The evolution of the Valanginian-lower Hauterivian ammonites of the Vocontian basin and the Provence platform (Southeastern France) : relations with the sequence stratigraphy and biostratigraphic implications.
L'évolution des ammonites du Valanginien-Hauterivien inférieur du bassin Vocontien et de la plate-forme Provençale (Sud-Est de la France) : relations avec la stratigraphie séquentielle et implications biostratigraphiques.
Résumé
A palaeontological review of the Valanginian to lower Hauterivian ammonite fauna from the Provence margin and the Vocontian Basin (South-East of France) has been carried out on the basis of a fairly abundant material (23600 individuals). The studied specimens, coming from new collections, have been recorded in 11 reference sections with a close stratigraphic control. This has allowed a detailed recording of the successive ammonite associations through this stratigraphic interval. The section of La Charce (Dr™me) appears to be the most interesting due to the abundance and the good quality of the collected material.
18 new species are described. From them, 11 are left in open nomenclature. the identification of dimorphic pairs within the assemblages has allowed many of the former nominal species to be re-grouped, leading to a systematic simplification in some groups. Among the studied ammonite families, the Neocomitinae have been the subject of a more detailed study, which has pemitted to place them within a precise evolutionary scheme.
The evolution of the ammonite fauna has been integrated in a sequence stratigraphy framework. Five major faunal renewals are recognized between the Campylotoxus and Nodosoplicatum Zone. They have been correlated to the the marine relative sea-level changes. First appearance phases and faunal crisis are contemporary to the third order lowstand and transgressive systems tracts respectively, defined in the Vocontian Basin. Such events seem to equally affect different families of ammonites. Phases of first appearance follow similar evolutionary heterochronic processes (Peramorphosis or Paedomorphosis) within the different families (Neocomitinae, Olcostephanidae, Oosterellidae). A quantitative study of the successive ammonite associations has been carried out in every section across the Vocontian Basin. The relative proportions of the successive ammonite spectra show some significant differences, between the limestone beds and the marly interbeddings (6th order elementary sequences), and also within the paracycles (5th order sequences). Both the evolution and quantitative composition of the ammonite associations show a close correspondence with the rythmic character of the lithological changes.
The abundant ammonite bed-by-bed collection has led to the establishment of a detailed biozonation. This includes 18 biohorizons extending from the top of Pertransiens to the top of Nodosoplicatum Zone. The high resolution reached by this biostratigraphic scale has allowed a precise correlation between the Vocontian Basin and the Provence Platform sections.
La revision de l'ensemble de la faune d'ammonites du valanginien-hauterivien inferieur du bassin vocontien et de la bordure provencale (se france) a ete realisee a partir d'un materiel tres abondant (23600 ammonites), entierement nouveau et tres bien repere stratigraphiquement sur 11 coupes de reference, qui permettent de suivre tres precisement la succession des peuplements. La coupe de la charce (drome) s'est revelee la plus interessante par la qualite et l'abondance du materiel recolte. 18 especes ont ete creees, dont 11 sont laissees en nomenclature ouverte. De nombreux couples dimorphes ont ete mis en evidence, ce qui a conduit a simplifier la systematique de plusieurs groupes. Les neocomitinae ont fait l'objet d'une etude approfondie qui a permis de les replacer dans un cadre evolutif detaille. L'evolution de la faune d'ammonites a ete integree dans un cadre sequentiel. Cinq renouvellements majeurs ont ete reconnus entre les zones a campylotoxus et a nodosoplicatum ; ils ont pu etre relies aux variations du niveau marin relatif. Les phases d'apparitions et les crises fauniques sont respectivement contemporaines des corteges de bas niveau marin relatif et des corteges transgressifs de troisieme ordre, tels que ceux-ci sont definis dans les series marno-calcaires de bassin. Ces evenements affectent simultanement plusieurs familles d'ammonites. Les phases de differenciation s'effectuent a partir des memes mecanismes evolutifs (peramorphose ou paedomorphose) chez differentes familles (neocomitinae, olcostephanidae et oosterellidae). L'etude quantitative de la succession des peuplements a ete realisee sur toutes les coupes du bassin vocontien. La composition des spectres fauniques montre des differences significatives entre le banc et l'interbanc (6eme ordre) et au sein des paracycles (5eme ordre). L'evolution et la composition quantitative des peuplements paraissent rythmees par les changements lithologiques. Les tres nombreuses recoltes, banc a banc, ont permis d'etablir une zonation tres detaillee, composee de 18 horizons, allant du sommet de la zone a pertransiens au sommet de la zone a nodosoplicatum. La precision de cette echelle biostratigraphique a permis de corriger et de completer les correlations entre les coupes du bassin vocontien et celles de la plate-forme provencale.
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