Evolution of the Eocene shallow carbonate platform in Tunisia: Carbon isotopes stratigraphy and petroleum implications

Abstract

Abstract This study aims to comprehend how the SW Neo-Tethys carbonate platform responded to Eocene global warming events and the possible relations with developing the middle Eocene carbonate oil reservoir (Reneiche member). The Eocene deposits of the Southern Neo-Tethys margin (Tunisia) are NW-SE-oriented facies belts with evident variations in thickness and facies. Along a homoclinal ramp, the Cherahil Formation's shallow carbonate platform facies progressively shift into the Souar Formation's globigerinids-clays. Major controls have been exerted on the central Tunisia platform by synsedimentary tectonics, sea-level oscillations, and warming events. Ten facies were identified and are attributed to at least three sedimentary models. The middle-upper Eocene sedimentary environments are wave dominant, influenced by storms and tide in a shallow water depth, passing during the Oligocene into a sandy fluvial and estuarine environment. Sequence stratigraphic analysis points towards shallowing upward trend interrupted during the lower Bartonian by a significant transgression. This sea-level rise coincides with a shift towards a short maximum nummulite carbonate production absent since the nummulitic limestones of the El Garia Formation (Ypresian). Proximal facies of dominant lumachellic marls shift to a thickening-upward sequence of transgressive facies culminating in a maximum nummulitic carbonate production during the middle Eocene transgression and coincides broadly at the global scale with the occurrence of the Middle Eocene Climatic Optimum (MECO). Previous biostratigraphic studies confirm the position of the Reneiche/Siouf member and here by the bulk carbon isotope record (δ13C) characterized by a negative carbon isotope excursion in the middle of the Siouf member followed by a positive excursion at the top of this member, which is comparable to the δ13C record through the MECO in the reference sections. In response to the MECO, the early Bartonian is recognized as crucial for the large benthic foraminifera in various shallow-marine settings along the Tethys margins.