Stratigraphical and lateral distribution of sedimentary organic matter in Upper Jurassic carbonates of SE France

Abstract

Abstract A detailed analysis of sedimentary organic matter (SOM) has been carried out in Upper Jurassic deep-water marls and carbonates of the Vocontian Basin (SE France). Its main purpose is to analyse the vertical and lateral trends of SOM distribution, in order to better understand processes and factors controlling the accumulation and preservation of organic constituents in basinal environments. The Vocontian Basin is characterized by widespread oxic depositional environments in the Upper Jurassic, as confirmed by the very low total organic carbon content (less than 0.25 wt.%). For such organic-poor deposits, palynofacies analysis represents the best tool to investigate SOM. Stratigraphic analysis of palynofacies trends has been performed within a pre-established sequence stratigraphic framework, defined by means of field sedimentology and, subsequently, complemented through lateral correlations, biostratigraphical constraints and geochemistry. This approach highlights the vertical signatures which are directly related to relative sea level changes. Furthermore, lateral correlations of palynofacies trends, at the scale of 3rd and 2nd order cyclicity, permit the signatures observable at basin scale to be distinguished from those detectable only at local scale. The former are controlled by factors affecting the whole basin, such as eustasy, regional tectonics and climate. The palynofacies signatures at the scale of 3rd order cyclicity reflect relative sea level changes and the degradation state of SOM. Taking into account the preservation state of organic constituents leads to a better understanding of the relationship between SOM and sequence stratigraphy. Usually, condensed sections about maximum flooding surfaces are characterized by a decrease in the relative proportion of the terrestrial organic constituents. As far as sequence boundaries are concerned, they are marked by an increase in the relative proportion of continental organic matter, which results from a renewal of the erosion related to relative sea level fall. An increase in fresh woody debris has not been systematically observed at sequence boundaries. Nevertheless, the abundance of fresh woody debris tends to decrease stratigraphically towards maximum flooding intervals and laterally in a distal direction. The vertical palynofacies signatures at the scale of 2nd order cyclicity appear to be controlled by tectonics. In the Upper Oxfordian to Lower Kimmeridgian interval, these signatures confirm the hypothesis of a regional uplift of the hinterland and/or a differential basin subsidence related to the intense rifting activity in the Proto-Atlantic or to the westward opening of the Tethys.