Microfacies, Depositional Model, and Distinct Aragonitic Peyssonelliales Red Algae Assemblages of the Asmari Formation in the Lorestan Region, Zagros Basin

The Asmari Formation (Oligo-Miocene) is one of the most significant carbonate reservoir units in the Zagros Basin, Iran, and has been extensively studied in Khuzestan and Fars provinces (Sadeqi et al., ۲۰۲۵; Hoseinzadeh et al., ۲۰۲۴). However, its sedimentological characteristics in the Lorestan region remain poorly understood (Rajabi, ۲۰۲۱; Maghfouri Moghaddam, ۲۰۲۲). This study investigates the microfacies, depositional environments, and facies architecture of the Asmari Formation in a representative well from the Lorestan sector, where the succession exhibits a mixed carbonate-evaporite nature (Abyat et al., ۲۰۲۱). Petrographic analysis of ۱۴۵ thin sections led to the identification of ۱۷ microfacies, which were grouped into tidal flat, restricted lagoon, carbonate shoal, open marine, and deep marine belts within a homoclinal ramp setting (Mahmoodabadi et al., ۲۰۲۳; Xu et al., ۲۰۲۳). The lower sequence of the Asmari Formation shows facies distributions comparable to typical Oligo-Miocene platforms in the Zagros (Karami et al., ۲۰۲۰; Yazdanpanah et al., ۲۰۲۵), whereas the upper sequence is characterized by a unique dominance of aragonitic red algae (Peyssonelliales), replacing the more common coralline red algae (Rajabi & Moghaddam, ۲۰۲۲). A detailed depositional model reveals significant environmental shifts linked to relative sea-level changes, which controlled the development of evaporitic intervals, the distribution of high-energy shoals, and the occurrence of diagenetic features such as dissolution-enhanced moldic porosity (Hallenberger et al., ۲۰۲۲; Huan et al., ۲۰۲۴).

Keywords:

Asmari Formation , Microfacies Analysis , Carbonate Ramp Depositional Model , Peyssonelliales Red Algae , Zagros Basin (Lorestan Region)