Stratigraphic Distribution, Geochemical, and Mineralogical Characterization of the Middle Jurassic in Châabet El Attaris, North–South Axis, Tunisia

The Early Toarcian Oceanic Anoxic Event was a time of the most dramatic paleoenvironmental changes, including magmatic, oceanographic, and climatic changes. This event was characterized by a rapid greenhouse. The phenomena are marked by marine deoxygenation, which favored the widespread accumulation of organic matter rich sediments in shelf settings, and deeper marine basins. In this study, we present the sedimentology and paleoenvironmental deposition of the Early Toarcian Nara formation of Châabet El Attaris located in central part of the North–South axis, central Atlas of Tunisia. During this period, an extensional tectonic event related to the Tethyan rifting evolution caused the dislocation of the carbonate platform into small sub-basins characterized by restricted conditions that may have favored organic matter deposition. The study cross-section contains an approximately 22 m thick sediments, which are composed of alternation of dark marls, marly limestone, and fossil-rich limestone. The aim of this work is to characterize the deposit conditions by different approaches, such as redox geochemical analysis, paleoproductivity detrital proxies, mineralogical analysis, scanning electron microscopy, and infrared spectroscopy. The results obtained from these analyses led to a number of conclusions. The geochemical data gave a precise characterization of paleoenvironmental changes during regional depositional of organic matter. The variation in Si/Al, Rb/Al, and Zr/Al ratios recorded in some samples indicate significant increases in redox indexes, and suggest that the sediments were probably O2-deficient. The presence of clear cyclicities in the Sr/Al and Ba/Al indicate a variation in the paleoproductivity, which is confirmed by oxic to anoxic conditions as revealed by the redox sensitive trace metals values such as V/Al, U/Al, and U/Th. Clay mineral assemblages were controlled by the intensity of chemical weathering, climate changes, depositional environment, and sedimentary regime without neglecting the diagenesis process. Indeed, the clay minerals indicate a warm humid climate in the Toarcian period. A detailed SEM observations show the occurrence framboidal pyrite, which confirms the anoxic environment, where the pyrite is formed from exergonic reaction of H2S with Fe produced by sulfate-reducing bacteria, which is related to the organic matter oxidation under anoxic conditions.