Redox, productivity and paleotectonic studies in the southern Tethyan margin of northern Tunisia

Abstract

Abstract In northern Tunisia, the Cenomanian-Turonian sediments are characterized by organic-rich material. These deposits are commonly attributed to the Bahloul Formation and record the worldwide OAE2 event. In general, the Bahloul Formation is related to paleogeographic and tectonic events, these results suggest that multiple causes, including primary productivity, sea level changes, climatically driven organic carbon burial and structural framework. This formation is composed of three differentiated units, the first unit (U1) is composed of laminated limestones with small intercalations of marls (%CaCO 3 varies between 48 and 83), the second unit (U2) is characterized by very thin beds of marls, argillaceous and laminated limestones (%CaCO 3 ranging from 57 to 85). Lastly, the third unit (U3) is represented by a set of thick-black shales (with some levels of marls). Isotopic δ 13 C carb study shows a large positive excursion in the Bahloul Formation. It is characterized by three peaks that exceed 3‰. First and the second peaks are identified at the lower unit U1 with isotopic values recording 3.2 and 3.3‰ respectively. Third peak is recorded in the middle of the unit U2 and it attains 3.2‰. This section records important TOC values reaching up to 3.84% in unit U1, with average values exceeding 3%. The Rock-Eval data shows a marine organic matter with kerogen Type II (429 max > 435 °C), which can generate oils. A significant enrichment of major and trace elements are also recorded which took place during the deposition of the Bahloul Formation. In general, high levels of P, Cu and Ni indicate a high surface primary productivity, which led to the accumulation of large quantities of organic matter on the sea floor.