Eocene lacustrine–volcaniclastic deposits at Aliabad (Central Iran): A possible Proto-basin of the Oligo-Miocene Qom Formation

Mahdokht Karimi, Abdolhossein Amini, Nasrollah Abbassi, Mohsen Ranjbaran

Central Iran hosts intricate Cenozoic successions where the Oligo‑Miocene Qom Formation forms a major hydrocarbon reservoir. The stratigraphic and paleoenvironmental relationship of the Eocene Aliabad deposits to this formation has been controversial. This study integrates stratigraphic logging, petrography, geochemistry (XRD/XRF), and ichnology on 46 thin sections from Aliabad and 157 comparative samples from the Qom Formation to determine age, composition, stratigraphic, petrographic, diagenetic, diagenesis and depositional environment. Results demonstrate that the Aliabad deposits are exclusively Eocene, dominated by hybrid volcaniclastic–carbonate facies (with a mean content of ~69 % volcaniclastic content), lacking Oligo‑Miocene index fossils. Geochemical data reveal primary volcanic minerals (albite, microcline) and abundant secondary alteration and evaporite phases (analcime, clinoptilolite, halite, carnallite, hematite). Diagenesis involves volcanic glass alteration to clays and zeolites, diverse cements, dissolution, neomorphism, compaction, pyritization and limited dolomitization. Sedimentological and ichnological evidence of Thalassinoides indicate a predominantly lacustrine environment with episodic marine incursions and pyroclastic events. Together, these results confirm that Aliabad is a distinct Eocene lacustrine–volcaniclastic basin unrelated to the Oligo‑Miocene marine carbonate ramp of the Qom Formation and highlight potential evaporite and iron‑oxide mineralization.