Middle Paleocene to early Eocene sedimentological evolution of the central-eastern Sirt Basin (Libya): biostratigraphy and depositional environment

Abstract The depositional history of the central-eastern Sirt Basin of Libya during the Middle Paleocene to Early Eocene is characterised by the deposition of shallow-marine carbonates and hemipelagic claystone and marlstone. To gain a better understanding of the depositional history during this period, we dated the hemipelagic marlstones using planktonic foraminifera and calcareous nannoplankton to refine the stratigraphic relationships with the intercalated shallow-marine limestones. During the Danian, a broad carbonate shelf formed in the eastern sector of the basin. Transgression in the Early Selandian resulted in the deposition of claystones and limestones. The shallow-marine carbonate factory recovered rapidly during the Early-Middle Selandian, forming the Upper Sabil carbonate ramp, however, it became subaerially exposed and eroded in the shallowest areas. The resulting domal structures represent carbonate ramp erosional relics as demonstrated by biostratigraphic results combined with microfacies studies. Whereas in the deeper-water areas of the Upper Sabil carbonate ramp, the deposition was probably continuous throughout the Paleocene–Eocene, the shallow-marine limestones were exposed for about 6 Ma until they were drowned in the Early Eocene. A distinct argillaceous unit, with intercalated shallow-marine limestone beds, was deposited in the basinal areas between the domal structures during the Late Selandian–Thanetian stages. In this study, we define this deposits as the Intisar Formation. The overlying Late Paleocene/Eocene Harash Formation consists of shallow-marine limestones. This trangressive unit infilled the Upper Sabil paleotopography with a 2.5 Ma stratigraphic gap. The hemipelagic marlstone of the Kheir Formation was deposited above the Harash Formation and partly above the Upper Sabil paleotopography during the Ypresian, driven by the major sea-level rise following the Eocene Thermal Maximum-2 (ETM-2). Therefore, a substantial stratigraphic gap of at least 6 Ma is documented on the crest of the domal structures, which were subsequently overlain by the Kheir Formation. Graphical abstract