Biostratigraphie et stratigraphie séquentielle du Bajocien supérieur-Bathonien inférieur dans le Nivernais

Abstract

The Bajocian-Bathonian boundary has been studied in three new sections in the Nièvre Dept. of France (fïgs. l-2). I- Loire section (figs. 3-4). A 7.50-m-thick series of alternating grey limestone and marl beds. Seventy-three levels are recognized, of which only levels 9 to 55 have yielded fauna: - Levels 9 - 46 contain species, such as Parkinsonia (Durotrigensia) oxonica Arkell and P. (P.) bomfordi Arkell, indicative of the Bomfordi subzone (top of the Parkinsoni zone, Upper Bajocian). - Level 47, which has a burrowed Upper surface, has yielded numerous ammonites. It contains both P. (P.) bomfordi Arkell, which would assigned this level to the Bomfordi subzone, and Partisonia (P.) pachypleura (Buckman), which is everywhere abundant at the base of the lower Bathonian, and would thus assigned this level to the Convergens subzone (Zigzag zone, lower Bathonian). - Level 48 is a marly-limestone bed yielding a faunal content that is both rich (more than 300 variably pyritized specimens) and variad (eight ammonite families or sub-families). The very large variety of species collected are all characteristic of the Macrescens subzone (Zigzag zone). - Levels 49 - 55 contain an impoverished and not very varied fauna that is thought to be representative of the Recinctus subzone (Aurigerus zone, lower Bathonian). 2- Nevers section (Figs. 5-6). A thin section of three levels over-lying a massive upper Bajocian limestone with a ferruginized burrowed surface. Level 1 chiefly contains species from the Macrescens subzone together with a few forms from the Recinctus sub zone. - Level 2, or Pygomalus (echinoid) level, has yielded rare ammonites of the Tenuiplicatus ,subzone (Aurigerus zone). Level 3 is poor in fossils and thought to be of similur age. 3- Mousseau section (fig. 7) The Bathonian series (1.20 m) lies on a crinoid limestone dated as Late Bajocian by R. Mouterde (Parkinsoni zone). The first 20 cm are rich in ferruginous ooliths and ammonites from the upper part of the Convergens subzone. The next 20 cm of marly limestone with ferruginous ooliths are assigned to the base of the Macrescens subzone. The overlying ferruginous oolitic limestone bed (20 cm thick), which is capped by a hard ferruginized surface, is assigned to the upper part of the Macrescens subzone and the Recinctus subzone. These three sections, and the previously published Saint Benin d'Azy section (fig. 7), extend from the upper Bajocian (Parkinsoni zone) to the top of the lower Bathonian (Aurigerus zone). In Dogger times, the Loire section was located at the margin of the shelf represented by the other three sections. Seguence stratigraphy applied to this region of marked paleotopography provides the following results: I- From the Bomfordi subzone (uppermost Bajocian) to the base of the Convergens subzone (lower Bathonian). Sedimentation was restricted to the Loire region and the faunal diversification over this period suggests a gradual deepening of the environment. Deposits for this period do not occur in the other three sections located on the shelf. This, was therefore a period of relative lowstand. 2- From the top of the Convergens zone to the Macrescens subzone (lower Bathonian). Sedimentation was more widespread but benthonic organisms are more abundant on the shelf in a shallower environment. In the Loire, sedimentation abruptly diminished (starved basin) and the environment became reducing (pyritized fauna). The transgression period that began at the top of the Convergens subzone would have ended with a maximum ,flooding surface at the top of the Macrescens subzone. 3- From the Recinctus subzone to the Tenuiplicatus subzone (lower Bathonia). Sedimentation was more marly and thicker, consistent with a relative highstand. The eustatic changes recognized at the Bajocian-Bathonian boundary in the Nièvre region, an area of marked palaeotopography, are consistent with those recognized by M. Rioult et al. (1991) in Normandy for the same period.