Sulphur-rich organic matter from bituminous laminites of Orbagnoux (France, Upper Kimmeridgian). The role of early vulcanization

Upper Kimmeridgian bituminous laminites outcrop in the southern Jura Mountains at Orbagnoux and the field section comprises five calcareous facies. Organic matter (OM) in all these facies shows high hydrogen index values (from 780 to 960 mg ''HC''/g TOC) whereas only four are OM-rich (TOC between 2 and 8.6%). A combination of petrographical and geochemical methods was applied to analyse a representative sample of one of the most OM-rich facies (dark parallel laminae). Transmitted-light microscopy and UV excitation microscopy indicated the presence of two lamellar organic constituents: a dominant orange one with an intense yellow-green fluorescence and a minor dark, non-fluorescing one. Backscattering scanning electron microscopy (BSEM) also revealed the occurrence of an organic network closely associated with the micritic matrix. The palynological residue consists of gel-like OM which is amorphous even when observed by transmisssion electron microscopy (TEM). Sulphur-mapping and energy dispersive spectrometry indicated that this element is solely associated with the OM. A high S-content is observed in both the organic lamellar constituents and in the organic network. All these petrographical features of the organic material in dark parallel laminae suggest a minor role of the selective preservation pathway whereas vulcanization of lipids probably played a major role. Such a process would explain the highly aliphatic nature of the kerogen. ''Off-line'' pyrolysis released mostly organic sulphur compounds (OSC), thus confirming the major contribution of vulcanized lipids. S-incorporation allowed these lipids to escape bacterial degradation. Moreover, the macromolecular compounds thus formed can survive strongly oxic, post-depositional conditions due to temporary and occasional sediment emergences. Nevertheless, the latter conditions should have induced some alterations reflected by the presence of the dark organic constituent. Analyses of pyrolysis products indicated that the vulcanized lipids were chiefly of algal origin. Since coccolithophorids constitute the mineral matrix, they could have provided the bulk of these lipids.