Les ammonites du Callovien–Oxfordien des sondages Andra dans l'Est du bassin de Paris : synthèse biochronostratigraphique, intérêts paléoécologique et paléobiogéographique

Abstract

The ammonites collected in nine deep bore-holes and two open wells made by ANDRA near Bure (Meuse Department, France) in the eastern Paris Basin have allowed the following: – a precise biochronostratigraphic calibration of the Callovian–Oxfordian clay-rich facies; – the correlation between the boreholes and wells by means of the ammonite biostratigraphic units that show an outstanding disparity in the sedimentary registration; – undertaking a palaeoecological and palaeobiogeographical investigation into the faunal associations; – highlighting their palaeoenvironmental and palaeogeographical applications. In the Middle Callovian–Middle Oxfordian interval, the following remarkable bio-chronostratigraphic elements are seen: – the position of the interpositum horizon (uppermost Middle Callovian, Coronatum Zone, Grossouvrei Subzone) and its importance for the localization of the Middle Callovian–Upper Callovian boundary; – the respective importance of the paucicostatum horizon (uppermost Upper Callovian, Lamberti Zone, Lamberti Subzone) and the thuouxensis horizon (lowermost Lower Oxfordian, Mariae Zone, Scarburgense Subzone) for the localization of the Callovian–Oxfordian boundary; – the validity of the woodhamense horizon based on the morphological variability of the corresponding species-index that corroborates its stratigraphic situation above the scarburgense horizon (Scarburgense Subzone, Mariae Zone, Early Oxfordian); – the consequences of the morphological variability of the successive species Cardioceras praemartini, C. alphacordatum, C. praecordatum and their selection as species-index for the three horizons of the Praecordatum Subzone (Mariae Zone, Early Oxfordian); – the difficulties of recognizing the Cordatum and Plicatilis Zones because of the scarcity, if not the total lack of characteristic ammonites of these units, which have consequences for the location of the Lower Oxfordian–Middle Oxfordian boundary. The correlations between bore-holes and wells underline that the intracratonic domain to which belongs the Paris Basin has been affected by an unceasing subsidence, which varied in both space and time from the Middle Callovian to the Middle Oxfordian. The subsidence seems to be stronger near the centre of the basin than near its margins, with a maximum activity taking place during the Lower Oxfordian, Mariae Zone, Scarburgense Subzone, woodhamense horizon. The palaeoecological interpretation of the faunal associations indicates a medium palaeodepth (lower offshore) in this eastern part of the Paris Basin, certainly a little more marked towards the centre than on its margins. If not considering the subsidence effects, the palaeodepth does not seem dissimilar to that of the southeastern basin of France. In comparison, the proximity of the former with the Tethyan oceanic domain allows us to deduce that variations of the ammonite assemblages may be related to the different modes of life of the collected taxa. Especially, if compared to the Ammonitina taxa, the selective distribution of the Phylloceratina and Lytoceratina is characteristic of open-sea distal platforms. From a palaeobiogeographical point of view, the Callovian–Oxfordian deposits of the eastern Paris Basin have recorded two boreal southward ammonite invasions: the Kosmoceratidae during the Middle–Upper Callovian, then the Cardioceratidae during the Upper Callovian and Lower Oxfordian. These invasions gave rise to heterogeneous biogeographic areas, which locate between the Boreal and Tethyan Provinces. Considered as transition areas, they are called Subboreal Province and Subtethyan Province; their ammonite associations are composite and their time and space ranges and extensions are variable. To cite this article: J. Thierry et al., C. R. Geoscience 338 (2006).