Wyniki wyszukiwania - BazTech

Ograniczanie wyników

2 Volumina Jurassica

Powiadomienia systemowe

Sesja wygasła!

Znaleziono wyników: 2

Liczba wyników na stronie

Wyniki wyszukiwania

Sortuj według:

Ogranicz wyniki do:

Indirect biostratigraphy in condensed successions: a case history from the Bajocian of Normandy (NW France)

Pavia G., Martire L.

Volumina Jurassica

2009

Vol. 7, no. 1

67-76

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.

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

Pavia G., Martire L.

Volumina Jurassica

2006

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.