The transgressive event in the Upper Cenomanian-Lower Turonian carbonate platform of central Tunisia: main indicator and precursor criteria

The Cenomanian–Turonian Oceanic Anoxic Event (OAE-2) is generally identified in basinal and restricted environments appropriate for the deposition of black shales, commonly used as the main criteria of this global event. Despite, it would not always be easy to identify with certitude this event in shallower and more open depositional environments, other features and indicators are needed. The main purpose of this study in central Tunisia was to point out the sedimentary record of the (OAE-2) event equivalent in shallow marine depositional environments. However, based on bed-by-bed logging, five stratigraphic sections have been logged, described and well-documented throughout the excellent and well-exposed Cenomanian–Turonian succession in central Tunisia. Our approach was also based on comparative studies undertaken elsewhere, providing features announcing this event, in both platform and basin environments. As a result, some sedimentary features in a platform setting have been recently documented and considered as indicators of the equivalent of (OAE-2) event. At a first glance, this transgressive event recorded near the Cenomanian–Turonian boundary is essentially expressed by the sharp change of the sedimentary facies from subtidal-emergent stromatolite-rich carbonates to pelagic marlstones. In fact, the uppermost carbonate beds of the Upper Cenomanian-Lower Turonian Gattar Member, showing stromatolites with emergence and erosion features, are unconformably overlain by the Lower Turonian Annaba pelagic marlstones. This sharp change in the sedimentary record illustrates a clear deepening of the depositional environment. The main transgressive surface related to the global Cenomanian–Turonian sea level rise could be superimposed on the emergent surface topping the Gattar carbonates. Below this main transgressive surface, other surfaces and tectonic marks are identified and interpreted as precursors of this latter. These precursors consist of sea level changes and tectonic tilting features. Overall, we present a new chrono-stratigraphical correlation of the different logged sections, taking the lowermost part of the Lower Turonian Annaba pelagic marlstones as the datum, and running from a deep-marine environment receiving black shales sedimentation to platforms, that will be very useful to underline the expression of the equivalent of the OAE-2 event in platform setting. © 2023, The Author(s), under exclusive licence to Springer Nature Switzerland AG.