Lithostratigraphy, biostratigraphy and diagensis of the Ruteh Formation, south of Amol

During the Late Carboniferous-Early-Permian, glaciers covered a large part of Gondwana (Aghanbati, 2009). In the Late Asselian-Early Sakmarian, due to the increase in temperature, glaciers have retreated from all of Gondwana (Aghanbati, 2010) and after that progress has occurred continuously at the beginning of the Permian. during the Early Permian, mostly mixed siliceous sediments Carbonate-debris of Droud formation was mainly deposited in continental and intermediate environment, with increasing depth gradually in the late Permian, Middle Permian and early Permian of Ruteh Formation with carbonate facies and then in the late Permian. Ruteh formation was identified and introduced for the first time by Assereto (Assereto, 1963) in the valley of Ruteh village in the northeast of Tehran. in this study of Ruteh formation based on detailed lithological characteristics (macroscopic and microscopic) and identification of benthic foraminifera in a stratigraphic section 30 km south of Amol in central Alborz, in terms of rocks Stratigraphy and biostratigraphy have been investigated.

After preparing microscopic thin-sections from all samples, the sections were stained with red alizarin solution according to Dickson's method (Dickson, 1965) to distinguish calcite from dolomite and then studied under a microscope to identify microfossils. After the identification of microfossils, the bio zonation of the sediments of Ruteh Formation was started based on the biozone of Leven and Okay (1996) belonging to the West of Paleo-tethys.

In the biostratigraphic studies of the Ruteh formation in the stratigraphic section of South Amol, whereas 31 genera and 57 species of benthic foraminifera were identified, three local biozones were identified and introduced as described below. Small Foraminifera and Fusulinids together with green and red algae constitute the most important microfossils of Ruteh formation. Neoendothyra bronnimanni-Neoendothyra reicheli assemblage zone This biozone, which is an accumulation type, is defined based on the existing symbiotic community in it, and it occupies 72 meters from the base of the Ruteh formation after the erosion boundary between the Dorud and Ruteh formations. The lower boundary of this biozone is determined by the beginning of the symbiotic complex below and its upper boundary by the appearance of the indicator foraminifera of Midian stage, such as Chusenella sinensis and Codonofusiella erki. The age of this biozone based on its identified species such as various speciesof Neoendothyra and the comparison of this cumulative biozone with Khachik Julfa layers and other areas of the Tethys region such as Afghanistan indicates the late Murgabian age for this zone. Chusenella sinensis - Codonofusiella erki Pachyphloia assemblage zone This cumulative biozone includes 282 meters of the thickness of Ruteh formation after biozone 1. Its lower boundary is determined based on the emergence of the midian Chusenella sinensis, and the upper boundary of this biozone is determined by the emergence of the foraminifera such as Dagmarita chanakchiensis and Paraglobivalvulina mira. The age of this formation is based on the presence of foraminifera such as Chusenella sinensis and the comparison of this accumulation biozone with midian foraminifera of Khuf Formation Paraglobivalvulina mira - Dagmarita chanakchiensis assemblage zone This biozone, which is defined based on the following symbiotic complex, inhabits 288 meters from the top of the Ruteh Formation in the studied section. Based on the symbiont complex, the age of this biozone has been identified as the early Julfian. However, an indicator species such as Pachyphloia iranica is accepted by most of the experts as the indicator species of Julfian stage (Bozorgnia, 1973).

The following results have been obtained in the Lithostratigraphic and biostratigraphic studies of the Ruteh formation in the section of the mansion:The thickness of the sediments of Ruteh formation in this section is 642 meters and in terms of lithology, it mainly consists of thick to medium and thin dark limestone along with dolomitic limestone, limy dolomite, sandy limestone, cherty limestone, shale and sandstone.The lower boundary of Ruteh Formation with Dorud Formation is disconformity, so that the limestones at the base of Ruteh Formation are placed on the eroded surface of the sandstones at the top of Dorud Formation. Its upper border is also with the Nesen Formation, which is discontinuous and eroded along with a laterite horizon. Based on the above biological zones and the set of fossils identified in them, the age of the Late Murgabin-Early Julfian Ruteh formation was determined.