Duration of the oceanic anoxic event at the Cenomanian Turonian boundary. Cyclostratigraphic interpretation of the Bahloul Formation in Central Tunisia

The Bahloul Formation of Central Tunisia was deposited on the proximal part of the southern Tethyan platform. It recorded the global oceanic anoxic ev ent (OAE 2) that occurred at the Cenomanian/Turonian boundary. The type sec tion of the Wadi Bahloul comprises 29 m of sediment showing a regular repet ition of an elementary depositional sequence including black, thinly-bedded limestones and bioturbated gray marls. These sedimentological fluctuations define two trophic states of the palaeoenvironment. The laminated black li mestones are associated with eutrophic conditions in the water column above an anoxic basin, while oligotrophic and at least temporarily oxic conditio ns at the sediment/water interface prevailed during the deposition of the b ioturbated grey marls. Each lithological cycle reflects short-term climatic fluctuations: arid conditions are associated with the deposition of the bl ack laminated limestones, whereas the grey marls correspond to sub-humid co nditions. These environmental changes are interpreted as being related to p recession cycles of 20 ky. According to the number of precession cycles ide ntified in the section of the type locality, the Bahloul Formation probably was deposited during one eccentricity cycle of 400 ky. This cyclostratigra phic interpretation, calibrated by biostratigraphy, constrains the duration of the anoxic event more precisely than radiometric ages which suggest a d uration of 500 +/- 400 ky.