Biostratigraphical, geochemical and microfaunal responses to environmental changes around the Cenomanian-Turonian boundary at Theniet El Manchar (Bellezma-Batna Range, NE Algeria)

The present study deals with the biostratigraphic and geochemical analysis of the Cenomanian-Turonian deposits of the Theniet El Manchar district in the Bellezma-Batna Range, and their vertical fluctuations. A 180 m-thick profile is described in this area. This reference section encompasses the 'Marnes de Smail' Formation, subdivided into four lithostratigraphic units (IA, IB, IC, and ID respectively) and dated of the Cenomanian (ammonites and coral), and the base of the 'Dolomies de I'Oued Skhoun' Formation (unit IIA), dated of the Lower Turonian (planktonic foraminifers). Both qualitative and quantitative analysis of foraminifers (planktonic and/or benthonic), ostracodes and of geochemical indices, lead us to reconstitute the evolution of both palaeoenvironmental and palaeobathymetrical changes during this time interval. Subsequently units IA, IB, IC and the lower part of unit ID (all Cenomanian) are displaying agglutinated benthonic foraminifers and complete ostracode shells, as evidence of a shelfal environment and weak hydrodynamism. Within these deposits microfaunal assemblages display a low-to-medium species diversity and a relatively high specimen abundance, supporting evidence of normal trophic conditions and water oxygenation. The upper part of unit ID (topmost Cenomanian), and unit IIA (basal Turonian) are documenting an environmental setting into deeper conditions, supported by the standard event succession already recorded in northern Africa, namely: the abundance of planktonic foraminifers, the occurrence of 'filaments' and the sudden reduction of ostracofauna. Furthermore the total carbonate isotope fluctuations (delta C-13 and delta O-18) are documenting isotopic anomalies related to palaeoenvironmental changes. The delta C-13 results, coupled with those of TOC, are evidencing a low primary productivity while the delta O-18 data are supporting a temperature rise as the main potential drive of the onset of OAE2. These paleoenvironmental assertions are consistent with the regional paleogeographic context and are highlighting tethysian features.