Micropaleontological study (foraminifera, ostracods) and characterization of the paleoenvironment of middle Cretaceous deposits (Djebel Chemla area, north-eastern Algeria)

The Djebel Chemla section is situated in the north-east of Algeria, close to the town of Tébessa and just a few kilometers from the Tunisian border. This section is composed mainly of a sequence of marl and marly limestone levels, alternating with recurring oyster levels which range in age from Vraconnian to the Early Turonian. Firstly, the micropaleontological study of the content (counting of foraminifera and ostracods, thin sections) allows the subdivision of the section into four biozones, with Rotalipora appenninica, Rotalipora globotruncanoides, Rotalipora cushmani, and Whiteinella archaeocretacea as bio-indicators; and, secondly, it provides the opportunity to specify the paleoenvironment. The supposed depositional environment is rather calm and deep, of an external platform type, with a maximum depth of 200 m. Bottom waters seem to be commonly poorly oxygenated. The end-Cenomanian anoxic crisis was also characterized by the presence of Bahloul facies levels, rich in TOC, in filaments and of globulous planktic foraminifera, including a considerable proportion of Heterohelix. The genus Rotalipora has disappeared a few levels lower down.