THE CENOMANIAN-TURONIAN BOUNDARY - BIOSTR ATIGRAPHY AND PALEOENVIRONMENT

The 'Dolomie du Gattar' (part of the Zebbag Formation of central and s outhern Tunisia) is, for the first time, accurately dated by means of newly-discovered ammonites in underlying and coeval strata. The underl ying beds are limestones with a pelagic fauna (ammonites, foraminifers , coccoliths) encompassing three ammonite zones: the Neolobites vibray eanus Zone (late Cenomanian); the Vascoceras cauvini Zone (end Cenoman ian); and the Pseudaspidoceras flexuosum Zone (early Turonian). The Ga ttar Member is not entirely dolomitic and corresponds to a carbonate p latform sequence deposited during a shallowing upwards cycle. It can b e divided into a lower part consisting of biogenic limestones rich in rudists (Durania) and gastropods, and an upper series of limestone and dolostones indicative of tidal flat environments. In the Gafsa Basin this carbonate platform sequence passes laterally into deeper water am monite-bearing facies. From this we deduce that the Gattar Member is e arliest Turonian (flexuosum Zone) in age. The member belongs to a thir d-order transgressive-regressive cycle. The fossiliferous pelagic lime stones that mark its base are interpreted as having been deposited dur ing the transgressive phase, the Gattar Member representing the highst and tract. In South Tunisia, the member is topped by an unconformity w ith evidence of emersion and is interpreted as a sequence boundary. Th is unconformity is missing in central Tunisia, where uninterrupted mar ine conditions continued across the Cenomanian-Turonian boundary. As a result, the overlying lower to middle Turonian deposits are diachrono us along a north-south axis in Tunisia. The third-order cycle that the Gattar Member encompasses is a major palaeogeographical event on the North African margin and needs to be taken into account in global reco nstructions.