Carnets de Géologie / Notebooks on Geology: Article 2009/03 (CG2009_A03)

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Contents

[Introduction] [Methodology of integrating biostratigraphic data]
[Integrated Upper Albian chronostratigraphy]
[Chronostratigraphic correlation of key sections]
[Re-examination of the rationale for a 'Vraconnian Stage']
[Conclusions] and ... [Bibliographic references]


Uppermost Albian biostratigraphy and chronostratigraphy

Robert W. Scott

Precision Stratigraphy Associates & University of Tulsa, RR3 Box 103-3, Cleveland, Oklahoma 74020 (U.S.A.)

Manuscript online since April 22, 2009

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Abstract

The Albian Stage is the highest chronostratigraphic unit of the Lower Cretaceous Series and underlies the Cenomanian Stage of the Upper Cretaceous Series. The Albian is divided into three substages, each of which is composed of two or three zones based on distinctive and phylogenetically related ammonite assemblages. The uppermost zone of the Upper Albian Substage, the Stoliczkaia dispar Zone, is found in many Western European condensed sections. The ammonite assemblage in the thin glauconitic sandstone near La Vraconne, Switzerland, was defined as the 'Vraconnian Stage' in 1868. However this concept has been little used and was abandoned in 1963 as part of the Cretaceous chronostratigraphic scale. A recent proposal to resurrect and redefine this stage is based on a number of criteria and very detailed and reliable stratigraphic data. A quantitative biostratigraphic analysis of the ammonite ranges in the key sections shows that the proposed subzones of the S. dispar Zone have discordant ranges. Furthermore, the utility of a 'Vraconnian Stage' between the Albian and Cenomanian stages is geographically limited and the concept embraces one of many depositional sequence cycles of the Albian. The reinstatement of a 'Vraconnian Stage' is not recommended.

Key Words

Albian; Vraconnian; ammonites; planktic foraminifers; graphic correlation; age calibration.

Citation

Scott R.W. (2009).- Uppermost Albian biostratigraphy and chronostratigraphy.- Carnets de Géologie / Notebooks on Geology, Brest, Article 2009/03 (CG2009_A03)

Résumé

Biostratigraphie et chronostratigraphie de l'Albien sommital/terminal.- L'étage Albien constitue l'unité chronostratigraphique la plus élevée du Crétacé inférieur et repose sous l'étage Cénomanien du Crétacé supérieur. L'Albien est divisé en trois sous-étages, chacun comprenant deux ou trois zones établies sur des associations d'ammonites distinctes mais phylogénétiquement reliées. La zone sommitale du sous-étage Albien supérieur, la Zone à Stoliczkaia dispar, a été identifiée au sein de nombreuses séries condensées en Europe occidentale. L'association d'ammonite reconnue dans le mince niveau de grès glauconieux des environs de La Vraconne (Suisse) a été définie comme 'étage Vraconnien' en 1868. Toutefois ce concept a été peu usité et fut abandonné en 1963 en tant qu'unité de l'échelle chronostratigraphique du Crétacé. Il a été récemment proposé de réhabiliter et de redéfinir cet étage sur la base d'un certain nombre de critères et de données stratigraphiques très détaillées et fiables. Or une analyse biostratigraphique quantitative des répartitions des ammonites dans les coupes clefs révèle que les sous-zones proposées pour la subdivision de la Zone à S. dispar correspondent à des intervalles non concordants. En outre l'intéręt de placer un 'étage Vraconnien' entre les étages Albien et Cénomanien apparaît géographiquement limité, et le concept correspond à une seule des nombreuses séquences de dépôt de l'Albien. Il n'est donc pas recommandé de restaurer un 'étage Vraconnien'.

Mots-Clefs

Albien ; Vraconnien ; ammonites ; foraminifères planctoniques ; correlation graphique ; calibration des âges.


Introduction

Recently Amédro (2002, 2008), Amédro & Robaszynski (2008) and Robaszynski et alii (2007) proposed to reinstate the 'Vraconnian Stage' between the Albian and Cenomanian stages. This proposition, which would significantly modify the Cretaceous geologic column, merits careful and thorough analysis. Our objectives are to provide a testable global biostratigraphic database with which to evaluate the ranges of key uppermost Albian species and to calibrate their ranges to a numerical time scale.

The Cretaceous System is composed of twelve stages that can be correlated world-wide and are of different durations (Hancock, 2003). The Albian Stage is the youngest chronostratigraphic unit of the Lower Cretaceous Series and one of the longest Cretaceous stages, about 13 to 15 myr. The Albian Stage was defined by d'Orbigny (1840-1842) to include the fossil assemblages in shale and glauconite sands cropping out along the Aube River in the Department of Aube on the eastern margin of the Paris Basin (Magniez-Jannin & Rat, 1980). A stratotype section along the Aube River was composited from outcrop sections and nearby boreholes (Larcher et alii, 1965; Amédro, 1992). The Albian biota in the Paris Basin is transitional between Boreal and Tethyan realms. The concepts of Albian ammonite zones have evolved over a period of time beginning in 1868 when the "Ammonites mammillaris" zone (de Rance, 1868) (now the Douvilleiceras mammillatum Zone) and the "zone of Ammonites inflatus" (de Lapparent, 1868) (now considered equivalent to the Stoliczkaia dispar Zone) were proposed (Rawson et alii, 1978). The current zonal scheme (Fig. 1 ) was composed by Spath (1923) and Breistroffer (1947) and modified by Owen (1971) and Amédro (1992). The zonal succession has been relatively stable since 1947 and most are interval zones defined by the first appearance of an ammonite species (FO; Amédro, 1992).

The Albian Stage is divided into three substages, Lower, Middle and Upper. Although stratotype sections for these substages are yet to be agreed upon, here they are used formally following Hart et alii (1996) and are capitalized. The zonal boundaries of the substages have been used consistently since 1947 (Fig. 1 ). Prior to 1947, however, two different criteria were used to define the base of the Upper Albian Substage. Spath (1923, 1941) placed the boundary at the top of the Dipoloceras cristatum Subzone, which directly overlies the Euhoplites lautus Zone (Owen, 1971). Alternatively Breistroffer (1947) placed the Cristatum Subzone in the basal Upper Albian Mortoniceras inflatum Zone. This later opinion has been followed since (Birkelund et alii, 1984; Owen, 1984a, 1984b; Hancock, 1991; Hart et alii, 1996; Rawson & Hoedemaeker, 1999; Hoedemaeker & Rawson, 2000; Hoedemaeker et alii, 2003). The rationale for this change is reviewed by Owen (1971) and Hart et alii (1996). In North America a regional transgressive unconformity coincides with the top boundary of the Cristatum Subzone, which is a widespread mappable contact (Scott et alii, 2003).

The Albian Stage comprises the evolutionary origins and/or diversification of seven families of ammonites. Zones of the Lower, Middle and Upper Albian substages are based mainly on species of the families of Lyelliceratidae, Hoplitidae, and Brancoceratidae. Four heteromorph families first appear in the Albian, Anisoceratidae, Baculitidae, Hamitidae, and Turrilitidae, but are not used to define zones until their appearance in the Upper Cretaceous.


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Figure 1: Chart comparing three stage and substage concepts of the Albian (P. Destombes, 1979 in Magniez-Jannin & Rat, 1980; Ogg et alii, 2004, and Amédro, 2008). The positions of Albian anoxic events and key microfossils were interpolated by Ogg et alii (2004).

The 'Vraconnian Stage' was proposed by Renevier (1868) for a 2 m-thick condensed interval of green glauconitic sand with a distinctive ammonite fauna between the Upper Albian and Lower Cenomanian substages near La Vraconne in western Switzerland. The ammonite assemblage is part of the Stoliczkaia dispar Zone and the lithostratigraphic unit correlates with the Upper Gault and Upper Greensand in England (Fig. 1 ). This stage was discarded in 1963 (Collignon, 1965; Rawson et alii, 1978; Hancock, 1991, 2003, among many others). However the 'Vraconnian Stage' has been resurrected and redefined by Amédro (2002, 2008), Amédro & Robaszynski (2008) and by Robaszynski et alii (2007). Amédro cites five reasons for recognizing 'Vraconnian' sedimentary strata as a stage between the Albian and Cenomanian stages: (1) the interval is mappable in Western Europe, (2) the interval is a third-order depositional cycle that records an important eustatic event, (3) the interval has a distinctive and diverse fossil assemblage that can be recognized outside Europe, (4) its 2 to 3 myr duration is equivalent to that of the Santonian Stage, and (5) in the Vocontian Basin the interval is more than 100 m thick, which is thicker than the underlying part of the Albian (Amédro, 2002, 2008). Amédro has presented detailed lithostratigraphic and biostratigraphic data of twelve sections in Europe, Tunisia, Madagascar, and California to support his proposal. For the first time he presents a regional lithostratigraphic correlation of many of these sections. Hancock (2003) reviewed the history of the rejection of the 'Vraconnian' concept by the community of Lower Cretaceous stratigraphers beginning in 1963 and believed that reasons to revive 'Vraconnian' were "trivial". He noted that most 'Vraconnian' sections are condensed intervals.

As a framework for discussing the wisdom of revising the Albian Stage by reinstating the 'Vraconnian Stage', a review of the stage concept is relevant. "A stage is a chronostratigraphic unit of smaller scope and rank than a series. It is most commonly of greatest use in intra-continental classification and correlation, although it has the potential for worldwide recognition" (NACSN, 2005, p. 1582). As a chronostratigraphic unit the Albian Stage has synchronous boundaries and is the physical evidence or 'material referent' of a time interval, the Albian Age. The Albian Stage has traditionally been defined by a set of ammonite biozones (Fig. 1 ): d'Orbigny, 1840-1842; Spath, 1923; P. & J.-P. Destombes, 1965; Owen, 1984a, 1984b; Hancock, 1991; Hart et alii, 1996, among others. The Global Stratotype Section and Point (GSSP) have yet to be selected to define the basal boundary although an excellent section has been proposed (Kennedy et alii, 2002). The upper boundary of the Albian Stage is defined by the base Cenomanian Stage GSSP at Mont Risou, France (Gale et alii, 1996; Kennedy et alii, 2004).

Methodology of integrating biostratigraphic data

The integrated ranges of select ammonites in seven key sections defined three zones of the Vraconnian (Amédro, 2008, Fig. 4 ; Fig. 1 , Table 1). The ranges of key planktic foraminifera in the Tunisian sections and the French Mont Risou section were added to that set of species by Robaszynski et alii (2007). However most of the ammonite species are found in only one or two sections (Amédro, 2008). To test the accuracy of the integration of species ranges the quantitative technique of graphic correlation is used in this report. Graphic correlation (GC) provides an objective method to compare the ranges of species in multiple sections and the outcome can be tested independently.

Graphic correlation (GC) is a quantitative, non-statistical, technique that determines the coeval relationships between two sections by comparing the ranges of event records in both sections (Carney & Pierce, 1995). A graph of any pair of sections is an X/Y plot of the FOs (first appearances) and LOs (last appearances) of taxa found in both sections. The interpreter places a line of correlation (LOC) through the tops and bases that are at their maximum range in both sections. This LOC is the most constrained hypothesis of synchroneity between the two sections and extends the fewest bioevents. The LOC also accounts for hiatuses or faults at stratal discontinuities indicated by the lithostratigraphic record. The position of the LOC is defined by the equation for a regression line. Explanation and examples of the graphic technique are illustrated by Miller (1977) and Carney & Pierce (1995). By iteratively graphing successive sections a database of species ranges is compiled. The accuracy of these ranges depends on the number of sections, preservation and correct identification of the species. Such a database is testable and the process is transparent so that the fossil occurrence in each section can be evaluated to determine its accuracy. This process compiles data of many specialists who have studied numerous global sections.

Mont Risou, France, Section FO meters LO meters
Base limestone interval 330 m
Actinoceramus sulcatus 50 78
Actinoceramus subsulcatus 25
Anisoceras salei 78
Anisoceras perarmatum 78
Arrhaphoceras briacensis 305
Dipoloceras cristatum 18 25
Hysteroceras orbignyi (as sp.) 78
Lechites moreti 155
Mantelliceras mantelli 205
Mariella gresslyi 155
Mortoniceras perinflatum 205
Rota globotruncanoides 295
Turrilitoides hugardianus 155
Folkestone, UK, Section
Base cenomanian ammonites 38
Inoc concentricus 0 10.5
Actinoceramus sulcatus 9.7 13
Anisoceras perarmatum 26 38
Arrhaphoceras substuderi 26 38
Callihoplites auritus 14.5 24.5
Callihoplites cantabrigense 25
Callihoplites leptus 25
Callihoplites tetragonus 26 38
Callihoplites vraconensis 26 38
Hyphoplites coelonotus 25
Lepthoplites falcoides 26 38
Mortoniceras inflatum 14.5 25
Pleurohoplites renauxianus 26 38
Merstham, UK
Anisoceras picteti 4.7 5
Callihoplites seeleyi 7 7.2
Callihoplites vraconensis 4.7 7.2
Idohamites elegantulus 4.7 5
Lechites gaudini 7
Lepthoplites falcoides 4.7 5
Lepthoplites pseudoplanus 4.7 7.2
Mortoniceras alstonensis 4.7 5
Mortoniceras fallax 4.7 5
Neophlycticeras blancheti 4.7 6.6
Ostlingoceras puzosianum 7 7.2
Pleurohoplites renauxianus 7 7.2
Harchies, Belgium
Anisoceras perarmatum -87.1
Anisoceras pseudoelegans -95.5
Callihoplites vraconensis -87.1
Hamites virgulatus -98.4
Hyphoplites subfalcatus (ID cf.) -86.1
Lepthoplites cantabrigiensis -102.1
Pleurohoplites subvarians -87.1
Strépy, Belgium, Outcrop
Callihoplites seeleyi 30
Callihoplites pulcher 30
Callihoplites tetragonus 20
Cantabrigites subsimplex 30
Hyphoplites valbonnensis 20
Lechites gaudini 20
Mortoniceras fallax 20
Neophlycticeras blancheti 20
ANDRA MAR 203 Core, France
Hamites virgulatus (ID cf.) -523.23
Hyphoplites coelonotus -776.75 -523.38
Hyphoplites falcatus -455.85
Hyphoplites valbonnensis -487.17
Lechites gaudini -487.17
Mariella bergeri (ID cf.) -467.25
Ostlingoceras puzosianum -561.54
Pleurohoplites renauxianus -581.8
Schloenbachia varians -430.8
Diégo core, Madagascar
Lechites gaudini -50
Mantelliceras mantelli -23 -5
Mariella bergeri -20
Neostlingoceras carcitanense -23 -5
Scaphites simplex -54
Sciponoceras roto -23 -5
Stoliczkaia dispar -50
Biti breggiensis -215 -118
Planomalina buxtorfi -106 -10
Planomalina praebuxtorfi -118 -90
Rota appenninica -131 -10
Rota brotzeni -23 -10
Rota globotruncanoides -23 -10
Tici praeticinensis -215 -180
Tici primula -236 -135
Tici subticinensis -215 -155
Tici ticinensis -180 -106

Table 1: Biostratigraphic species and ranges in meters of each section from Amédro (2008).

Integrated Upper Albian chronostratigraphy

Process of Graphic Correlation Experiment

To begin the GC experiment the Mont Risou section, southeastern France (Gale et alii, 1996; Fig. 2 ) was selected as the standard reference section because it records continuous basin deposition at a uniform rate of accumulation and it yields diverse ammonites, inoceramids, planktic foraminfera, and nannofossils. This section was cross plotted to itself setting its thickness in meters as the relative time scale. Subsequently seven other sections, which yielded precise and abundant biostratigraphic data, were plotted to it through multiple rounds and the ranges of the fossils extended to the thickness scale at Mont Risou. The first section plotted was the Kalaat Senan, Tunisia (Amédro, 2008, Fig. 20 ), which spans from uppermost Albian to Cenomanian; this section also represents uniform deposition. The result is that the scale was extended from the base of the Upper Albian to the Cenomanian/Turonian contact.


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Figure 2: Biostratigraphic range chart of Mont Risou Albian-Cenomanian reference section showing key ammonites, planktic foraminifera and nannofossils (from Gale et alii, 1996).

The second graphed section was the Diégo well, N Madagascar (Amédro, 2008, Fig. 22 ). This 300 m cored interval is mainly marl that spans an interval from lowermost Cenomanian to middle Albian (Fig. 3D ). The section yields several key planktic foraminifera and in three samples a few ammonites. The LOC of the graph is constrained by a small number of first occurrences and it extends the bases of many foraminifera. The section appears to record continuous Late Albian deposition.

The third section graphed was the cored interval in the ANDRA MAR 203 well (Amédro, 2008, Fig. 8 ). This interval is about 560 m thick and spans from the Cenomanian to the Aptian stages (Fig. 3C ). A significant unconformity, probably a sequence boundary, is at the base of the Albian Valbonne Formation (-827 m), which is a glauconitic sandstone that overlies Aptian marls. A second unconformity, a transgressive contact, is at the base of a phosphatic interval in the upper part of the Valbonne at -797.9 m. The Valbonne grades up into the Marcoule Formation. Biostratigraphic data appear 18 m above this upper break and span an interval about 350 m thick. In this section Amédro placed the base of the 'Vraconnian' at the unconformity in the upper part of the Valbonne so that the 'Vraconnian' is 334 m thick. The LOC of the section is constrained by the FO and LO of Hyphoplites coelonotus and the LOs of three other ammonites (Fig. 3C ). A short hiatus is suggested by the offset of the LOC in the basal Cenomanian. An alternative interpretation is that the interval recorded a reduced rate of sediment accumulation beginning in latest Albian times. At the Marcoule locality the complete section, from its base at -781 m to the base of the Cenomanian at about -464 m, correlates with zones XII-XIII in the near shore Folkestone section, where these same zones are only 13.8 m thick.

The fourth section to be added was the Mont Risou section in Amédro (2008, Fig. 11 ), which was based on data from multiple sources including Gale et alii (1996). Thus the slope of the LOC is the same as that of the standard reference section. This plot is well constrained by several first and last occurrences; it extends some ranges and it adds three new species not found in other sections (Fig. 3A ).

The next section plotted was the Gault Clay in the Folkestone section in Amédro (2008, Fig. 14 ). This section spans several significant unconformities (Fig. 3B ), one at the base of the Gault Clay overlying the Lower Greensand, a second at the condensed interval VIII zone, a third at the XII zone, and the highest at the Albian/Cenomanian contact between the Gault and the glauconitic marl (Hart, 2000). The LOC of the lower interval of zones I-VII is poorly constrained at the top by the FO of Actinoceramus sulcatus and its slope is the same as that of the higher LOCs. The unconformity break is placed at the top of zone VIII. The LOC in the next higher interval of zones IX-XI is constrained by the range of Callihoplites auritus. In the next higher section of zones XII-XIII the LOC is constrained by several FO and LO bioevents (Fig. 3B ). This section plot defines the relative duration of the hiatus of two unconformities as meter thicknesses in the Mont Risou section; these intervals can now be projected into other graphed sections.

An older ammonite data set from Folkestone by Amédro (1992) is graphed by several FO and LO's. The unconformities are well constrained as in the newer section in Amédro (2008). The Merstham, England (Amédro, 2008, Fig. 15 ) repeats the uppermost part of the Upper Greensand Formation that is present at Folkestone. The data are confined to two horizons so the LOC is not well constrained. However these data adjust nine FO's and nine LO's. The Strépy and Harches sections were graphed next (Amédro, 2008, Fig. 17 ) and have limited data so that the LOC of each is not tightly constrained.


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Figure 3: Graphic correlation interpretations of four key 'Vraconnian' sections. First occurrences (FO) of species are indicated by a square symbol (□) and the last occurrence (LO) by a plus sign (+). Superposed symbols of a species indicates a single occurrence. Symbols on the left of the Y axis indicate that the occurrence is only in the section they accompany. CSU is a composite standard unit calibrated in meters to the thickness of the Mont Risou section (file MIDK24). Horizontal solid and dotted lines are stratigraphic contacts; inclined dashed lines project a position in the section into the position of a species in the set of sections in the database of "Vraconnian" sections; these lines are 'lines of correlation' (LOC). Roman numerals indicate ammonite zones in the English sections.

Process of calibration to numerical ages

The graphic correlation experiment resulted in a list of species and their ranges relative to each other in the metric scale of the Mont Risou section. The next step in the experiment was to convert the metric scale to a numerical age scale in mega-annum units. This was accomplished by plotting the composited range data set of Amédro's 'Vraconnian' data set, VRACCS.1, to the MIDK45CS.1 composited range data set (Fig. 4 ). The MIDK45CS.1 data set is the next development stage beyond MIDK3 (Scott et alii, 2000) and MIDK42CS.1 (Scott, in press; see also data of MIDK42 on website precisionstratigraphy.com). It is composed of more than one hundred sections and nearly 3000 bioevents, geochemical events, magnetochrons, and sequence stratigraphic contacts. The scale of this range data set is the numerical time scale of Harland et alii (1990), as revised by Ogg et alii (2004), with the exception of the age ascribed the base Cenomanian, 97.13 Ma, because correlations of radiometrically dated bentonites and bioevents have been re-evaluated (Oboh-Ikuenobe et alii, 2007).

The two unconformities in the uppermost Albian Upper Greensand Formation are well defined by the data plot (Fig. 4 ). The two hiatuses separate the VRACCS.1 section into three intervals; the lower break is at -355 meters and the upper break is at -249 meters, which is the base of the 'Vraconnian Stage'. The duration of the lower hiatus is 103.70 to 102.91 Ma, and that of the upper hiatus 100.15 to 98.26 Ma. The LOC of the lower interval is well constrained by numerous bioevents. This lower interval represents two rates of sediment accumulation, a slower basal interval and a faster upper interval. This rate change is indicated by the change in the LOC slope. The LOC of the middle interval is tightly constrained by ammonites and planktic foraminifera. The upper interval includes numerous bioevents and the LOC is very tightly constrained. The age of the 'Vraconnian Stage' spans from 98.26 to 97.13 Ma. The black shale, organic-rich Breistroffer marker beds were deposited at the beginning of the 'Vraconnian'.


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Figure 4: Graphic correlation plot of VRACCS.1 database to the global integrated MIDK45CS.1 database.

Integrated biostratigraphic ranges

The stratigraphic ranges of 46 ammonites that were recorded in the seven key sections of Amédro (2008) were integrated into a single range chart at the scale of the thickness of the Mont Risou section (Fig. 5 ). This scale was converted to a numerical age scale by graphing the composited VRACCS.1 data set to the MIDK45CS.1 data set as explained above. The three Middle Albian Substage zones are defined by the FO of the nominate species (Fig. 1 ; Table 2). The base of the Upper Albian Substage is defined by the FO of Dipoloceras cristatum, which is the basal subzone of the Inflatum Zone. Two of the next overlying subzones are defined in this dataset by the FOs of Hysteroceras orbignyi and Callihoplites auritus; Hysteroceras varicosum is not included in this dataset. The base of Stoliczkaia dispar defines the base of the uppermost Upper Albian Dispar Zone.

Amédro (2008) proposed that the base of the 'Vraconnian Stage' be defined by the FOs of Mortoniceras fallax and Neophlycticeras blancheti. He included three ammonite zones in this stage in ascending order: Mortoniceras (Mortoniceras) fallax Interval Zone, Mortoniceras (Subschloenbachia) perinflatum Total Range Zone, and the Arrhaphoceras (Praeschloenbachia) briacensis Interval Zone (Fig. 1 ). The composited dataset of his sections shows that M. inflatum first appears lower in the section than M. fallax (Fig. 5 ) because Gale et alii (1996) reported it lower in their Mont Risou section than did Amédro (2008, Fig. 11 ). The FO of A. briacensis is at the base of the Cenomanian Stage based on this set of sections. The ranges of Amédro's key zonal species (Amédro, 2008, Fig. 4 ) are dashed lines on Fig. 5 ; some ranges are similar and others are quite different from the ranges derived by graphic correlation of his data. S. dispar has a short range because it occurs in two sections, the Diégo well and Gale's et alii Mont Risou section. Both Mariella bergeri and Stoliczkaia clavigera are low in the Mont Risou section (Gale et alii, 1996). Some species have longer ranges in Amédro's dataset because they occur with species of other zones in condensed intervals in other sections not graphed, such as at the 'Vraconnian' stratotype (Renz & Jung, 1978) and near Drap in the Alpes-Maritimes, France (Delanoy & Latil, 1988).

This graphic correlation experiment shows that the base of the S. dispar Zone and the FO of M. perinflatum are significantly lower (31 m) and older (380 kyr) than the FO of either M. fallax or N. blancheti. Thus the concept of a 'Vraconnian Stage' is not equal to the Dispar Zone. However the FOs of M. perinflatum, M. fallax/N. blancheti, and A. briacensis may be used to divide the Dispar Zone into three subzones.


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Figure 5: Chronostratigraphic range chart of ammonites in 'Vraconnian' composited database of key 'Vraconnian' sections. Ages interpolated by graphic correlation of the VRACCS.1 database with the MIDK45CS.1 database. The ages of some ranges are younger in the subset of 'Vraconnian' sections than in the complete set of sections in the MIDK45CS database used in Figure 7 .

TAXA VRACCS: FO - LO MIDK45CS: FO - LO
Acanthoceras amphibolum 65.0472 65.0472 94.3454 92.6212
Acanthoceras rhotomagense 5.9825 43.151 94.9483 92.5959
Actinoceramus subsulcatus -406.00 -406.00 104.79* 104.79*
Actinoceramus sulcatus -381.00 -329.0148 104.4776 101.5134
Algericeras boghariense -103.4988 -93.7467 96.5874 96.2374
Algericeras proratum -92.6427 -64.6744 96.4245 95.578
Amphizygus brooksii -236.00 -80.00 124.717 65.505
Anahoplites daviesi -414.8517 -410.6029 105.1156 104.8296
Anahoplites intermedius -465.8373 -450.2584 107.1613 106.32
Anisoceras perarmatum -353.00 -180.0123 98.426 97.5888
Anisoceras picteti -248.88 -247.2 94.8091 94.3577
Anisoceras pseudoelegans -188.2678 -188.2678 97.68* 97.68*
Anisoceras salei -353.00 -353.00 102.7656 102.7656
Arrhaphoceras briacensis -136.00 -132.00 97.0865 97.0865
Arrhaphoceras substuderi -236.8768 -180.3551 98.18* 97.6*
Axopodorhabdus albianus -236.00 102.7677 109.77 90.6992
Axopodorhabdus dietzmannii -139.916 35.7909 128.6386 82.56
Beudanticeras beudanti -410.6029 -321.1158 104.9638 103.0425
Biticinella breggiensis -367.6759 -250.0185 105.757 97.5059
Calculites anfractus -140.00 -84.00 97.1702 96.5842
Callihoplites auritus -321.1924 -269.043 103.0146 100.4255
Callihoplites cantabrigense -280.00 -230.00 98.6353 98.1121
Callihoplites leptus -241.587 *** 98.23* ***
Callihoplites pulcher -170.00 -170.00 97.49* 97.49*
Callihoplites seeleyi -236.00 -170.00 98.17* 97.49*
Callihoplites tetragonus -236.8768 -180.00 98.18* 97.60*
Callihoplites vraconensis -248.88 -180.0123 97.49* 97.60*
Cantabrigites subsimplex -170.00 -170.00 97.49* 97.49*
Carbon peak OAE 2 100.9276 104.9757 93.52 93.45
Chiastozygus bifarius -236.00 -84.00 98.1749 65.505
Chiastozygus litterarius -236.00 -80.00 128.2573 65.5535
Chiastozygus platyrhethus -236.00 -80.00 125.5054 88.3575
Corollithion kennedyi -80.00 101.6637 96.55 92.00
Corollithion madagaskarensis -236.00 -80.00 122.358 65.3262
Corollithion signum -236.00 -80.00 114.0455 70.4437
Cretarhabdus conicus -236.00 -80.00 133.1464 64.5992
Cretarhabdus striatus -236.00 -80.00 117.1136 65.505
Cribrosphaerella ehrenbergii -236.00 -80.00 116.0163 94.3685
Dicarinella algeriana 38.73 118.2239 95.105 89.7362
Dicarinella hagni 80.3194 118.2239 93.8363 89.1675
Dimorphoplites niobe -449.5502 -436.8038 106.3502 105.8965
Dipoloceras cristatum -413.00 -406.00 105.5386 104.1574
Discorhabdus ignotus -236.00 -80.00 128.6386 65.505
Eiffellithus turriseiffelii -236.00 -80.00 101.856 64.4399
Ellipsagelosphaera ovata -216.00 -112.00 121.959 88.1021
Eprolithus apertior -236.00 -80.00 121.1235 96.5423
Eprolithus floralis -236.00 -84.00 122.8468 85.5995
Euhoplites lautus -433.9713 -410.6 105.9188 104.6453
Euhoplites loricatus -465.8373 -436.8038 106.9309 105.8965
Euhystrichoceras nicaisei -103.4988 -19.04 96.5874 94.5395
Euomphaloceras septemseriatum 108.6557 108.6557 93.88 93.15
Favusella washitensis -139.916 11.8706 114.1975 94.84
Forbesiceras beaumontianum -127.8239 -128.8911 96.9524 96.2621
Gartnerago nanum -236.00 -96.00 98.296 94.23
Gartnerago obliquum 107.1837 128.16 99.2536 64.743
Gartnerago praeobliquum -84.00 *** 96.5842 ***
Gartnerago stenostauron -139.916 -128.3391 97.588 96.864
Gartnerago theta -108.00 -80.00 96.8353 89.0722
Graysonites azregensis -127.82 -127.2379 96.95* 96.94*
Graysonites cobbani -126.0738 -122.2903 96.92* 96.87*
Hamites virgulatus -191.1179 -165.9805 97.71* 97.45*
Hayesites albiensis -232.00 *** 118.6477 97.3296
Hedbergella libyca -236.00 -212.00 100.6217 94.66
Helenea chiastia -224.00 102.7677 133.9695 92.7
Helicolithus trabeculatus -236.00 -80.00 122.8533 65.505
Hoplites dentatus -472.9187 -465.8373 107.5987 106.93
Hyphoplites coelonotus -242.942 -165.00 97.4319 97.4319
Hyphoplites falcatus -132.0523 -132.0523 96.2621 96.08
Hyphoplites subfalcatus -179.0295 -179.0295 97.51* 97.51*
Hyphoplites valbonnensis -180.00 -155.0337 97.5888 97.5888
Hypoturrilites gravesianus -128.8911 -112.3309 96.9684 94.84
Hypoturrilites schneegansi -127.8239 -98.9 96.9524 96.3536
Hysteroceras orbignyi -353.00 -318.414 104.226 101.71
Idohamites elegantulus -248.88 -247.2 98.31* 98.29*
Inoceramus anglicus -130.00 -96.00 104.1574 96.7098
Inoceramus concentricus -435.00 -375.0571 107.5807 101.71
Inoceramus crippsi -97.00 -38.00 96.7471 94.2
Lechites gaudini -236.00 -151.2174 98.6353 96.9133
Lechites moreti -276.00 -276.00 98.59* 98.59*
Lepthoplites cantabrigiensis -194.7543 -194.7543 97.75* 97.75*
Lepthoplites falcoides -248.88 -180.3551 98.58* 98.31*
Lepthoplites pseudoplanus -248.88 -234.88 98.31* 98.16*
Lithraphidites acutus -90.8027 101.6637 96.3969 92.7
Lithraphidites alatus -139.916 -105.5228 98.296 93.2
Lithraphidites carniolensis -236.00 -80.00 134.1294 65.4901
Lithraphidites pseudoquadratus -236.00 21.0707 98.1749 93.87
Lyelliceras lyelli -475.7512 -475.7512 107.84 107.5987
Manivitella pemmatoidea -236.00 -80.00 133.9695 65.2514
Mantelliceras dixoni -41.4901 -41.49 95.6585 95.4
Mantelliceras mantelli -132.00 -38.00 97.0656 95.65
Mantelliceras saxbii -127.8239 -54.0022 96.9643 95.5
Mariella bergeri -280.00 -129.6378 98.6353 96.8887
Mariella cenomanensis -127.8239 -112.3952 96.9524 95.7203
Mariella gresslyi -276.00 -276.00 98.59* 98.59*
Markalius circumradiatus -228.00 -80.00 134.0985 82.2933
Marker bed Breistroffer -235.00 -224.00 98.1644 98.0493
Mojsisovicsia subdelaruei -436.8038 -436.8038 105.8965 105.8965
Mortoniceras alstonensis -248.88 -247.2 98.31* 98.29*
Mortoniceras fallax -248.88 -180.00 98.31* 97.58*
Mortoniceras inflatum -321.1158 -241.59 103.0425 99.1149
Mortoniceras perinflatum -280.00 -226.00 98.6353 98.0702
Mortoniceras pricei -342.7298 -318.414 104.1574 102.1977
Neolobites vibrayeanus 101.6637 102.7677 93.7514 93.12
Neophlycticeras blancheti -248.88 -180.00 98.31* 97.58*
Neostlingoceras carcitanense -132.00 -38.00 97.0656 96.1028
Ostlingoceras puzosianum -236.00 -129.6378 98.17* 96.97*
Oxytropidoceras roissyanum -472.9187 -472.9187 108.0147 107.58
Percivalia hauxtonensis -144.00 -144.00 97.2121 93.47
Placozygus fibuliformis -230.00 -80.00 98.1121 65.505
Planomalina buxtorfi -236.00 -119.00 100.606 96.0093
Planomalina praebuxtorfi -250.0185 -216.0556 100.7341 97.9467
Pleurohoplites renauxianus -236.8768 -180.3551 98.18* 97.60*
Pleurohoplites subvarians -180.0123 -180.0123 97.59* 97.59*
Praeglobotruncana delrioensis -222.9565 101.6637 108.0343 92.00
Praeglobotruncana stephani -235.7826 118.2239 100.509 90.41
Prediscosphaera columnata -236.00 -80.00 122.8533 90.7
Prediscosphaera cretacea -84.00 -80.00 119.3508 64.523
Prediscosphaera spinosa -220.00 -80.00 121.9656 64.4953
Pseudaspidoceras flexuosum 109.7598 109.7598 93.044 93.00
Quadrum gartneri 107.1837 128.16 93.5451 68.3833
Rhagodiscus achlyostaurion -236.00 -80.00 124.6741 92.00
Rhagodiscus angustus -236.00 -80.00 123.278 65.505
Rhagodiscus asper -236.00 -80.00 128.6386 92.95
Rhagodiscus splendens -228.00 -80.00 123.278 65.551
Rotalipora appenninica -265.787 23.6467 100.5159 94.48
Rotalipora brotzeni -136.00 -119.00 97.0842 94.62
Rotalipora cushmani 33.9508 101.6637 96.17 93.07
Rotalipora deeckei 23.6467 37.9989 95.3597 93.2
Rotalipora evoluta -139.916 -52.1622 97.588 95.05
Rotalipora gandolfi -222.9565 88.7835 98.9078 93.52
Rotalipora globotruncanoides -136.00 101.6637 97.13 93.15
Rotalipora greenhornensis -100.0028 99.8236 97.0842 93.07
Rotalipora montsalvensis -59.5223 101.6637 97.384 93.15
Rotalipora reicheli -1.0096 32.1108 96.706 94.42
Rotelapillus crenulatus -236.00 -80.00 124.612 65.597
Rotelapillus laffittei -224.00 -80.00 134.0402 66.407
Scaphites simplex -172.3889 *** 97.51* ***
Schloenbachia varians -119.9188 -38.00 97.0648 95.12
Sciponoceras roto -132.00 -50.00 97.0656 96.2284
Sharpeiceras laticlavicum -76.0825 -42.2261 96.8389 95.45
Sharpeiceras schlueteri -127.8239 -103.4988 96.9524 96.4478
Staurolithites glaber -232.00 -112.00 98.133 96.8772
Stoliczkaia africana -222.9565 -131.2738 98.04* 97.00*
Stoliczkaia clavigera -280.00 -132.00 98.6353 97.0656
Stoliczkaia dispar -280.00 -280.00 98.6353 97.116
Stoverius achylosus -172.00 -80.00 123.3079 97.116
Tegumentum stradneri -230.00 -80.00 129.3543 83.3875
Tetrapodorhabdus coptensis -230.00 -80.00 128.4545 93.05
Ticinella praeticinensis -367.6759 -325.2222 110.9318 97.4606
Ticinella primula -393.1481 -270.6389 111.614 97.3667
Ticinella subticinensis -367.6759 -232.00 103.1399 97.82
Ticinella ticinensis -325.2222 -140.00 101.8279 96.8779
Tranolithus gabalus -230.00 -80.00 123.9877 83.5584
Tranolithus minimus -220.00 -132.00 98.0074 65.505
Tranolithus orionatus -230.00 -80.00 110.9137 69.1743
Turrilites acutus 26.5908 71.6713 94.64 93.98
Turrilites costatus -23.8259 26.5908 95.3943 94.14
Turrilites scheuchzerianus -14.8097 -0.6416 96.1343 94.07
Turrilitoides hugardianus -276.00 -260.00 98.426 98.426

Table 2: Biostratigraphic ranges of Albian Stage ammonites, planktic foraminifera and nannofossils zones in 'Vraconnian' sections in metric units in the Mont Risou section (VRACCS.1) and numerical ages in the global MIDK45CS.1 database; ages with asterisk are interpolated by plotting the 'Vraconnian' database, VRACCS.1 to the MIDK45CS.1 database.

Integration of key planktic foraminifera and calcareous nannofossils (Fig. 6 ) based on the limited dataset of Amédro (2008) shows that R. appenninica is below the base of Mortoniceras fallax, and the first occurrence of E. turriseiffelii is slightly above it. In the larger MIDK45 dataset the FO of E. turriseiffelii is projected at 101.86 Ma and R. appenninica at 100.37 Ma, both of which are significantly older than the FO of M. fallax at 98.26 Ma. So neither pelagic species is useful as a proxy guide to the base of a 'Vraconnian Stage'.

Five inoceramid species are recorded in the sections at Mont Risou and at Folkestone (Fig. 6 ). Actinoceramus concentricus, Actinoceramus sulcatus, and Actinoceramus subsulcatus characterize the lower part of the Upper Albian and Inoceramus anglicus and Inoceramus crippsi characterize the Lower Cenomanian.


Click on thumbnail to enlarge the image.

Figure 6: Chronostratigraphic range chart of planktic foraminifera, calcareous nannofossils, and inoceramids in 'Vraconnian' composited database of key 'Vraconnian' sections. Vertical axis is meters in the Mont Risou section. Ages in Ma are interpolated by graphic correlation of the VRACCS.1 database with the MIDK45CS.1 database. Highest part of composite section not converted to ages. Foraminifer zones are defined by FO of named taxa. Nannofossil zones are defined by FO of E. turriseiffelii and C. kennedyi respectively. The FO of Lithraphidites acutus is in the middle part of the range of the Lower Cenomanian ammonite Mantelliceras mantelli in the Kalaat Senan section, Tunisia and 103.5 m above the base of Rotalipora globotruncanoides (MIDK.10 section; Robaszynski et alii, 1994).

The Albian stage is divided into seven zones and twenty-five subzones. The ranges of key fossils that define these zones can be calibrated to numerical Ma ages by graphic plots to sections bearing dated bentonites and geochemical events. This process measures the durations of the zones as proposed by Amédro and Robaszynski (2008). This database is anchored to bentonites in the U.S. Western Interior dated by Obradovich (1993) and projected to the age of Magnetochron M0 at the base of the Aptian. As new radiometric ages are accrued this database can be tested and adjusted to accommodate new data.


Click on thumbnail to enlarge the image.

Figure 7: Chronostratigraphic chart of the original Albian Stage. Ammonite zones from Ogg et alii (2004), Hoedemaeker et alii (2003), Hoedemaeker & Rawson (2000), Hoedemaeker et alii (1993), Hancock (1991), and Owen (1984a, 1984b). Planktic foraminifera zones based on FOs from Premoli Silva & Sliter (2002). Nannofossil zones based on FOs from Bown et alii (1998) and Bralower et alii (1995). Numerical ages interpolated by graphic correlation of MIDK45 database (Scott et alii, 2000). NA indicates species not in the MIDK45 database.

Chronostratigraphic correlation of key sections

The graphic correlation method provides data for chronostratigraphic correlation of the key 'Vraconnian' sections (Fig. 8 ). The proposed 'Vraconnian Stage' correlates with the upper part of the Upper Albian Substage (Fig. 4 ). The cross section datum is the Albian/Cenomanian stage boundary as defined by the FO of R. globotruncanoides. Amédro used the FOs of Mortoniceras fallax and Neophlycticeras blancheti to define the base of the 'Vraconnian', which is approximated by the -250 m position in the VRACCS.1 database. This position correlates with the unconformity at the base of zones XII-XIII in the Folkestone section and with the transgressive facies between -800 and -781 m in the MAR 203 core at Marcoule. The same time line also correlates within the lowstand limestone bundles in the Mont Risou and Diégo core. The entire interval of the Bracquegnies Formation in the Strépy boatlift section and in the Harchies N° 1 well (Amédro, 2008, Fig. 17 ) correlates with Folkestone zones XII-XIII. The organic-rich Breistroffer interval in the Mont Risou section lies within the lower part of the transgressive interval equivalent to zone XII; it may represent maximum flooding. In the Mont Risou section a lower bundle of bioclastic-glauconitic limestone from about 50 to 80 m (Amédro, 2008, Fig. 11 ) correlates with the lower condensed section between zones VIII and IX at Folkestone.

The thickness of the 'Vraconnian Stage' varies greatly among the sections as noted by (Amédro, 2008). In the MAR 203 core at Marcoule the equivalent 'Vraconnian' interval is 317 m thick, eight times than thicker than at the Strépy section, more than twenty-three times thicker than at Folkestone, and nearly 160 times thicker than in the 'Vraconnian' reference section in Switzerland. The rates of sediment accumulation varied from 4.5 m/myr at Folkestone to 37.6 m/myr at Mont Risou, and 111.6 m/myr at Marcoule. The rate of accumulation is based on the compacted section and is not a sedimentation rate. These rates are based on the duration of about 3 myr for the 'Vraconnian' interval. This great range in rates is based on very different basin subsidence histories and tectonic conditions of each section.


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Figure 8: Stratigraphic correlation of key 'Vraconnian' sections using base of Cenomanian Stage as datum. CSU values are meters in the Mont Risou section of Gale et alii (1996, MIDK.24). Ages interpolated by graphic correlation of the VRACCS.1 database with the MIDK45CS.1 database. Age of lower part of Valbonne Formation below intra-formational unconformity at -800 m not projected because of the absence of fossils.

Re-examination of the rationale for a 'Vraconnian Stage'

  1. The interval is mappable

This criterion is essential to the definition of lithostratigraphic units such as formations (NACSN, 2005). However, it is not part of the definition of a stage, in which lithofacies change from basin to basin. The lithologies that comprise the 'Vraconnian' interval are quite different from section to section (Amédro, 2008), and thus do not make up a mappable lithostratigraphic unit.

  1. The interval records an important eustatic event

Indeed the 'Vraconnian' interval records a third-order three myr depositional cycle of transgression and regression on a regional even global scale. This feature, however, defines sequence stratigraphic units, not stages (NASCN, 2005). The Upper Albian Stage records five such sequences of this scale (Scott et alii, 2003), but it would be impractical to divide the Albian into five stages.

  1. The interval has a distinctive and diverse fossil assemblage

This property is an essential feature of a stage concept. Three species of Mortoniceras and one species of Stoliczkaia comprise the 'Vraconnian Stage' according to Amédro (2008) and Amédro & Robaszynski (2008). Amédro (2008) characterizes the 'Vraconnian' by the abrupt diversification of heteromorph ammonites of the Turrilitidae, Hamitidae, Anisoceratidae, and Baculitidae in the condensed La Vraconne section, where they comprise 60% of the specimens. However, species of three of these families appear earlier in the Late Albian between 102.47 and 98.64 Ma (Fig. 5 ) earlier than the FO of Mortoniceras fallax and Neophlycticeras blancheti.

The three heteromorph species restricted to the 'Vraconnian' in Amédro's database (2008) are primarily found in Western Europe. In the Carpatho-Balkan region of Eastern Europe the 'Vraconnian' is represented by a condensed interval of glauconitic and phosphatic sandy limestone less than one meter thick; the only ammonite species of the zone is Stoliczkaia notha (Kutek & Marcinowski, 1996). One North American section, Dry Creek in northern California (Amédro, 2008, Fig. 24 ), yields three ammonite species of the Dispar Zone, only one of which, Lechites gaudini, is found at the type section of the 'Vraconnian' in Switzerland (Renz & Jung, 1978). Few species characteristic of the 'Vraconnian' are found outside of Western Europe. The ammonite assemblage is not widespread in the Tethyan or Boreal Realms.

  1. The interval duration is similar to that of other stages

The proposed duration of the 'Vraconnian Stage' is 2-3 myr, which is equivalent to that of the Santonian Stage. However the durations of Cretaceous stages vary from 2.3 to 13 myr (Ogg et alii, 2004) and duration is not a criterion for defining a stage.

  1. The interval locally is quite thick

Sections bearing the 'Vraconnian' fauna are locally thicker than the underlying part of the Albian. The thickness difference is highly variable from basin to basin and within basins. Such thickness differences are found between condensed sections and coeval basin margin and basin center sections of many zones.

Conclusions

The diverse ammonite assemblage in the uppermost part of the Upper Albian Substage comprises a distinctive zone in Western Europe that can be subdivided into three subzones. These subzones are recognized in many Tethyan and transitional Boreal sections in Europe. However, at the 'Vraconnian' type section in Switzerland Stoliczkaia dispar and Mortoniceras perinflatum are the only zonal named taxa present. The strata bearing the S. dispar Zone are bounded by unconformities in many sections and they are but one of five Upper Albian depositional sequences. The thickness of this sequence varies from condensed sections of two meters to expanded basinal sections more than 300 meters thick. The S. dispar Zone represents a time interval of about three myr, which is about the same duration as the briefest Cretaceous ages. This interval is very useful as a biostratigraphic unit and a third-order sequence stratigraphic unit. However this interval is not a practical chronostratigraphic unit such as a stage because its boundaries cannot be demonstrated to be synchronous and the interval is not isochronous, nor is it globally recognizable. Defining this interval as a stage equivalent to the Albian and Cenomanian stages would materially alter the concept of the Albian Stage by deleting its uppermost zone. The concept of a 'Vraconnian Stage' is not a practical subdivision of the Cretaceous System.

Acknowledgments

Michael Wagreich and anonymous referees offered very useful and constructive suggestions that clarified this contribution.

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