La limite Cénomanien-Turonien en Tunisie centrale et méridionale: biostratigraphie et paléoenvironnements

Abstract

The 'Dolomie du Gattar' (part of the Zebbag Formation of central and southern Tunisia) is, for the first time, accurately dated by means of newly-discovered ammonites in underlying and coeval strata. The underlying beds are limestones with a pelagic fauna (ammonites, foraminifers, coccoliths) encompassing three ammonite zones: the Neolobites vibrayeanus Zone (late Cenomanian); the Vascoceras cauvini Zone (end Cenomanian); and the Pseudaspidoceras flexuosum Zone (early Turonian). The Gattar Member is not entirely dolomitic and corresponds to a carbonate platform sequence deposited during a shallowing upwards cycle. It can be 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 ammonite-beating facies. From this we deduce that the Gattar Member is earliest Turonian ( flexuosum Zone) in age. The member belongs to a third-order transgressive regressive cycle. The fossiliferous pelagic limestones that mark its base are interpreted as having been deposited during the transgressive phase, the Gattar Member representing the highstand tract. In South Tunisia, the member is topped by an unconformity with evidence of emersion and is interpreted as a sequence boundary. This unconformity is missing in central Tunisia, where uninterrupted marine conditions continued across the Cenomanian-Turonian boundary. As a result, the overlying lower to middle Turonian deposits are diachronous 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 reconstructions.