Changes in carbonate sedimentation and faunal assemblages in the Tunisian carbonate platform around the Cenomanian-Turonian boundary

Abstract

In Tunisia the Cenomanian-Turonian boundary coincides with the transition between two carbonate deposits: the Late Cenomanian Bahloul Formation at the bottom and the Early Turonian Gattar Member at the top. According to field study and microscope examination of the studied Formations throughout Tunisia, the Bahloul Formation consists of a deep-water condensed carbonate platform largely extended throughout Tunisia. Detailed analysis shows a particular richness in planktonic microfauna and ammonites. Thin bedded limestones within the Upper part exhibits a high TOC ratio and commonly called the black shales, whereas the Gattar Member displayed a rudist-bearing carbonate ramp which pinches northward while being progressively relayed by hemipelagic marls forming the so-called Annaba Member. The Cenomanian-Turonian transition was marked by relevant changes the main features of which are well recorded in the Tunisian carbonate platform. The changes consist of consecutive (i) shutdown and recovery of the carbonate factory and (ii) extinction, development and diversification of many faunal species. As matter of fact, during Late Cenomanian many planktonic foraminifers (Rotaliporidae) and rudists (Caprinidae) were extinct, and simultaneously the carbonate production rate fallen into 0.003–0.017 m k y −1. During Early Turonian the carbonate platform recovered contemporaneously with the development and diversification of many planktonic foraminifera and rudists (Hippuritidae); the carbonate productivity rose to 0.125 m k y −1 in average. The whole previous changes are to be linked with the significant sea-level fluctuation and the contemporaneous oceanic anoxic event (OAE2) having occurred at the Cenomanian-Turonian boundary.