Biofacies Analysis and Depositional Environments of Mid-Eocene Larger Benthic Foraminifera-Rich Deposits in Northern Tunisia

In NW Tunisia, the lateral facies equivalents of the mid-Eocene (Bartonian) "Reineche Limestones" member of the shaley Souar Formation, which constitute a proven reservoir hydrocarbon bearing in Gulf of Gabes, are still poorly known and characterized in NE Tunisia and the Salt Dome zone. Recent sedimentological investigations conducted on: (1) the "Reineche Limestones" member in its type locality in NE Tunisia and (2) three correlative sections, first described herein NW Tunisia, allow improving our knowledge about the Middle Eocene biofacies distribution as well as their depositional environment at a regional scale. Fossil assemblages, rock texture, and fabrics lead to the characterization of eight micro-biofacies (Mf1-Mf8) corresponding to "shoal" inner ramp and middle-to-outer ramp depositional settings. This study demonstrates that the Bartonian carbonates of NE Tunisia bear LBF-dominating assemblages and subordinate planktic and small benthic foraminifera, gastropods, algae, and echinids are indicative of marine ramp context under oligotrophic conditions. However, correlative successions from NW Tunisia are represented by relatively thinner carbonate intervals, including either skeletal limestone facies or phosphorite-rich carbonates. The first facies type, dominated by LBF assemblages, suggests the same depositional conditions as the "Reineche limestones". The phosphorite-rich carbonate facies is characterized by the presence of peloids, bone fragments, lithic components with subordinate nummulitids, planktic and small benthic foraminifers, diversified open marine fauna, and fine siliciclastic grains, all suggesting oxic-sub-oxic conditions favorable for phosphorite genesis. The Bartonian phosphorite-rich carbonate of northwestern Tunisia represents a good example of phosphatic sediment production and accumulation during the latest episode of the Paleogene phosphatogenesis around the paleohigh structures of the Salt Dome zone in the south-Tethyan margin of Tunisia.