Organic facies of the early Albian Niveau Paquier, a key black shales horizon of the Marnes Bleues formation in the Vocontian Trough (Subalpine Ranges, SE France)

The marnes Bleues Formation was deposited in the central, hemipelagic part of the Vocontian Trough during Aptian-Albian times. It contains several levels of black shales which are characterized by abundant organic matter of both continental and marine origin, as well as by some qualitative and quantitative variations in planktonic and benthic foraminifera contents. We concentrate here upon the most striking of these horizons: the Niveau Paquier (early Albian). The Niveau Paquier has a laminated, paper shale facies. Varying in thickness from a few decimeters to 5 m, it can be traced over a large part of the Alpine Tethys. It is one of the most organic-rich levels in the Marnes Bleues Formation. It displays a subtly fluctuating organic content of both marine (dinocysts, algae, foraminifera linings, amorphous organic matter or AOM) and terrestrial origins (inertinite, vitrinite, cutinite, spores and pollen). Many characteristics of the Niveau Paquier (lithology, microfauna, organic matter, presence of resident dysaerobic faunas, etc.) suggest frequent oscillations between anaerobic, dysaerobic and aerobic conditions. A model involving subtle modifications in the water stratification can be suggested: the fluctuating position of the redox-potential-discontinuity layer (RPD-layer) or of the pycnocline would have affected epi-meso-planktonic, nectonic and epi-endo-benthic life and the nature and preservation of organic matter (mainly of AOM). Organic parameters (palynofacies and Rock Eval data) of the Niveau Paquier seem to have recorded some four ca. 100,000-yr long cycles. These cycles may be related to climatic variations affecting both the emerged lands supplying the Vocontian Trough in clay minerals, organic residues and nutrients and the conditions in the marine milieu. These variations seem to be in the Milankovitch frequency band.