Mineralogical and geochemical signatures of Yeşilvadi clays: Implications for the paleoenvironmental evolution of the Şile Basin, Northern Türkiye

Eight stratigraphically constrained Yeşilvadi clay layers (GE-1–GE-8) from the Oligo–Miocene Meşetepe and Istanbul formations in the Şile Basin (northeast Istanbul, Türkiye) were assessed using XRD (with glycolation/heating), petrography and SEM-EDS, pH measurements, and bulk and trace-element geochemistry to reconstruct their depositional history and paleoenvironmental evolution, while resolving clay-mineral variability, provenance, weathering intensity, and redox-controlled diagenesis. XRD reveals mineralogical heterogeneity dominated by poorly ordered kaolinite and non-expandable illite, with subordinate dioctahedral smectite and minor chlorite; expansion of the 14 Å basal reflection to ∼16.7 Å after glycolation and its collapse upon heating confirm smectite. Geochemical trends mirror mineralogy: MgO is higher in smectite-richer horizons, whereas K₂O (2.7–4.0 wt%) reflects widespread illite/mica. SiO₂/Al₂O₃ ratios (1.47–2.99) track quartz dilution versus kaolinite enrichment; the lowest ratios coincide with high loss-on-ignition and strong alkali depletion, consistent with prolonged leaching and enhanced refractory character. Provenance and alteration proxies indicate compositionally mixed source rocks spanning felsic to intermediate–mafic fields, overprinted by extreme chemical weathering (CIA = 86–99; CIW = 98–100). Layer-scale pH variability (∼4.2–8.3) links acidic, kaolinite-rich intervals to more alkaline, smectitic and spherosiderite-bearing beds. Spherosiderite is the only carbonate phase identified by XRD/SEM-EDS. Trace elements record alternating detrital input and redox states, with Zn–Pb–As enrichment in GE-5 and Zr–Ba enrichment in GE-6. Spherosiderites preserve a three-stage diagenetic history from microbially mediated FeCO₃ nucleation, to chemically zoned growth (Fe-rich cores, Mn-enriched rims), and partial burial homogenization, consistent with freshwater, sulfate-limited lacustrine–wetland conditions modulated by basin tectonics and sediment supply.