THE BAHLOUL FORMATION IN CENTRAL NORTHERN TUNISIA - LATERAL VARIATIONS, NEW DATING, NEW INTERPRETATION IN TERMS OF SEQUENTIAL STRATIGRAPHY

From various geological traverses made in North-central Tunisia and no rthern 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 foliat ed 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, White inella, Dicarinella,etc). This formation overlies a massive 2 m thick limestone unit; the base of which is eroded, rich in phosphate and qua rtz grains. The same unit consists of wackestone-packstone carbonate r ich in Calcisphaerulidae, with filaments and benthonic debris at the t op of the formation. Thus, the base of this unit which covers the Fahd ene 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. Microfaun a and microfacies analyses cofirm that the Bahloul Formation, at its t ype locality, began in the late Cenomanian. This is confirmed by Ammon ites (Calycoceras naviculare Mantell, with Eucalycoceras gothicum Koss mat, Mantelliceras villei Coquand) and ended in the early to middle Tu ronian. The richness and the pelagic nature of the fauna prove that th e Bahloul Formation sediments were deposed in a continental-slope envi ronment during a transgressive interval within the UZ-25 sequence (Vai l et al., 1987).