New insights into the age and depositional environments of the Messinian salinity crisis in the sedimentary record of the Kechabta foreland basin, Northern Tunisia

The Neogene Kechabta foreland basin of northern Tunisia contains well-exposed Messinian outcrops, including the Oued Bel Khedim (OBK) and Chaabet Et Tabbela (CET) formations. Through bed-by-bed logging and comprehensive analyses that combined biostratigraphy, sedimentology, and petrography, the age and depositional environments were established. The Messinian succession of the studied sections includes four distinct lithological units that have produced a rich and well-preserved ostracod fauna. Species such as Cyprideis ruggierii, Callistocythere antoniettae, Dorukella aff. bireticulata, Leptocythere sanmarinensis, and Costa batei collected from the OBK1 and OBK2 units provide biostratigraphic evidence supporting an Early Messinian attribution for the first time. In contrast, the OBK3 unit of the Oued Bel Khedim Formation, along with the overlying CET unit of the continental Chaabet Et Tabbela Formation, exhibits a significant fauna scarcity, tentatively indicating a Late Messinian age for these units. The facies analysis enabled the characterisation of eight distinct facies (F1-F8) organised into four associations (FA1, FA2, FA3, and FA4), representing different depositional environments. FA1 and FA2 include gypsum, clay, as well as oolitic and peloidal limestone mixed with predominantly clay-rich sediments containing both brackish and marine benthic foraminifers (Ammonia tepida and Elphidium, along with ostracods including Cyprideis, Loxoconcha, and Xestoleberis). These sediments likely formed in shallow lagoonal settings that experienced fluctuations between restricted and fully marine conditions. In contrast, FA3 features varved micritic to peloidal limestone and gypsum/clays, with very few bivalves and ostracods, and was formed in a saline lake environment that later shifted to a sebkha setting. Lastly, FA4, composed of conglomerates and coarse-grained sandstones, indicates continental environments dominated by alluvial fan and fluvial deposits. The correlation of lithostratigraphic units and their associated facies across the Kechabta foreland basin, located in front of the Maghrebian chain, allows reconstruction of the depositional history and emphasises the main local tectonic and regional eustatic factors that influenced the evolution of the Mediterranean basin during the Messinian. The revised stratigraphic framework for Messinian deposits in the studied section, when compared with the Gulf of Tunis and the Cap Bon Peninsula in northeast Tunisia, suggests that the three-stage model consisting of Lower Evaporites, Upper Evaporites/Terminal Carbonate Complex, and Lago Mare, which is proposed for the central Mediterranean basin, is applicable in northern Tunisia. This study also shows that the Messinian deposits in the Kechabta foreland basin, despite being formed at the foot of the Maghrebian chain and in a proximal margin, may record several major phases associated with the Messinian salinity crisis across the Mediterranean.