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Paleoecological Variations Recorded in a Shallow Lacustrine Formation: The Upper Eocene of the Issirac Basin (SE France)

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 Issirac basin (SE France) is a small-sized (5×15 km), N110 syncline basin formed during the Eocene NNE-SSW Pyrenean compression and the subsequent West European Cenozoic Rifting. This basin is filled by a 200 m thick carbonate succession, deposited in a shallow lacustrine environment (0-10 m depth). A detailed multi-proxy analysis was conducted on multiple outcrops; it includes sedimentological data (petrographic analysis, facies description,…), paleontological contents (gastropods, ostracods, charophytes, foraminifera…) and geochemical signatures (δ18O and δ13C). This allows us to determine depositional models and the paleoenvironmental evolution of the basin. 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. Three main facies associations are interpreted in terms of trophic regimes: 1) the macrophyte-related micritic facies association (charophyte-bearing wackestones, chalky mudstones) corresponding 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 (wackestones with planar mats, grazed by gastropods; low-relief stromatolites; evaporite-rich facies) during eutrophic regime. Thus, this work proposes new assumptions on sedimentary facies control of a shallow microbial carbonate lacustrine system submitted to fluctuating salinity and trophic regime. 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.