The rudist-rich carbonate units in central Tunisia as markers of Late Cretaceous transgressive events

Rudist-rich carbonates are common in central Tunisia. They constitute frequent intercalations in Upper Cretaceous series, particularly within the Cenomanian-Campanian interval. They are generally transgressive upon shallower deposits, such as Lower Cretaceous siliciclastic sediments, or Upper Cretaceous tidal flat deposits. The sequence stratigraphy analysis of the study carbonate series reveals discontinuities and unconformities that mark the boundaries between the identified sequences. These unconformities, which are mainly represented by oxidized hard-grounds with several borings and micro-karstification features, are in relation with a sea level drop or a tectonic uplift of central Tunisia during the Late Cretaceous. Sea-level changes, tectonic movements, and climatic events appear acting a lot for the growth but also for the demise of Upper Cretaceous rudist and coral communities. Globally, the Upper Aptian-Lower Albian shallow-water carbonate deposits are unconformably overlain by Upper Albian-Cenomanian transgressive deposits, locally rich in organic matter, represented by black-shales. This unconformity, which is identified at least in the whole central Tunisia, consists of a regional sequence boundary that announces typical greenhouse conditions and the beginning of a carbonate-rich sedimentation admitting frequent intercalations of rudist-sponge-coral-rich deposits. Above this main regional unconformity, the Upper Cretaceous series show additional sequence boundaries separating 3rd-order sequences. Overall, six transgressive-regressive sequences were identified within the Cenomanian-Campanian interval. The identification of these 3rd-order sequences including rudist-rich deposits has helped a lot to establish correlations at least at regional, north African (Algeria, Egypt), and Tethyan scales (Italy, Germany and other areas). In central Tunisia, Rudist-coral-rich carbonates appear both during shallowing and at the beginning of deepening stages. However, according to our study, they are obviously more common and well represented during transgressions. In this context, they constitute deepening-upward cycles in transgressive systems tracts. For these reasons, they could be considered as markers of most Late Cretaceous transgressive events, essentially the Cenomanian-Turonian and the Campanian global transgressions. Commonly, the Upper Cenomanian-Lower Turonian transgressive event, for example, is globally marked by the deposition of black-shales. However, in shallower environments, more closed to carbonate platforms and their margins, regionally extended rudist-coral-rich carbonates could be used as markers of such transgressive event.