An overview of the Cretaceous oceanic anoxic events in Egypt, southern Tethys

Abstract

Abstract The Cretaceous Period ( c. 143–66 Ma) attested to successive phases of regional tectonic activities and large-scale submarine volcanism resulting in increased atmospheric CO 2 concentrations, enhanced greenhouse climate and global warming, surface water bioproductivity and well-developed pore and bottom water anoxia, especially during the mid-Cretaceous (Aptian to Turonian). These conditions triggered widespread deposition of organic carbon (OC)-rich black shales during episodes of severe oxygen exhaustion, commonly known as oceanic anoxic events (OAEs). Oceanic anoxic events had many important consequences, including the widespread mass extinction of marine and terrestrial organisms. Several intervals of enhanced oxic ventilation and deposition of OC-poor pelagic–hemipelagic red carbonates and claystone interrupted Cretaceous OAEs, which are commonly known as Cretaceous oceanic red beds (CORBs). The mid-Cretaceous black shales from many parts of the world and the corresponding global carbon cycle changes indicated that OAE1 (Selli Level) and OAE2 (Bonarelli Event) are the most widespread and well-defined OAEs compared with the more regional OAE3 and Valanginian–Hauterivian black shales (Weissert Event). These appear to have been more restricted to the Atlantic and adjacent areas, a result of more regional conditions rather than being forced by global environmental perturbations. In the course of this review, the Egyptian territory in the southern Tethys shelf is taken as a case study area for the development, evolution and regional expression of mid-Cretaceous OAEs. The region is characterized by a thick Cretaceous sedimentary succession of OC-rich black shale and OC-poor carbonates and mudstone interbeds. The late Albian OAE1d was reported in the North Western Desert (upper Kharita Formation) based on a positive carbon isotope excursion and moderate total sulfur, total organic carbon and redox-sensitive elements. The end-Cenomanian OAE2 was reported in several regions of Egypt based on positive δ 13 C excursions. However, differential deposition between OC-rich and OC-poor facies took place, especially in marginal to shallow marine settings where black shales are absent, such as in the Eastern Desert and Sinai. During the Coniacian–Santonian, OC-poor limestone, calcareous shale and red claystone were deposited under enhanced water column respiration in Egypt. This indicates that enhanced oxygenated ocean circulation controlled organic matter decomposition during weak continental weathering and enhanced carbonate production, all of which led to the deposition of OC-poor CORBs in this region.