From various geological traverses made in North-central Tunisia and northern Tunisia the Bahloul Formation (Burollet, 1956) is considered to be variable in thickness and facies. This formation is characterized essentially by its lithology and structure, consisting of alternating laminated and darkish micritic limestones interbedded with dark foliated marls which are rich in organic matter (mean T.O.C. 2.66%). At the type locality (Oued Bahloul), the Bahloul limestones are wackestones rich in pelagic fauna (Ammonites, Calcisphaerulidae, Rotalipora, Whiteinella, Dicarinella, etc). This formation overlies a massive 2 m thick limestone unit; the base of which is eroded, rich in phosphate and quartz grains. The same unit consists of wackestone-packstone carbonate rich in Calcisphaerulidae, with filaments and benthonic debris at the top of the formation. Thus, the base of this unit which covers the Fahdene Formation corresponds to an unconformity, shown by a rapid change in the depositional regime. The base of the overlying Kef Formation is also marked by an abrupt change in the depositional regime. Microfauna and microfacies analyses cofirm that the Bahloul Formation, at its type locality, began in the late Cenomanian. This is confirmed by Ammonites ( Calycoceras naviculare Mantell, with Eucalycoceras gothicum Kossmat, Mantelliceras villei Coquand) and ended in the early to middle Turonian. The richness and the pelagic nature of the fauna prove that the Bahloul Formation sediments were deposed in a continental-slope environment during a transgressive interval within the UZ-25 sequence (Vail et al., 1987).