Facies Architecture of a Mixed Siliciclastic–Carbonate Ramp: The Barremian–Lower Aptian Gadvan Formation on the Northeastern Arabian Plate Margin

The Gadvan Formation (Barremian–lower Aptian) is an important Lower Cretaceous unit in the Zagros Basin and the Persian Gulf region. In this study, a detailed microfacies analysis of the Gadvan Formation was conducted based on thin sections from core and cutting samples, as well as wireline-log data from five wells across the A, B, C, D, and E oilfields. Fourteen microfacies (MF-1 to MF-14) were identified and grouped into distinct depositional belts, including coastal-plain, inner ramp, middle ramp, and outer ramp settings. These microfacies are dominated by peloids, bioclasts, orbitolinids, benthic foraminifera, green algae, Lithocodium, and variable amounts of clay and terrigenous material, reflecting deposition within a mixed siliciclastic–carbonate ramp system. Integration of microfacies distribution with depositional models suggests that the Gadvan Formation developed on a homoclinal ramp, which locally evolved into a distally steepened geometry in the northwestern Persian Gulf. In some areas, shoal complexes separate restricted inner-ramp settings from more open-marine environments. The presence of argillaceous sandstone and scattered quartz grains in certain intervals indicates episodic influxes of siliciclastic material from adjacent continental sources. Diagenetic features have partially modified the original depositional textures. These findings contribute to the refinement of the regional depositional model of the Gadvan Formation mixed siliciclastic–carbonate ramp in the Persian Gulf.