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

Abstract

Une répartition biostratigraphique des orbitolinidés calée sur la biozonation à ammonites a été établie pour l'Hauterivien supérieur et le Barrémien inférieur pro parte du SE e la France. Elle est basée sur l'étude de huit coupes de terrain relevées dans le SE de la France, qui ont livré des orbitolinidés, encadrés ou surmontés par des faciès à ammonites associées parfois à des échinides et des dasycladales significatifs sur le plan biostratigraphique. Il est établi à cette occasion que trois espèces d'orbitolinidés caractérisent l'Hauterivien supérieur : il s'agit de Praedictyorbitolina busnardoi, Paleodictyoconus beckerae et Valserina primitiva.

Some key Lower Cretaceous sites in Drôme (SE France)

“…Thus, L'Estellon section can be regarded as a "Rosetta Stone" for the Urgonian biostratigraphy. A comparison of these updated microfossil appearances with their ranges shown on current orbitolinid charts CLAVEL et al, 2007CLAVEL et al, , 2010a provide contrasting results: for instance, our results call for the definitive withdrawal of one mented chart (ARNAUD-VANNEAU's that denies lower Barremian occurrences as documented herein) and lead to question the conclusions of the several publications relying on such biostratigraphic framework (see similar discussions in CONRAD et al, 2012, andCHAROLLAIS et al, 2013). …”

“…3) we also collected in the argillaceous and muddy limestones a rather diversified ammonite fauna composed of late Hauterivian and (early and late) Barremian forms . The co-occurrence of both shallow-and deeper-water fossils allowed us to reconfirm the calibration of the First Appearance Datum -FAD -(and eventually the Last Appearance Datum -LAD -, because this parameter may be affected by late reworking phenomena) of these benthic foraminifers (MOUL- LADE, 1966LADE, , 1974CLAVEL et al, 2007CLAVEL et al, , 2010aCLAVEL et al, , 2010bCLAVEL et al, , 2010c and algae (CLAVEL et al, 2007;GRANIER, 2013a) using the ammonite biozones. In other words L'Estellon section helped us correlating the distribution of allochtonous neritic ("shallow-water") assemblages with time-equivalent subautochtonous pelagic ("deeper-water") components of the fossil record and it provides therefrom a mean to transcript them into ages.…”

“…In order to test the validity of existing scales (SCHROEDER et al, 2002;ARNAUD-VANNEAU et al, 2005;CLAVEL et al, 2007CLAVEL et al, , 2010a, we needed a reference framework. We selected the "distribution chart of Orbitolinids" given by AR- NAUD-VANNEAU et al (2005) because with almost 50 taxa it was intended to give the most comprehensive list of species.…”

“…Theorically, it is almost excluded that such synsedimentary events occur at times of faster relative sea-level rise, i.e., during transgressive intervals (TST); instead, they may occur at times of slow relative sea-level rise, i.e., mostly during both late hightstand (HST) and lowstand (LST) intervals, and obviously at times of relative sea-level fall, i.e., mostly during "falling-stage" intervals (early LST). In a parallel paper we identified a number of sand flow units at L'Estellon, ascribing them ammonite-derived ages and referring them to discrete LST in a set of sequences labelled Ba1 to Ba5, as identified by CLAVEL et al (2010aCLAVEL et al ( , 2010bCLAVEL et al ( , 2010cCLAVEL et al ( , 2012. The list of units, their relative datings and their labellings follow:…”

“…the first calcarenitic bed at 26.5m (Niklesi Zone) is referred to the LST "Ba1" of CLAVEL et al (2010aCLAVEL et al ( , 2012, B. the second set starting from 49.3m (Pulchella Zone) to the LST "Ba2", C. the third starting from 72.4m…”