Contents
[Bibliographic references]
In the stratotype of the Lower Aptian substage the position of the assemblage that includes Roloboceras spp. and Megatyloceras spp. has been clearly established as being in the middle part of the upper Bedoulian. Some authors have erected this horizon as a subzone, or even a zone, with R. hambrovi as species-index. In other areas (southern England, eastern Spain, the Ardèche in southeastern France), where taphonomic conditions are not always as favourable as they are in the stratotype, the level at which this assemblage occurs seems to be confined to the upper part of the lower Bedoulian. Various hypotheses are considered in an attempt to explain this divergence.
Biostratigraphy; ammonites; Cretaceous; Aptian.
P., M., G. & G. (2008).- About the stratigraphic position of the Lower Aptian Roloboceras hambrovi (Ammonoidea) level.- Carnets de Géologie / Notebooks on Geology, Brest, Letter 2008/03 (CG2008_L03)
Sur la position stratigraphique du niveau à Roloboceras hambrovi (Ammonoidea) dans l'Aptien inférieur.- L'association à Roloboceras spp. et Megatyloceras spp. est clairement positionnée dans la partie moyenne du Bédoulien supérieur dans le stratotype du sous-étage, au point d'avoir été érigée en sous-zone ou même en zone, avec R. hambrovi comme espèce-index, par certains auteurs. En d'autres régions (S. Angleterre, Espagne orientale, Ardèche), où les conditions de gisement ne sont cependant pas toujours aussi favorables que dans le stratotype, cette association semblerait plutôt confinée à la partie supérieure du Bédoulien inférieur. Diverses hypothèses sont considérées en vue de tenter d'expliquer cette divergence.
Biostratigraphie ; ammonites ; Crétacé ; Aptien.
In 1984, after intensive research concerning the ammonites collected in the Cassis railway station-Comte Quarry section of the Bedoulian (=
Lower Aptian) stratotypic area, introduced a new succession of "zonal subdivisions" for this substage. These zones were not formally defined but simply indicated graphically in a table together with a log of the type-section
(Fig. 1 
).
Click on the image to enlarge it.
Figure 1: Bedoulian stratotype: Cassis-La Bédoule railway station section and ammonite zonation, as published by (1984).
In 's (1984) zonal succession one can notice a Hambrovi Zone, thus implying that Roloboceras hambrovi be regarded as its species-index. The corresponding level is the base of the upper Bedoulian, in compliance with a ternary mode for the subdivision of this substage. This Hambrovi Zone overlies a Matheroni Zone (= upper middle Bedoulian) and is below a Grandis Zone (= middle upper Bedoulian). Note that in the table of species ranges provided by this author
(Fig. 2 ), Deshayesites deshayesi is absent. Later, the ternary mode of subdivision of the Bedoulian was replaced by a binary one ( et alii,
1997) and the boundary between lower and upper Bedoulian placed at the top of bed 128 (top of the Weissi Zone) in the stratotype section.
Click on the image to enlarge it.
Figure 2: Stratigraphic range of the main species of Aptian ammonites, as published by (1984).
The presence of the genus Roloboceras in the stratotype area had already been mentioned by several authors. Between Cassis and La Bédoule, (1927, p. 8) had identified Douvilleiceras cf. hambrovii in the upper part of the Lower Aptian succession, above a "typical" Parahoplites deshayesi level. At Cassis, in the Comte Quarry section, (1975) described a Roloboceras gr. transiens from the upper part of the Bedoulian, more precisely "à quelques 12 m. de la partie supérieure du Bédoulien (niveau à Tropaeum), au dessus de la partie moyenne dans laquelle abondent les D. deshayesi" (op. cit., p. 108). This occurrence was confirmed by (pers. comm., 1996), who identified in the Comte Quarry section, in bed 148: Roloboceras gr. hambrovi, Megatyloceras aff. coronatum, M. aff. ricordeanum, Pseudohaploceras liptoviense, Cymatoceras neocomiensis, and in bed 150: Roloboceras sp., Pseudohaploceras sp.
Our own record of ammonites in the Comte Quarry section ( et alii,
2000, 2006) can be summarized as follows (see also
Fig. 3 ):
Click on the image to enlarge it.
Figure 3: Cassis-La Bédoule railway station section: stratigraphic range of the Deshayesitidae and Roloboceratinae (from et alii, 2006).
The findings of all these authors are concordant in the premise that a Roloboceras and Megatyloceras level is indisputably present at Cassis-La Bédoule. More precisely, in the 110m thick succession representing the Lower Aptian substage there, these ammonites occur very high in the series, that is between 80 and 100m, and are found above and together with certain species of Deshayesitidae that characterize the upper Bedoulian (in compliance with the most frequently used binary mode of subdivision).
However, a compilation of the principal discoveries of Roloboceratinae in both the Tethyan (Ardèche, Spain) and Boreal (southern England) Aptian reveals a stratigraphic position which sometimes differs from that observed in the stratotype.
Thus & (1915) mention (p. 8) the presence of Douvilleiceras Hambrowii [sic] in the calcareous complex located between Le Teil and the Lafarge quarries on the right bank of the Rhone; the authors draw a parallel between this complex and the limestones of the l'Homme d'Armes section, located on the left bank, near Montélimar, which are dated early Bedoulian on the basis of an ammonite fauna that in particular includes Parahoplites Weissi. In their summary list of the Lower Aptian fauna from southeastern France, & (1915) mention (p. 118) Douvilleiceras Hambrowii in such outcrops as "Bourg-Saint-Andéol, Lafarge", in the text attributed to the lower Bedoulian. However, p. 176, in the locality of Opoul, near Perpignan, the same authors report Douvilleiceras Hambrowii in association with typical and abundant Parahoplites furcatus, and relate their level to that of the "marnes inférieures" of La Clape (near Narbonne), which they attribute to "du Bédoulien très supérieur" [uppermost Bedoulian].
These somewhat divergent data appear to support 's (1961a) opinion who wrote (p. 496): "Roloboceras hambrovi has too long a range".
In England, as early as 1923 "Roloboceras hambrovi was used by "
(...) "as a guide fossil for the middle part of the old deshayesi Zone, which corresponds more or less to the
Lower Aptian of present usage" (quotation in ,
1961b, p. 182). From the work of
(1961a, 1961b) and et alii
(1998) on the Aptian of the Lower Greensand, it is clear that in southern England the several species of Roloboceras (associated with Megatyloceras) are confined to the Forbesi Zone, which can be taken as the equivalent of the Tethyan Weissi Zone and represents the upper part of the lower Bedoulian
(Fig. 4 ).
Click on the image to enlarge it.
Figure 4: Correlation between the Lower Aptian ammonite zonations of (1961a, 1998), (1984) and et alii (2006, 2008).
The various Roloboceras and Megatyloceras collected and studied by & (1972) in Spain (Tejeria de Josa section, Oliete basin, prov. Teruel) are said by the authors to indicate the upper lower Bedoulian, without further explanation ("toute cette faune indique le sommet du Bédoulien inférieur, un niveau plus élevé étant exclu"). One might assume that the authors are led to this categorical assignment on the basis of 's work. But & , working in the same sector, wrote in 1971: "les attributions stratigraphiques ont été faites en tenant compte de la zonation admise en Europe septentrionale (Grande-Bretagne). Il n'est pas absolument certain, compte tenu de migrations faunistiques possibles, que cette zonation soit strictement applicable à la Péninsule ibérique" [Stratigraphic attributions were made taking into account the zonation accepted in northern Europe (Great Britain). It is not absolutely certain in view of possible faunal migrations that this zonation is strictly applicable on the Iberian peninsula].
Let us add that the age of the series underlying the fossiliferous bed of the Tejeria de Josa section is not constrained, for this stratum rests on a thin marly level (that overlies a massive undated limestone through a significant hardground) in which the only "marker" is Palorbitolina lenticularis; the range of this orbitolinid is upper Barremian and Bedoulian (, 1981; et alii, 1995; et alii, 1998), the Furcata Zone included. In 1965, , for this same area of the Maestrazgo of the Celtiberic Chain (including the Oliete basin), indicated (Table III) above an "Aptien I" with Deshayesites weissi and D. consobrinus, an "Aptien II" (or "Rolobocératien") including Roloboceras hambrovi, Dufrenoya cf. lurensis, Deshayesites deshayesi, Pseudosaynella, various nautiloids, etc., thus dated upper Bedoulian. These data were not taken into account by & (1971), stating only: "il semble" (...) "que les divisions récemment proposées [par ] pour cette région d'Espagne ne puissent être retenues".
Lower Cretaceous ammonites of this eastern Iberian cordillera were also studied by et alii (1994), who wrote (in Lower Aptian, p. 346-347): "La Zona Deshayesi" (...) "se halla identificada en la Fm Forcall de las cuencas de Oliete, Maestrat, El Perelló y Sallou-Garraf. Quince especies identifican, prácticamente todas, esta biozona. Pertenecen a los géneros Deshayesites, Roloboceras, Megatyloceras, Toxoceratoides y Pseudosaynella" (...) ". En el yacimiento de la Tejeria de Josa" (...) ", aparece la especie indice Deshayesites deshayesi junto a gran cantidad de ejemplares de varias especies del género Pseudosaynella" (...) ". Destacan también las poblaciones de Roloboceras" (...) "junto a Megatyloceras sp." Their summary table (op. cit., fig. 3, p. 341) shows without ambiguity a Deshayesi Zone including the species-index accompanied by Roloboceras and Megatyloceras, that follows a Forbesi Zone including Deshayesites weissi. So these data agree with the work of (1965), which in Spain places the Roloboceras level in the upper Bedoulian, not in the lower Bedoulian as & (1972) had advocated.
The stratigraphic assignment of the Margas del Forcall Formation in the Oliete basin has been re-examined recently by et alii
(2007). The authors insist clearly on the fact that the main fossiliferous level, a thin
(0.3 m) bed of red nodular limestone located at the base of the formation, represents a condensed, probably even lacunar facies and that the assemblages of ammonoids and nautiloids that it contains are redeposited elements. A hardground separates this level from underlying massive undated limestones. In this thin bed the authors collected several species of Roloboceras (hambrovi, transiens, hispanicum), Megatyloceras coronatum, Pseudosaynella spp.and three species of Deshayesites which they determine as fittoni, euglyphus and spathi. They conclude that this level is to be attributed to the Hambrovi Subzone. But they point out:
1) that the association of Roloboceras and Megatyloceras mentioned above was attributed by to the Forbesi [# Weissi] Zone;
2) that the Deshayesitidae which accompany them at the base of the Margas del Forcall Formation in the northeastern part of the Iberian peninsula are attributed by
(2007) to the upper part of the Weissi Zone. Nothing is said about the previous attribution of the same level to the Deshayesi Zone by et alii
(1994). Finally the authors mention that in the
Lower Aptian sections of Le Teil (Ardèche, southeastern France) a rich assemblage with Roloboceras spp., Megatyloceras, Subsaynella sp., nautiloids,
etc. and Deshayesites consobrinus, the latter species indicating lower Bedoulian, was recently collected [collector unspecified; probably the authors but this is not specifically stated]. In fact these findings corroborate those made in the same area by &
(1915) (a reference not listed in et alii'
paper). et alii (2007) conclude that "El análisis
bioestratigráfico de los Deshayesítidos permite atribuir la Subbiozona Hambrovi a la parte alta de la Biozona Weissi cuando en otros trabajos se proponia que esta Subbiozona pertenecía a la Biozona
suprayacente: Deshayesi" and " et al.
(2000, 2006) incluyen la Subbiozona Hambrovi en la Biozona
Deshayesi. Este cambio" [from the opinion of ,
1961a, 1961b]
"se apoyaba en las determinaciones de las especies de Deshayesítidos presentes en este
intervalo. Pensamos, sin embargo, que esas determinaciones deben ser revisadas".
We think that the problem is not only to determine to which zone the Hambrovi level must be attributed, nor to question specific identifications of the Deshayesitidae, but to ascertain whether or not Roloboceras spp. and their accompanying assemblage have any stratigraphic value at all, without regard to possible hiatuses, reworking and mixing of faunas in condensed outcrops, all of which can lead to misinterpretation.
We have shown ( et alii,
2000, 2006; et alii,
1998, 2000)
(Fig. 3 ) that in the stratotypic area of Cassis-La Bédoule the Roloboceras/Megatyloceras assemblage occurs in an expanded section, devoid of hiatuses, where the fossil content (ammonites, foraminifers, calcareous nannofossils) is continuous and the markers succeed one another in a logical succession. Everywhere else, with the exception of southern England, there are problems: condensed facies, possible mixing and/or redeposition of faunas, poor or undated framing levels, diversity in the determination and/or placement of the
Deshayesitidae, etc. Several potential solutions of the problem can be considered, with no clear cut explanation possible in the present state of knowledge:
To assign a chronostratigraphic significance to the Hambrovi assemblage and to discard et alii's determinations of certain species of Deshayesitidae, as et alii (2007) suggested, would imply an unlikely stratigraphic interpretation of the 110m thick Cassis-La Bédoule Lower Aptian succession: an 80 m lower Bedoulian would include an anomalously thick (60 m) Weissi (= Forbesi) Zone, the entire Deshayesi Zone would be absent, and an upper Bedoulian would have a thickness of only 30 m. Such a chronologic interpretation is not consistent with the micropaleontological (planktonic foraminifera, calcareous nannofossils) and geochemical (stable isotopes, trace elements) evidence ( et alii, 1998, 2000; et alii, 2005).
This paper benefited from helpful comments from Mikhail (Geological Institute of the Academy of Sciences of Georgia, Tbilisi), Robert (University of Lyon, France) and one anonymous reviewer. We are also much obliged to Nestor for his suggestions about linguistics of this paper.
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