Biofacies analysis and depositional environments of Mid-Eocene Larger Benthic Foraminifera-rich deposits in Northern Tunisia

Abstract In NW Tunisia, the lateral equivalents of the Mid-Eocene “Reineche Limestone” member of the Souar Formation (Bartonian) are still poorly known. Sedimentological investigations in the “Reineche Limestone” type locality in NE Tunisia; and three correlatives, first described here from NW Tunisia, allow the reconstruction of their depositional settings and to propose a regional geodynamic context. Fossil assemblages, rock texture and fabrics plead in favor of eight micro-biofacies (Mf1 to Mf8) indicating “shoal” inner ramp to outer ramp depositional settings. The Bartonian carbonates of NE Tunisia bear LBF-dominating assemblages and subsidiary planktic and small benthic foraminifera, gastropods, algae and echinids indicating a progressive marine ramp context under oligotrophic conditions. However, correlative successions from NW Tunisia are represented by relatively thinner carbonate intervals including two main facies: 1) bioclastic limestone facies and 2) phosphorite-rich carbonates. The first is dominated by LBF assemblages suggesting the same depositional conditions as the “Reineche limestone”. However, phosphorite-rich carbonate facies is mainly characterized by peloids, bone fragments and lithoclast components with subsidiary nummulitids, planktic and small benthic foraminifers, echinids, oyster shells, brachiopod fragments and fine siliciclastic components, all suggesting oxic-suboxic to anoxic conditions for phosphorite genesis. This would have implied microbial decomposition of organic compounds and/or bacterial reduction of sulfate in shallow marine environment. These phosphorite-rich carbonate of northwestern Tunisia may serve as an example of phosphatic sediment production and accumulation during the latest episode of the Paleogene phosphatogenesis first described here from the south-Tethyan margin of Tunisia.