L'excursion isotopique du carbone organique (delta 13 C org ) dans les paleoenvironnements continentaux de l'intervalle Paleocene/Eocene de Varangeville (Haute-Normandie)

The late Palaeocene carbon isotope excursion (C.I.E.) is often regarded as the best means of correlating marine and continental deposits. The few isotopic studies carried out in continental environments were based on pedogenic carbonate [Koch et al., 1992], or on organic matter. Sinha [1997] took up this subject starting from the outcrops on the coast of the English Channel at Varangeville, where marine sequences biostratigraphically constrain the isotopic excursion. His work documents a negative delta 13 C org excursion value approximately -27 per mil PDB. The present work points out that it is necessary to study more complete sections than those studied by Sinha. A section named Phare d'Ailly has been sampled in detail (figs. 1, 2, 3). In this section, for which detailed analysis of sedimentology, palaeontology and organic matter facies establish the continental nature of the palaeoenvironment, isotopic analysis of organic matter reveals a very negative delta 13 C org excursion value approximately -30 per mil PDB. The P/E interval synthesized in figure 2 shows three main groups, the stratigraphy of which is strongly constrained between the calcareous nannofossil Zones NP8 and NP11. The marine Thanetian facies belong to Zones NP8 and NP9. Above, the "Sparnacian" (Mont Bernon Group) is divided into 5 units referred to as SP. Unit SP2 is attributed to the Peckichara disermas Charozone, equivalent to Zone NP9. For unit SP4, an indirect correlation with Zone NP10 may be deduced. The upper part of the Varangeville Formation is known for its nannofossil association attributed to Zone NP11. We may conclude from these observations that the sparnacian sediments are synchronous with NP9-10 Zones and that they are effectively located in the time interval of the delta 13 C excursion. The SP1 and SP2 sediments were analysed for carbonate content, grain size distribution, clay mineralogy and total organic content (T.O.C.), using standard laboratory methods (fig. 3). An optical specification of the organic matter has been obtained from semiquantitative analysis of the organic matter facies. The survey of macro- and microfossils has supplied complementary supports for isotopic analysis (seed, wood and charcoal). The isotopic measurements were carried out on the bulk sediment and on complementary supports with a mass spectrometer in continuous flow delta + (Finnigan Mat). The measurement precision is better than 0.1 per mil and the reproducibility is about 0.15 per mil. Washing residues provide Gasteropoda, Bivalvia, Ostracoda, Characea, as well as fruits and seeds, microcodiums and otolith. All the palaeontological data illustrate a continental biota of a pure lacustrine environment, quiet and shallow, in a hot and humid climate. The organic matter facies analysis confirms the absence of any marine influence and documents a palaeoenvironment of lakes and ponds having an anoxic floor. Thus the carbon of the organic matter is considered to be continental and their isotopic variations linked to those of the atmospheric carbon.