Biostratigraphy and facies architecture of Jurassic successions in the Ionian Basin (Epirus, NW Greece) with preliminary source-rock screening

The opening of the Tethys Ocean during the Early Jurassic led to the development of several sedimentary basins. Among these, the Ionian Basin developed along the southeastern margin of the Tethys Ocean. The Jurassic–Cretaceous formations of the Ionian Basin have been extensively studied as potential petroleum source rocks. Accordingly, this study combines fieldwork, biostratigraphy, microfacies analysis, and a preliminary assessment of source-rock potential in the Jurassic succession of the Middle Ionian Zone. The study area comprises two sub-basins, Elataria and Saloniki, located in central Epirus, north of the active strike-slip Petousi Fault. The Elataria sub-basin consists mainly of Jurassic–Cretaceous formations of the Middle Ionian Zone, which exhibit substantial variations in thickness and color compared with equivalent formations exposed in the Ionian Zone. The Saloniki sub-basin comprises Jurassic–Eocene formations of the Middle Ionian Zone. The aims of this study are to establish the biostratigraphy of the carbonate and shale formations, characterize the paleoecological signatures of the studied deposits, and reconstruct their paleoenvironmental evolution, with particular emphasis on the Elataria sub-basin, while comparing the results with equivalent nearby successions. A total of 76 samples were collected and thin sectioned. Microfacies analysis identified six microfacies types, including micrites, fossiliferous micrites, packed biomicrites, unsorted biosparites, crystalline microfacies, and mixed microfacies consisting of micritic and biomicritic matrices with fossiliferous micritic to packed biomicritic extraclasts. These microfacies correspond to four depositional environments: deep sea, toe of slope, slope, and platform-margin reef environments. During the Jurassic, deposition in the studied sub-basins is interpreted to have occurred mainly in toe-of-slope to deep-sea settings, with pronounced lateral variations in thickness, color, and bitumen content. Total organic content values from the Elataria sub-basin range from 4.1 wt% to 5.5 wt%, indicating very high organic matter content. The pronounced differences observed in the Jurassic succession of the Elataria sub-basin are interpreted to reflect the restricted character and tectonic evolution of the basin, which were influenced by normal and transfer faults. These structural controls affected accommodation space, sediment accumulation, and depositional conditions, resulting in the development of a thicker and lithologically distinct Jurassic succession compared with equivalent successions in the Ionian Zone.