Microfacies and Environmental Characteristics of Nummulitid-Rich Carbonate Deposits: A Case Study from the Jahrum Formation, Northwest Bandar Abbas
Subject Areas :Jahanbakhsh Daneshian 1 , S. Ghafouri 2 , Seyed Ali Moallemi 3 , Elham Asadi Mehmandosti 4
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Keywords: Microfacies , ,
Abstract :
The Jahrum Formation, with a Late Eocene age and a thickness of 344.42 m, was studied in the Suru subsurface section, located in northwest Bandar Abbas. This formation contains abundant and diverse Nummulitids, which are observed throughout the section. The morphological analysis of Nummulitids indicates that these foraminifera respond to environmental factors such as light intensity and hydrodynamic conditions by altering their size, thickness, and shell shape. Therefore, investigating their shell characteristics contributes significantly to interpreting the depositional environment. Thin-section analysis led to the identification of five microfacies: Porcelaneous foraminifera Packstone, Bioclast Peloidal Packstone, Miliolids packstone to Grainstone, Bioclast Nummulites Wackestone to Packstone, and Nummulitid Packstone. These microfacies were deposited in lagoonal, sand shoal, restricted marine, and open marine environments within the inner ramp setting. Based on the obtained data, the depositional environment of the Jahrum Formation is interpreted as a homoclinal ramp. The morphological study of Nummulitids revealed that they are most abundant in the Nummulitid packstone microfacies of the open marine setting. The lenticular forms with flat test indicate deeper open marine environments with normal salinity. Additionally, Nummulites with thick test, appearing as inflated lenticular to lenticular forms, along with some bioclasts, are characteristic of restricted open marine conditions. In contrast, Nummulitids with greater thickness and more inflated shells occur with very low abundance in deeper lagoonal settings. Finally, the presence of imperforate foraminifera and the absence of Nummulitids indicate high-salinity lagoonal environments, reflecting salinity fluctuations within the depositional setting.