foraminifers, and nannofossils are lacking. In our stratigraphical interpretation, all Urgonian deposits are Late Hauterivian in age in the western Swiss Jura, whereas in the French Subalpine Chains the lower member of the Urgonian limestones dates from the Late Hauterivian, the Early Barremian or the Late Barremian, depending on its paleogeographic location (Clavel et al. 2012). This interpretation relies on a wider set of palaeontological data and different approach to sequence stratigraphy. However, according to Godet et al. (2012), ‘‘The sequence-stratigraphic interpretation forwarded by Conrad et al. (2012) is not consistent with modern sequence-stratigraphic interpretation of platform carbonates’’. Since we understand from this statement that there will be no agreement on the best practice in sequence stratigraphy, we will not discuss the topic further. We focus here of the western Swiss Jura and on the difference in age between the interpretation of the first group (represented by the paper by Godet et al. 2011) and our interpretation. The difference between these two interpretations is significant, because it covers a full stage, with implications that go far beyond the study area, referring to the whole of the northwestern Tethyan domain. In particular it is whether, Editorial handling: A. G. Milnes.

Refining Urgonian Biostratigraphy: A Key Section at L’Estellon, Drôme, France

In a basinal section of the Vocontian Trough, “freshly” reworked shallow-water assemblages (orbitolinids and dasycladales) can be calibrated directly with ammonite zones. The section documents, for instance, the oldest record of Palorbitolina lenticularis and those of the so-called “typical early Aptian” representatives of the genus Orbitolinopsis in lower Barremian strata.