Comparative studies of a high resolution sampling of the different facies of the organic-rich Orbagnoux deposit (Upper Kimmeridgian, Jura): petrographic and bulk geochemical approach. Extent and origin of interfacies and intrafacies variations

A large set of high resolution samples (thickness down to a few millimetres ), corresponding to the five facies of the Orbagnoux deposit, were examined for their compositional (major and trace elements, organic matter (OM) con tent), petrographic (transmitted- and reflected-light microscopy, scanning and transmission electron microscopy), pyrolytic (Rock-Eval, Geoelf Sulphur Analyser and Pyromat pyrolyses) and spectroscopic (Fourier transform infra red) features. A number of similarities were thus observed although, as pre viously shown, the formation of the light undulated laminae was associated with the development of cyanobacterial mats on sediment surface, whereas th e other four facies originated from coccolith settling. Nevertheless, three substantial differences were also noted: - Lower OM contents in the light undulated laminae Likely due to the extensive mineralization of mar biomass before sulphurization. - Relatively less reducing conditions during the de position of the light undulated laminae and massive limestones. - Large sem i-quantitative differences in FTIR spectra (the kerogens from the dark-colo ured samples tend to be richer in C = O groups while those from the light-c oloured ones tend to be richer in OH groups and C = C bonds). Furthermore, the FTIR spectra show, for each facies, large intrafacies differences that may reflect, in part, differences in the degree of post-depositional oxidat ive alteration during emergences.