Influence of sea level changes on the deposition of carbonaceous sediments

The Mid-Cretaceous deposits (Aptian-Lower Cenomanian) of the Provence basin (SE France) show a predominance of interbedding of limestone and marl which indicates a deep marine environment. The stratigraphic intervals of this series have been dated with ammonites, foraminifera, dinocysts and calcareous nannofossils. The sedimentological, paleobiological and paleoecological data indicate the occurrence, during the Aptian-Lower Cenomanian period in Basse-Provence, of an open marine environment or at least in communication with open sea, subjected to some land influence fairly distant from the site of deposition. The sediments studied have a high carbonate content and include several levels rich in organic matter (0.5–11% of TOC). The nature of this organic material makes it possible to distinguish three different origins for the organic matter: 1. (1) a marine origin represented by phytoplankton and zooplankton, 2. (2) a continental origin with plant cuticles, tracheophytes, ligneous tissues, spores and pollen, 3. (3) a bacterial origin, with amorphous material. Strontium and manganese content fluctuations in the carbonated phase of the facies studied reflect variations in the chemical composition of the sea water function of the geological time. These fluctuations coincide globally with the sea level variation curves. Sedimentological, paleontological and geochemical analyses of the Mid-Cretaceous series in Provence have made it possible to detect positive and negative fluctuations in the sea level within this series, and to correlate deposition and nature of the facies rich in organic matter, with high and low sea level periods. The results of this study indicate that the sedimentation sources of facies rich in organic matter are many, and independant of sea level variations, as this type of deposit appears equally at high and low sea levels.