Support for a Vraconnian Stage between the Albian sensu stricto and the Cenomanian (Cretaceous System)
Résumé
The geological scale for the middle Cretaceous currently used throughout the world was proposed by Alcide d'Orbigny in the XIXth century between the years 1842 and 1847 and establishes the succession of stages as Albian, Cenomanian and Turonian. In 1868 Renevier proposed that a supplemental chronostratigraphic division be intercalated between the Albian and the Cenomanian: the Vraconnian stage. This term was not generally accepted and after a period when it was referred to by Breistroffer (1936) as a substage constituting the upper part of the Albian, as an equivalent of the Stoliczkaia dispar ammonite Zone, its abandonment was "recommended" by the Conference on the Lower Cretaceous held in Lyon in 1963. The conditions that led to this "decision" will be discussed herein. Historically, for almost a century the Vraconnian was studied only in the condensed levels of the platform where ammonites are abundant, but the succession is thin and not mappable. The type section of the Vraconnian in the Vaud canton of Switzerland is only 2 meters thick. In France the situation is the same in the northern Alps, in most of central Europe, in Russia, in the Crimea, and as far as the Caspian sea. The examination of a certain number of sections located in other parts of the world: in the southeastern basin of France (Salazac, Marcoule, Mont-Risou), in the Anglo-Paris basin (Folkestone, Merstham, Grandpré), in the Mons basin (Harchies, Strépy-Thieu), at the southern edge of the Tethys in central Tunisia (Kaalat Senan), in Madagascar (Diégo Suarez) and North America in California (Dry Creek) demonstrates, however, that in a number of regions of the globe the sedimentary record of the Vraconnian is sometimes rather considerable, often much more important than that of the Albian sensu stricto. In addition, the Vraconnian represents a very important eustatic event between an Albian transgression and the great Cenomanian transgression (third order cycle and the peak of transgression in a second order cycle). It involves a period of rapid ecologic expansion both as regards the macrofauna (ammonites in particular) and microfauna (planktonic foraminifera). These are the main reasons why a rehabilitation of the Vraconnian as a true stage is proposed here. The Vraconnian in 2008 * Lower boundary. The base of the Vraconnian is taken at the first appearance of the ammonite Mortoniceras (Mortoniceras) fallax (Breistroffer). A substitute index species might be the ammonite Neophlycticeras (Neophlycticeras) blancheti. * Upper boundary. It is the base of the Cenomanian stage which coincides with the first appearance of the planktonic foraminifer Rotalipora globotruncanoides Sigal (= R. brotzeni (Sigal)) in accordance with the recommendations of the "Second International Symposium on Cretaceous Stage Boundaries" held in Brussels in 1995 (Tröger & Kennedy, 1996). * Type Section. To conform to the criteria set up by the International Stratigraphic Commission, any candidate for defining the boundary of a stage should be uncondensed, have no important discontinuities, be easy of access, permanent and fossiliferous. Two sections currently under investigation seem to have good potential: Mont-Risou in the southeast basin of France where the lower boundary of the Cenomanian is already defined and in the neighborhood of Kaalat Senan in central Tunisia. * Duration. Cyclostratigraphic analyses (Fiet et alii, 1998) give the Vraconnian a duration of 2.4 ± 0.2 Ma. This length is equivalent to that of a stage like the Santonian (2.3 Ma). If the lower boundary of the Cenomanian is 98.9 Ma (Gradstein et alii, 1994) then by extrapolation the base of the Vraconnian is at 101.3 Ma. * Ammonite zonation. In the Vraconnian of northwestern Europe three ammonite zones are now accepted. From bottom to top they are: * Mortoniceras (Mortoniceras) fallax IZ; * Mortoniceras (Subschloenbachia) perinflatum TRZ; * Arrhaphoceras (Praeschloenbachia) briacensis IZ. However, note that the first occurrence of the planktonic foraminifer Rotalipora globotruncanoides which marks the Vraconnian-Cenomanian boundary is just below the upper limit of the A. (P.) briacensis Zone. As A. (P.) briacensis is one of the Hoplitidae of which the geographic distribution is confined to the North European province of the Boreal realm an alternative solution to the existing zonation in the future could be founded on the phyletic line of the cosmopolitan Stoliczkaiinae with the succession Neophlycticeras (Neophlycticeras) blancheti, Stoliczkaia (Stoliczkaia) dispar, Stoliczkaia (Lamnayella) tetragona or S. (Shumarinaia) africana. N.B. Many have correlated the Vraconnian with the range of the Stoliczkaia dispar Zone auct.. In fact, S. dispar is confined to the M. (S.) perinflatum TRZ. * Zonation by foraminifera. The calibration of foraminiferal zones with those of ammonites is not yet completely established. Following Sigal (1977, 1987) and Robaszynski & Caron (1979), the working group on planktonic foraminifera has held the appearance of Rotalipora appenninica (Renz) to be a marker of the base of the Vraconnian. * Zonation by calcareous nannofossils. The lower rate of speciation in calcareous nannofossils as compared that of ammonites and foraminifera leads to the definition of broad zones that pass beyond the boundaries of the Vraconnian. Nevertheless, the appearance of Eiffellithus turriseiffeli (Deflandre) is close to the boundary between the M. (S.) perinflatum and A. (P.) briacensis ammonite zones.
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