Depositional settings and basin-fill evolution of the restricted platform system on the Intra Pontide belt during the Late Eocene-?Early Oligocene transition, western black sea region, Turkey

Abstract

This study presents an integrated sedimentological, microfacies, and palynological analyses of the Late Eocene–?Early Oligocene (Priabonian–?Rupelian) Akçapınar Formation, exposed along the Soğanlı Stream in the Intra-Pontide Belt of the Western Black Sea region, Turkey. The formation, which overlies the evaporitic Pürçükören Formation and locally the Soğanlı Formation, records deposition in restricted shallow-marine to a lagoonal setting during a phase of significant tectonic and climatic reorganization. Detailed field mapping and measured stratigraphic sections, supported by petrographic, XRD, and SEM analyses, reveal mud-supported textures, limited faunal diversity, and thin-shelled gastropods, consistent with low-energy, brackish conditions. Palynological assemblages, identified here for the first time, refine the age constraints and document freshwater influence, whereas mineralogical data indicate alternating arid (smectite-rich) and humid (chlorite–illite-rich) intervals. Microfacies patterns (Mf1–Mf4) and lithofacies associations (Lf1–Lf8) were synthesized into three-dimensional depositional models, illustrating progressive basin restriction, episodic subaerial exposure, and evaporite precipitation. The multi-proxy approach, integrating sedimentology, microfacies, palynology, and mineralogy, refined the depositional model of the Akçapınar Formation, revealing facies heterogeneity driven by the interplay of tectonically induced basin isolation, relative sea-level fluctuations, and paleoclimatic oscillations during the Eocene–Oligocene transition, and providing new insights into the paleogeographic evolution of the northern Neo-Tethys during the final stages of its closure.