Analysis of biomicrofacies in the interpretation of the sedimentary environment of Middle Eocene-Early Miocene deposits in Isa Abad, and Imamzadeh Gahroo (surrounding Chaharmahal and Bakhtiari), Central Zagros

Abstract

The middle Eocene-early Miocene deposits have been sampled in the stratigraphic sections of Isa Abad and Imamzadeh Gahroo, Chaharmahal, and Bakhtiari, to determine the sedimentary environment based on microfacies analysis. Nine carbonate microfacies have been identified based on microscopic studies, where the inner, middle, and outer ramps are located in three facies belts. In the inner ramp, the MF1 and MF2 microfacies indicate an enclosed lagoon considering the presence of perforate benthic foraminifera in huge numbers, corallinaceae algae, ostracods, gastropods, and peloid. Also, the MF3 and MF4 microfacies indicate a lagoon with free water rotation and the absence of a continuous reef due to the simultaneous presence of perforate and imperforate benthic foraminifera. In Shoal (Microfacies MF5), bioclasts and ooids are the major grains, along with peloids, intraclasts, corallinaceae algae, echinoids, bryozoans, and corals. In the middle ramp (microfacies MF6 to MF8), considering the presence of large hyaline species (Lepidocyclina, Miogypsina, and Nummulitidae), the stretched shape of these species, the large size of the allochems, their association with corallinaceae algae, the diversity and abundance of species, the small presence of planktonic and porcelaneus species are evident. In these sections, the outer ramp (Microfacies MF9) can be identified by the presence of planktonic species, the small number of benthic species, and the abundance of the matrix. Due to the gradual conversion of microfacies into each other, the lack of abundance of shoaling ooid and peloid microfacies, and the absence of turbidities and collapse breccia’s, the sedimentary environment of these deposits can be attributed to a homoclinal ramp.