The lagoonal Upper Kimmeridgian sediments
outcropping near Orbagnoux (France, Southern Jura) are characterised by the presence of organic-rich bituminous laminites which organic matter (OM ) exhibits a high sulphur content (up to 15% wt). These sediments are processed to obtain S-rich oils. Five facies have been recognized in Orbagnoux sediments: decimetric to metric massive
limestones, millimetric to centimetric dark- or light-coloured, parallel or undulating, laminae (Tribovillard et al., 1992). Their mineral matrix is always essentially composed of coccoliths overwhelmed by only one species. The purpose of the present study was to derive information on the source organisms of the OM and the depositional environments corresponding to such facies.
Thirty samples were examined by combination of various methods: identification of mineral constituents (X-ray diffraction); quantitative study of major and trace elements (EDS-equipped SEM and ICP-OES spectroscopy); light microscopy observations on thin sections and on isolated kerogens; identification of the organisms with mineralised walls (light and scanning electron microscopy); transmission electron microscopy (ultrastructural features of the OM); determination of TOC and of the bulk chemical features (elemental analysis, HI, d 13C) of this OM; examination of the isolated kerogens by Infrared spectroscopy.
Mineralogical results and microscopy observations indicated that (i) deposition took place under very quiet conditions, (ii) the detrital contribution was always low and did not undergo important qualitative changes through the sedimentary sequence (quartz and, occasionally, illite), (iii) the water column and the water-sediment interface were probably not, or not always, anoxic (presence of oxygen-demanding benthic foraminifera, echinoderms, ostracods and holothuria), (iv) strongly reducing conditions occurred in the sediment (marked enrichment in redox-sensitive elements such as V, Mo or Cr, depletion in Mn and Fe, occurrence of pyrite framboids), however the presence of gypsum pseudomorphs suggests that highly reducing conditions did not occur at any time in the whole sediment (the presence of such sulphate pseudomorphs and of proxies of reducing conditions raises questions about the chronology of the succession of the diagenetic events and (v) light-coloured undulating laminae likely correspond to microbial mats (network of endolithic filaments). In addition, the above results allowed to discuss the possible relationships of the different types of facies occurring in Orbagnoux sediments with the bulk chemical (quantity, quality, d 13C) and spectroscopic (various IR indices) features of the OM.
On the basis of all the above results five samples, considered as representative of these facies, were selected for a complete chemical study of their OM. One of the selected samples, corresponding to dark-coloured parallel laminae (TOC 7.2%, HI 909 mg HC/g TOC) has been so far examined. This study was carried out via solid state 13C NMR spectroscopy and "off-line" pyrolysis at 400°C under an Helium flow. The constituents of the very complex pyrolysate thus obtained were identified by GC/MS after fractionation and/or desulphurisation. Taken together the derived results indicate that "natural vulcanisation" (Sinninghe Damsté & de Leeuw, 1990) occurred in the sediment and played a major role in OM preservation in this organic-rich facies (in agreement with electron microscopy observations indicating a "nanoscopically amorphous OM", as defined by Boussafir et al., 1995) and that the corresponding sulphur incorporation mainly took place in algal lipids.
Boussafir, M., Gelin, F., Lallier-Vergès, E., Derenne, S., Bertrand, P. & Largeau, C., Geochim. Cosmochim. Acta 59, 3731-3747 (1995).
Sinninghe Damsté, J. S. & de Leeuw, J. W., Organic Geochemistry 16, 1077-1101 (1990).
Tribovillard, N-P., Gorin, G. E., Belin, S., Hopftgarner, G. & Pichon, R., Palaeogeogr. Palaeoclimat. Palaeoecol. 99, 163-177 (1992).