In spite of significant discoveries (e.g. in the Campos Basin - Brazil, in Cabinda - Angola, in the Uinta Basin - USA) that demonstrated the prolific oil reservoirs of the syn-rift lacustrine carbonates, little is known about... [ view full abstract ]
In spite of significant discoveries (e.g. in the Campos Basin - Brazil, in Cabinda - Angola, in the Uinta Basin - USA) that demonstrated the prolific oil reservoirs of the syn-rift lacustrine carbonates, little is known about the processes driving the sedimentary architecture and the factors controlling such systems.
The Tertiary Issirac sub-basin (Southern France) is a small-sized (15x5 km), N110 syncline basin formed during the Eocene NNE-SSW Pyrenean compression. This sub-basin is filled by a 200 m thick carbonate succession that deposited in a shallow lacustrine environment (0-10 m depth) during the West European Cenozoic Rifting. The outcrop analysis allows us to determine the paleoenvironmental evolution, from a detailed multi-proxy analysis taking into account sedimentological features (petrographic analysis, facies description,…), paleontological contents (gastropods, ostracods, charophytes, foraminifera…) and geochemical signatures (δ18O and δ13C).
Based on these data, depositional and lake evolution models were built. Sedimentary facies are typical of palustrine to shallow lacustrine paleo-environments, organized in a low-energy, ramp margin-type lake. Despite the classical relationship between sedimentation and external controlling parameters (e.g. climate, tectonics impacting lake morphometry…), it appears that in such shallow lakes, sedimentary facies distribution and architectures may be drastically affected by internal parameters relative to lake ecology. Indeed, shifts in trophic regime, mainly controlled by nutrient availability in the lake water, have a strong impact on the carbonate production and sedimentary record. Three main facies associations are interpreted in terms of trophic regimes: 1) the macrophyte-related micritic facies association (charophyte-bearing wackestones, chalky mudstones) corresponds to oligotrophic regime, 2) the granular oncoidal-peloidal facies association (oncoid/ooid-rich porous grainstones) developed under mesotrophic regime and 3) the stromatolite facies association (wakestones with planar mats, grazed by gastropods; low-relief stromatolites; evaporite-rich facies) during eutrophic regime. Furthermore, it seems that moldic porosity and intercrystalline microporosity development as well as pore occlusion by sparry calcite cements are linked to early meteoric diagenetic transformations and may drastically modified the original texture.
In this way, this work proposes new assumptions on sedimentary facies control of a shallow microbial carbonate lacustrine system submitted to a fluctuating salinity. The recognition and the study of the interplays between sedimentation, large-scale paleoenvironmental constraints, but also internal parameters (lake ecology, faunal/floral competition…) are fundamental in order to provide an accurate view of lake sedimentation in the past. Stratigraphic forward modeling will be considered to test assumptions on carbonate factories and carbonate production and to derive conceptual rules in lacustrine carbonate systems.
