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Relative biostratigraphy in condensed succession: a case history from the Bajocian of Normandy

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Pavia G. , Martire L.

tom Vol. 4, no. 1

201-202

The classic dm-thick sections of the Bajocian of Normandy, NW France, such as those of St. Honorinedes- Pertes (formerly d’Orbigny’s stratotype), Sully and St. Vigor in the surroundings of Bayeux are affected by taphonomic and stratigraphic condensations (Pavia 1994). The use of their fossils for biostratigraphic purpose to date the embedding sediments must be avoided because of the taphonomically reworked state of most of them. Such beautiful fossils, ammonites in particular, would become useful again as taxonomic reference if their relative position is recognized. This goal is what Gauthier et al. (1996) tried to achieve in the more than 2 m thick section of Feuguerolles, south of Caen; but the fossil preservation is poor and the Quaternary weathering altered the Fe-oolitic limestone so that beds are not clearly discernible and taphonomic analysis is difficult. A succession similar to that of Feuguerolles was exposed more than ten years ago during a highway construction at Bretteville, SW Caen (Martire & Torta 2000). The succession of the Oolite Ferrugineuse de Bayeux Formation consists of evenly bedded limestones which total the thickness of 140 cm. Even more important beds are lenticular so that, correlating two separated sections, 20 metres far one to the other, we numbered 13 interfingered layers each one with a distinct fossil assemblage. Beds are rich in fossils and a large collection of them is stored in the Museo di Scienze Naturali di Torino. Most ammonites, if not all, are reelaborated and the taphonomic reworking could have been repeated many times; consequently beds contain fossils belonging to different taphorecords, each one characterized by a definite state of fossil preservation discernible from that of other mixed fossils. The taphonomic condensation (sensu Gomez & Fernandez-Lopez 1994) does not allow any reliable biostratigraphy. Nevertheless, the detection of bed interfingering and the careful collection of fossils, bed by bed, led us to consider the 13 successive fossil assemblages as the product of spatially restricted sedimentary events whose age could be outlined by the biochronologic relationships of ammonites contained in superposed beds. Each bed gets in this way a minimal relative age. This provided a relative biostratigraphic succession even if all fossils are reelaborated, which helps in using fossils at least for taxonomic purposes. It is worth noting that such a practice does not constitute any reference either for standardizing biozonation nor for correlation purpose except at regional extent. An example from the Bretteville section could put into practice this procedure. Bed 9 delivered more than 35 identified ammonites; all specimens are reelaborated and biochronologically span the Humpriesianum and Niortense zones; no taxa of the above Garantiana Zone are present. The overlying bed 10 is equally rich in reelaborated ammonites with taxa again referable to the Humpriesianum and Niortense Zones, but also taxa such as Pseudogarantiana and Prorsisphinctes stompus which point to the Garantiana Zone. In conclusion, we are confident to assign the bed 9 to the passage between Niortense and Garantiana zones.