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
Résumé
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) content), petrographic (transmitted-and reflected-light microscopy, scanning and transmission electron microscopy), pyrolytic (Rock-Eval, Geoelf Sulphur Analyser and Pyromat pyrolyses) and spectroscopic (Fourier transform infrared) features. A number of similarities were thus observed although, as previously shown, the formation of the light undulated laminae was associated with the development of cyanobacterial mats on sediment surface, whereas the 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 mat biomass before sulphurization. - Relatively less reducing conditions during the deposition of the light undulated laminae and massive limestones. - Large semi-quantitative differences in FTIR spectra (the kerogens from the dark-coloured samples tend to be richer in C = O groups while those from the light-coloured 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 oxidative alteration during emergences.