Plaidoyer pour un étage Vraconnien entre l'Albien sensu stricto et le Cénomanien (système Crétacé)

Le calendrier géologique en usage actuellement dans le monde pour la partie moyenne du Crétacé a été construit au XIXème siècle, entre les années 1842 et 1847 par Alcide d'ORBIGNY et montre la succession des étages Albien, Cénomanien et Turonien. En 1868, une division chronostratigraphique supplémentaire, intercalée entre l'Albien et le Cénomanien, a été proposée par RENEVIER : l'étage Vraconnien. Ce terme n'a pas été pérennisé et, après avoir été ramené dans un premier temps au rang d'un sous-étage formant la partie supérieure de l'étage Albien par BREISTROFFER (1936), son abandon a été "recommandé" lors du Colloque sur le Crétacé inférieur tenu à Lyon en 1963. On développera la façon dont la "décision" a été prise. Historiquement, le Vraconnien a été étudié pendant près d'un siècle uniquement dans des niveaux condensés de plate-forme, riches en ammonites mais peu épais et qui ne se prêtaient pas à une cartographie. Dans le gisement-type de La Vraconne situé dans le canton de Vaud en Suisse par exemple, le Vraconnien mesure seulement 2 mètres d'épaisseur. La situation est la même en France dans le Nord des Alpes, dans une grande partie de l'Europe centrale, en Russie, en Crimée et jusqu'à la mer Caspienne. L'examen d'un certain nombre de coupes réparties dans d'autres parties du monde : dans le bassin du Sud-Est de la France (Salazac, Marcoule, Mont-Risou), dans le bassin anglo-parisien (Folkestone, Merstham, Grandpré), dans le bassin de Mons (Harchies, Strépy-Thieu), sur la marge méridionale de la Téthys en Tunisie centrale (Kalaat Senan), à Madagascar (Diégo) et en Amérique du Nord en Californie (Dry Creek), montre cependant que dans de nombreuses régions du globe, l'enregistrement sédimentaire du Vraconnien est parfois considérable, souvent beaucoup plus important que celui de l'Albien sensu stricto. En outre, le Vraconnien correspond à un événement eustatique global d'une réelle importance entre une transgression albienne et une grande transgression cénomanienne (cycle de 3ème ordre et pic de transgression d'un cycle de second ordre). Enfin il s'agit d'une période d'épanouissement écologique à la fois en ce qui concerne les macrofaunes (ammonites en particulier) et les microfaunes (foraminifères planctoniques). Ce sont les principales raisons pour lesquelles une réhabilitation du Vraconnien comme étage à part entière est proposée ici. Le Vraconnien en 2008 • Limite inférieure. La base du Vraconnien est prise à la première apparition de l'ammonite Mortoniceras (Mortoniceras) fallax (BREISTROFFER). • Limite supérieure. C'est la base de l'étage Cénomanien qui coïncide avec la première apparition du foraminifère planctonique Rotalipora globotruncanoides SIGAL (= R. brotzeni (SIGAL)) selon les recommandations du Second Symposium sur les limites des étages du Crétacé tenu à Bruxelles en 1995 (TRÖGER & KENNEDY, 1996). • Coupe-type. Pour répondre aux critères élaborés par la Commission stratigraphique internationale, toute coupe candidate pour la définition d'une limite d'étage doit être dilatée, dépourvue de discontinuité importante, facile d'accès, permanente et fossilifère. Deux coupes en cours d'investigation semblent présenter un bon potentiel : le Mont-Risou dans le bassin du Sud-Est de la France où est déjà définie la limite inférieure du Cénomanien et les environs de Kalaat Senan en Tunisie centrale. • Durée. Les analyses cyclostratigraphiques (FIET et alii, 1998) confèrent au Vraconnien une durée de 2,4 ± 0,2 Ma. Cette durée est équivalente à celle d'un étage comme le Santonien (2,3 Ma). En considérant la limite inférieure de l'étage Cénomanien à 98,9 Ma (GRADSTEIN et alii, 1994), la base du Vraconnien peut être placée par extrapolation à 101,3 Ma. • Zonation par les ammonites. Trois zones d'ammonites sont admises actuellement dans le Vraconnien du Nord-Ouest de l'Europe, du bas vers le haut les : • IZ à Mortoniceras (Mortoniceras) fallax ; • TRZ à Mortoniceras (Subschloenbachia) perinflatum ; • IZ à Arrhaphoceras (Praeschloenbachia) briacensis. A. (P.) briacensis étant un Hoplitidae dont la répartition géographique est limitée à la province nord-européenne du domaine boréal, une solution alternative à la zonation actuelle pourrait être fondée dans l'avenir sur la lignée phylétique des Stoliczkaiinae, cosmopolites, avec la succession Neophlycticeras (Neophlycticeras) blancheti, Stoliczkaia (Stoliczkaia) dispar, Stoliczkaia (Lamnayella) tetragona ou S. (Shumarinaia) africana. • Zonation par les foraminifères. L'étalonnage des zones de foraminifères par rapport aux zones d'ammonites n'est pas encore complètement élucidé. Cependant, dans l'état actuel des connaissances, le groupe de travail sur les foraminifères planctoniques a retenu l'apparition de Rotalipora appenninica (RENZ) pour marquer la base du Vraconnien. • Zonation par le nannoplancton calcaire. Le taux de spéciation plus faible du nannoplancton calcaire par rapport aux ammonites et aux foraminifères conduit à définir des zones larges qui débordent les limites du Vraconnien. Toutefois, l'apparition d'Eiffellithus turriseiffeli (DEFLANDRE) se situe aux confins de la limite entre les zones d'ammonites à M. (S.) perinflatum et à A. (P.) briacensis.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 sup-plemental 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 St-oliczkaia 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 plat-form 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 south-eastern basin of France (Salazac, Marcoule, Mont-Risou), in the Anglo-Paris basin (Folkestone, Mers-tham, 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 (ammo-nites 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 globo-truncanoides 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

The global bio-events at the Cenomanian-Turonian transition in the reduced Bahloul Formation of Bou Ghanem (central Tunisia)

In central Tunisia, the thickness of the Bahloul Formation varies generally between 20 to 40 m such as for example, respectively, at wadi Smara near Kalaat Senan and at wadi Bahloul near Maktar. The thickness is also of several tens metres at Koudiat el Azreg near Jerissa and at Tajerouine. In these sites where the thickness of the Bahloul Formation is dilated, in addition to five geochemical events (δ¹³C), nine bio-events were defined near the Cenomanian-Turonian transition, successively in ascending order: 1. LO Rotalipora cushmani, 2. Heterohelix bio-event, 3. FO Pseudaspidoceras pseudonodosoides, 4. LO P. pseudonodosoides, 5. LO Thalmanninella multiloculata, 6. “filaments” bio-event, Cenomanian-Turonian transition, 7. FO Watinoceras sp., 8. FO Pseudaspidoceras flexuosum, 9. FO Helvetoglobotruncana helvetica. Even though the Bahloul Formation in the Bou Ghanem site is only 7.7 m thick (from which only 2.4 m of laminated black limestones), all nine marker bio-events were recorded, almost in the same order except some small differences. A good number of these bio-events are global as they are also present at Pueblo (Colorado, USA), the stratotype for the base of the Turonian stage. Consequently, at a global scale, it is possible to place the Cenomanian-Turonian boundary with one or several of these bio-events, with a good level of confidence

Zones d'ammonites et de foraminifères du Vraconnien au Turonien : Une comparasion entre les domaines boréal et téthysien (NW Europe / Tunisie centrale)

Depuis la fin du 19ème siècle, l'intervalle comprenant l'Albien supérieur élevé, le Cénomanien, le Turonien et le Coniacien basal a été subdivisé d'abord en zones d'ammonites puis, à partir du milieu du 20ème siècle, en zones de foraminifères planctoniques, deux groupes de macro- et de microfossiles particulièrement efficaces en bio-chronostratigraphie grâce à leur taux d'évolution rapide. Toutefois, des différences de compositions fauniques entre le domaine boréal (Europe du Nord-Ouest) et le domaine téthysien (Méditerranée) ont longtemps empêché des corrélations précises entre ces deux domaines. Aujour-d'hui, dans un intervalle de temps couvrant environ 16 millions d'années, on dénombre 29 zones d'am-monites en domaine téthysien contre 24 en domaine boréal parmi lesquelles 16 sont communes aux deux domaines. Pour les foraminifères planctoniques, on compte 11 zones en domaine téthysien et 10 en domaine boréal, dont 7 communes.Since the end of the 19th century the interval comprising the uppermost Upper Albian, the Cenomamian, the Turonian and the basal Coniacian has been subdivided, first into ammonite zones, then, beginning in the middle of the 20th century, into zones of planktonic foraminifera. These two groups, one macrofossil, the other microfossil, are particularly effective for bio-chronostratigraphy thanks to their rapid rates of evolution. But differences in the faunal makeup between the Boreal domain (northwestern Europe) and the Tethyan domain (Mediterranean) have for a long time hindered precise correlation of the two domains. Today, in a time interval covering about 16 million years, there are 29 ammonite zones in the Tethyan domain versus 24 in the Boreal one, of which 16 are common to both domains. For the planktonic foraminifera the Tethyan domain has 11 zones, the Boreal domain 10, with 7 in common

Zonation by ammonites and foraminifers of the Vraconnian-Turonian interval: A comparison of the Boreal and Tethyan domains (NW Europe / Central Tunisia)

Since the end of the 19th century the interval comprising the uppermost Upper Albian, the Cenomamian, the Turonian and the basal Coniacian has been subdivided, first into ammonite zones, then, beginning in the middle of the 20th century, into zones of planktonic foraminifera. These two groups, one macrofos-sil, the other microfossil, are particularly effective for bio-chronostratigraphy thanks to their rapid rates of evolution. But differences in the faunal makeup between the Boreal domain (northwestern Europe) and the Tethyan domain (Mediterranean) have for a long time hindered precise correlation of the two domains. Today, in a time interval covering about 16 million years, there are 29 ammonite zones in the Tethyan domain versus 24 in the Boreal one, of which 16 are common to both domains. For the planktonic foraminifera the Tethyan domain has 11 zones, the Boreal domain 10, with 7 in common.Depuis la fin du 19ème siècle, l'intervalle comprenant l'Albien supérieur élevé, le Cénomanien, le Turonien et le Coniacien basal a été subdivisé d'abord en zones d'ammonites puis, à partir du milieu du 20ème siècle, en zones de foraminifères planctoniques, deux groupes de macro- et de microfossiles particulièrement efficaces en bio-chronostratigraphie grâce à leur taux d'évolution rapide. Toutefois, des différences de compositions fauniques entre le domaine boréal (Europe du Nord-Ouest) et le domaine téthysien (Méditerranée) ont longtemps empêché des corrélations précises entre ces deux domaines. Aujourd'hui, dans un intervalle de temps couvrant environ 16 millions d'années, on dénom-bre 29 zones d'ammonites en domaine téthysien contre 24 en domaine boréal parmi lesquelles 16 sont communes aux deux domaines. Pour les foraminifères planctoniques, on compte 11 zones en domaine téthysien et 10 en domaine boréal, dont 7 communes

The Albian Stage in its type area, the Aube (France): a synthesis in a global sedimentary context

Le département de l'Aube est la région-type de l'étage Albien créé par d'Orbigny (1842). Deux formations sont reconnues dans les faciès argileux ou "Gault" auct. du stratotype : les Argiles tégulines de Courcelles (82 m) surmontées des Marnes de Brienne (43 m). La limite entre les deux formations est définie au sommet d'un niveau induré (hardground L'Étape) aisément identifiable à la fois sur le terrain et en sondages. La région-type de l'étage Albien est particulièrement intéressante en raison de l'épaisseur exceptionnelle des faciès argileux (plus de 120 m), une situation unique dans le bassin anglo-parisien. Aujourd'hui, 82 % de la succession lithologique sont connus précisément grâce à 16 affleurements. Dans deux intervalles épais respectivement de 33 m et 28,50 m, une suite lithologique composite continue est construite à l'aide de coupes qui se suivent et se relaient, corrélées l'une à l'autre sur le terrain à l'aide de niveaux repères. Six faciès successifs sont décrits : dans les Argiles tégulines de Courcelles et du bas vers le haut : 1 - lit de nodules gréso-phosphatés ; 2 - argiles silteuses ; 3 - argiles et bancs calcaires ; dans les Marnes de Brienne : 4 - marnes argileuses ; 5 - gaize ; 6 - marnes argileuses. À une échelle plus fine, une rythmicité est démontrée dans la sédimentation. Elle se traduit dans les Argiles tégulines de Courcelles par des cycles pluri-décimétriques débutant par des niveaux silteux et limités au sommet par des surfaces perforées. Dans les Marnes de Brienne, les cycles, métriques (sauf dans la gaize où ils sont décimétriques), montrent des alternances de marne argileuse gris foncé et de marne gris pâle annonçant les craies rythmées du Cénomanien. Ces couplets correspondent probablement à des cycles de précession des équinoxes (20 ka). Treize événements à caractère lithologique (lits de nodules phosphatés, fonds durcis, niveaux riches en sable quartzeux et glauconie) et écologiques (horizons caractérisés par l'abondance momentanée d'un certain nombre de fossiles) sont identifiés dans la suite stratigraphique. Ces niveaux repères sont très utiles pour des corrélations précises à travers le bassin. Une comparaison est proposée entre les faciès sableux de l'Yonne et les faciès argileux de l'Aube. La continuité de plusieurs lits de nodules phosphatés est démontrée depuis l'Yonne au sud-ouest jusqu'à l'Aube et le Perthois au nord-est, sur une distance supérieure à 130 km. Ces niveaux repères sont interprétés comme des surfaces d'inondation de cycles eustatiques de 3e ordre. Enfin, une étude détaillée de la lithologie, complétée par une analyse des faunes d'ammonites et d'inocérames, est utilisée pour une interprétation séquentielle. Le résultat est l'identification de huit séquences eustatiques de 3e ordre dans la moitié supérieure des Argiles de Courcelles et les Marnes de Brienne, dont deux nouvelles séquences indexées AL 5a et AL 6a.The Aube department is the type locality of the Albian stage created by d'Orbigny (1842). Two formations are recognised in the clay facies (the "Gault" auct.) of the stratotype, the Argiles tégulines de Courcelles (82 m), which is overlain by the Marnes de Brienne (43 m). The boundary between the two formations is defined at the top of an indurated bed (hardground L'Étape) that is readily identifiable, both in the field, and from boreholes. The type area of the Albian stage is of great interest because of the size of the clay facies (more than 120 m) unique in the Anglo-Paris basin. Today, 82 % of the lithological succession is documented accurately from 16 outcrops. In two intervals of respectively 33 m and 28.50 m thick, composite sections are made of overlapping sections which are correlated in the field with the help of lithological marker beds. Six successive facies are described: in the Argiles tégulines de Courcelles and from bottom to top: 1 - phosphatic nodules bed; 2 - silty clays; 3 - clays and limestone beds; in the Marnes de Brienne: 4 - clay marls; 5 - gaize; 6 - clay marls. On a smaller scale, a decimetre to metre-scale rhythmicity can be identified in the sedimentation. This is reflected in the Argiles tégulines de Courcelles by pluri-decimetric cycles that start with a silty level and are bounded at the top by a bored surface. In the Marnes de Brienne, the metric cycles (except for the gaize where they are decimetric) exhibit alternating dark grey clay marl and a pale grey marl that herald the rhythmic chalks of the Cenomanian. Individual couplets probably represent the precession cycles (20 kyr). On the other hand, 13 lithoevents as phosphatic nodules beds, hardgrounds, glauconitic and sandy beds and ecoevents characterised by the brief abundance of some fossils. These marker beds are very useful for fine correlations across the basin. A comparison is proposed between sandy units of Yonne and clay facies of Aube. The continuity of several phosphatic nodules beds is shown from Yonne in the South West to Aube and Perthois in the North East, that is a distance exceeding 130 km. These marker beds are interpreted as Flooding Surfaces of 3rd-order cycles. Finally, a detailed study of the lithology, complemented with the analysis of the ammonite and inoceramid faunas, is used for a depositional sequence interpretation. As a result, eight sequences are identified in the upper half of the Argiles tégulines de Courcelles and in the Marnes de Brienne in the type area of the Albian stage, including two new sequences indexed AL 5a and AL 6a

Sedimentary record of the “Austrian” tectonic pulse around the Aptian–Albian boundary in SE France, and abroad

A tectonically-controlled forced regression occurred on the western margin of the French subalpine basin around the Aptian–Albian boundary. It nearly emptied the western part of the Vocontian Trough during the early and middle Albian. A compressional pulse associated with vertical movements along the Cevennes fault row and its satellites is inferred. This forced regression is correlated with an inverse transgressive trend, both in the Paris Basin and the northern subalpine chains. The black shales of the early Aptian OAE1a and Albian OAE1b of the Vocontian Trough succession occur within opposite regional sea level trends, transgressive for the former, regressive for the latter. The inferred tectonic pulse is also recorded on a broader scale from literature data. The overall picture also shows outphasings in relative sea level changes on a large scale, likely controlled by tectonics

Sedimentary record of the “Austrian” tectonic pulse around the Aptian–Albian boundary in SE France, and abroad

A tectonically-controlled forced regression occurred on the western margin of the French subalpine basin around the Aptian–Albian boundary. It nearly emptied the western part of the Vocontian Trough during the early and middle Albian. A compressional pulse associated with vertical movements along the Cevennes fault row and its satellites is inferred. This forced regression is correlated with an inverse transgressive trend, both in the Paris Basin and the northern subalpine chains. The black shales of the early Aptian OAE1a and Albian OAE1b of the Vocontian Trough succession occur within opposite regional sea level trends, transgressive for the former, regressive for the latter. The inferred tectonic pulse is also recorded on a broader scale from literature data. The overall picture also shows outphasings in relative sea level changes on a large scale, likely controlled by tectonics

Support for a Vraconnian Stage between the Albian sensu stricto and the Cenomanian (Cretaceous System)

International audienceThe 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

Les variations du niveau marin au cours du Turonien.

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