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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/279341822 Indirect biostratigraphy in condensed successions: a case history from the Bajocian of Normandy (NW France) 67-76 Article in Volumina Jurassica · December 2009 CITATIONS READS 14 209 2 authors, including: Giulio Pavia Università degli Studi di Torino 86 PUBLICATIONS 1,495 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Bajocian ammonites View project All content following this page was uploaded by Giulio Pavia on 19 October 2015. The user has requested enhancement of the downloaded file. Indirect biostratigraphy in condensed successions: a case history from the Bajocian of Normandy (NW France) 67 Indirect biostratigraphy in condensed successions: a case history from the Bajocian of Normandy (NW France) Giulio PAVIA and Luca MARTIRE Dipartimento di Scienze della Terra, via Valperga Caluso 35, 1015 Torino, Italy; e-mail: [email protected], [email protected] Key-words: taphonomy, condensation, biostratigraphy, ammonites, Middle Jurassic, Normandy. ABSTRACT: The fossil assemblages of the Bajocian of Normandy are affected by taphonomic condensation, and the use of these assemblages for biostratigraphy must be carried out with great care because of the taphonomic reworking (reelaboration) of most of them. Nevertheless such beautiful fossils, ammonites in particular, retain their value at least as taxonomic references if their relative stratigraphic position is recognized. Such a goal has been achieved in the Bretteville section, where the “ Oolithe Ferrugineuse de Bayeux ” Formation ( OFB ), consisting of 14 beds contained within 170 cm of strata, is exposed. Each bed contains a condensed ammonite assemblage in which the overall chronologic interval represented by fossils is longer than the time of sedimentation of the bed, and no ammonites can be defined as contemporaneous with the enclosing matrix, except for the topmost two beds. The timing of biologic and sedimentary events in this Fe-oolitic succession must thus be established in an indirect way, through a stratigraphy of fossils based on their taphonomically delayed first occurrence. In particular, we can only define the maximum age of each layer, constrained by the youngest recorded fossil. On the basis of this approach, the lower part of the OFB is shown to fall within the middle to upper part of the Bajocian Stage (Humphriesianum to Parkinsoni chrons), whereas the uppermost OFB can be referred to as the uppermost Bajocian (latest Parkinsoni Chron). We conclude that, at Bretteville and in general for the Fe-oolites of Normandy, fossiliferous horizons with condensed assemblages have no value for definition of the biostratigraphic standard scale, and that indirect biostratigraphy is useful to assign minimal chronologic values to taxa when (1) their stratigraphical range has not yet been established in expanded successions, (2) they are new taxa, and (3) they are present only in a condensed succession that suffered from the same taphonomic constraints as the Bajocian of Normandy. INTRODUCTION and the lithostratigraphy of the Oolithe Ferru- gineuse Formation outcropping around Bayeux The Bajocian succession of the Bayeux area in from which the name Bajocien was derived, first western Normandy (Fig. 1) has been intensively used in 1849 (1849-52, p. 157) and formally defined studied since the mid-nineteenth century because in 1850 (d’Orbigny 1842-51, p. 606). Much later, of its rich fossil content. In particular, the famous Rioult (1964) proposed the section of Sainte- French author d’Orbigny, in classic monographs Honorine on the English Channel coast as the (1842-51, 1849-52, 1850-52), described many fossils stratotype of the Bajocian Stage. However, many 68 Volumina Jurassica, Volumen VII coordinated the analysis of the section at Feuguerolles, some 8 km south of Caen, where the OFB totals 3 meters (Gauthier et al. 1996). These authors described a seemingly continuous biostratigraphic sequence from the middle to the upper part of the Bajocian Stage. However, the rocks are superficially altered by Pleistocene ice weathering, so that textural features could not be observed and taphonomic analyses was not undertaken. In this respect, we note that the latter is essential in reduced succe- ssions such as those of the Normandy Bajocian (Pavia and Martire 1997). In 1995, during the excavation of the highway around Caen, a new section was exposed at Bretteville. The succession, Fig. 1. Location of the study area; the arrow in the top right insert refers to the intermediate in distance between Sully Bretteville section. and Feuguerolles, is similar though a bit thinner than that of the latter locality. authors (Gabilly and Rioult 1971; Fürsich 1971; The fine state of preservation of the abundant Rioult et al. 1991) recognized that discontinuities fossils, mainly ammonites, and the absence of any are present through the OFB succession and some superficial alteration of rocks have produced fossil assemblages are condensed. In addition information that, integrated with that coming from to that, more recently Pavia (1994) discussed Feuguerolles (Gauthier et al. 1996), enables a the stratigraphic condensation ( sensu Gómez and better understanding of the OFB in Normandy. Fernández-López 1994) of the sections around In particular, the Bretteville section assumes Bayeux, where the OFB is reduced to a few importance to (1) verifying the biostratigraphic decimetres, and demonstrated that their rich potential of the fossil assemblages which, though ammonite assemblages show high taphonomic not useful for Standard Zone purposes, at least condensation (see also Martire and Pavia 1996) so could spell out the rhythm of the sedimentary that they are of no use in defining biostratigraphic events recorded in the Oolithe Ferrugineuse de subdivisions. Nevertheless, fossils, particularly Bayeux Formation, (2) assigning a more precise ammonites, from the Bajocian of Normandy are biochronologic significance to the ammonites which extremely important in taxonomic studies for would constitute references of Middle Jurassic systematics and phylogenetic interpretation. taxonomic studies and phylogenetic interpretation. Consequently, thicker and less condensed sections In this paper we summarize the results of the would be very useful. taphonomic analyses carried out on the section at In this respect, it is worth remembering the note Bretteville and the implications for the biostra- by Gabilly and Rioult (1971; see also Parsons 1974) tigraphic interpretation of the fossil assemblages who briefly described the classic outcrop at May- of this condensed succession. No attempt is made sur-Orne quarry, south of Caen (Fig. 1). Although either to carefully describe the taphonomic chara- that study was focussed on the stratigraphic cteristics or to list the entire ammonite contents, relationships of beds and ammonite assemblages at which will be detailed in a separate paper. the Lower-Middle Jurassic boundary, it demonstra- ted that the OFB could be considerably thicker at May-sur-Orne than on the Bessin coast (Fig. 1). THE BAJOCIAN SUCCESSION OF NORMANDY The May-sur-Orne section was not analysed AND THE BRETTEVILLE SECTION further; by now, the quarry is out of work and it is impossible to get any detail on its ammonite The current knowledge of the Middle Jurassic succession. Later on, Rioult, attempting to get a stratigraphy of Normandy is essentially based on a complete picture of the inland OFB succession, few sections, some close to the English Channel Indirect biostratigraphy in condensed successions: a case history from the Bajocian of Normandy (NW France) 69 coast (Sully, St. Honorine-des-Pertes near Port-en- rolles the Couche Verte is represented by a 10 cm Bessin: Fig. 1) and two others inland south of thick bed, whereas in the other sections it fills the Caen, May-sur-Orne and Feuguerolles (Fig. 1). dense network of partially truncated firm ground The Bajocian succession comprises three forma- burrows that occur at the top of the Maliére Forma- tions (Rioult 1964; Gabilly and Rioult 1971; tion. This layer was assigned to the Sauzei Zone by Rioult et al . 1991; Pavia 1994): from base to top the Parsons (1974). Although all the fossils contained Maliére , the Oolithe Ferrugineuse de Bayeux , in the Couche Verte are taphonomically reworked and the Calcaires ∫ Spongiaires . (reelaborated sensu Fernández-López 1985, 1991), The Maliére Formation is represented by grey the absence of any ammonite referable to the Hum- cherty limestones consisting of biomicrites with phriesianum Chron of the Early Bajocian leads to sponge spicules and glauconite grains. The base the confirmation of Parsons’s dating to the Sauzei contains ammonites of the Early Aalenian. For the Zone. Bajocian, the Discites and Laeviuscula European Above, the Oolithe Ferrugineuse de Bayeux Standard Zones are documented (Gabilly and (OFB ) consists of the well known and most Rioult 1971; Parsons 1974). characteristic facies of the Bajocian of Normandy. At the very top of the Maliére Formation, thin The OFB consists of grey to yellow limestones rich and discontinuous sediments have been considered in ferruginous or phyllitic ooids. Ooids range in size a distinct member and given the name Couche Verte from 0.3 to 2 mm and may be variably abundant after their greenish colour. It consists of a float- in different beds, but they never give rise to stone rich in commonly