The Cenomanian–Turonian boundary in the Gafsa-Chott area (southern part of central Tunisia): biostratigraphy, palaeoenvironments

Abstract

This study is based on biostratigraphical and palaeoenvironmental analyses of four columnar sections exposed in the Gafsa-Chott area of central Tunisia. The Cenomanian-Turonian deposits in this area are divisible into Zebbag s.s. (at base), Bahloul and Gattar Formations or their equivalents. The Cenomanian-Turonian boundary is placed within the Bahloul Formation s.s.on the basis of the occurrence of several index ammonite faunas including Choffaticerasaff. massipianum(Pervinqui ère). The Gattar Formation and its equivalents are assigned to a part of the late Early Turonian (Thomasites rollandiand Choffaticeras luciaeIntervals). The occurrence of Spathites (Jeanrogiceras)aff. revelieranus (Courtiller) encrusting the top of the Gattar equivalent probably indicates the top of the Lower Turonian. The meaning of the association C. luciaePervinquière, Thomasites rollandiforma jordaniPervinquière and S. (J.)aff. revelieranuswith respect to the M. nodosoidesZone needs to be clarified. A wide lateral variation in facies is the characteristic feature of the sequence, caused by a rapid change in the depositional setting. It has been concluded that the Cenomanian-Turonian transition of the area under consideration was deposited in a dysaerobic environment, as evidenced by the occurrence in a basin setting of a micritic limestone unit rich in organic matter with planktonic and nektonic organisms and certain stunted neritic fossils. Oxygen impoverishment was less drastic on the carbonate platforms, as indicated by the occurrence of pelagic white limestones devoid of organic matter and containing cephalopods as well as planktonic foraminifers and minute bivalves.