Paleogeography and regional interpretation of facies in Tirgan Formation, west of Kopet-Dagh sedimentary basin, northeast of Iran

In order to study the lithostratigraphy, biostratigraphy, microfacies, depositional setting and also to reconstruct the paleo-environmental conditions of the Tirgan Formation, three outcrop sections were selected in the western part of the Kopet-Daghh sedimentary basin. These sections locate at the Estarkhi village (30 km East of Shirvan city), the Jozak (40 km West of Bojnourd city) and the Zaw Mountain (45 km far from the Kalaleh city). Geographical coordinates of these outcrop sections are: 57o, 51’, 31’’ longitude and 37o, 11’, 29’’ latitude in Estrakhi section, 56o, 42’, 25’’ longitude and 37o, 24’, 55.8’’ latitude in Jozak section, and finally, 55o, 45’, 10.6’’ longitude and 37o, 31’, 20’’ latitude in Zaw section. The Tirgan Formation has a thickness of 129 m in Estarkhi, 362 m in Jozak and 639 m in Zaw section, in which measured and samples to be used in this study. A total number of 62, 102 and 226 rock samples was taken respectively from the Estarkhi, Jozak and Zaw sections, and used for the microscopic studies. Therefore, a total number of 700 thin-sections were prepared from the rock samples and studied under the microscope. In Estarkhi section, the Shurijeh Formation is overlain conformably by the Tirgan Formation. Moreover, the upper contact of the Tirgan Formation with the upper rock unit, the Sarcheshmeh Formation, is continues and conformable. In this outcrop section, the Tirgan Formation begins with siliciclastic deposits with intercalations of marls, representing a gradational passage from the continental Shurijeh Formation to the marine deposits of the Tirgan Formation. The Jozak section represents a conformable and continues contact between the Tirgan Formation with the underlying Shurijeh Formation, but, its upper contact with the overlying Abderaz Formation is cut by a fault. In Zaw section, the lower contact of the Tirgan Formation with the Zard Formation is continues and conformable, but, its upper contact with the younger deposits is covered and unclear. Based on the distribution of facies belts, three paleogeographical maps were prepared for the study area. They are: Hauterivian?-lower Barremian, lower Barremian-upper Barremian and upper Barremian-lower Aptian. Analysis of these maps shows that during the Hauterivian?-lower Barremian, in the far eastern part of the study are (e.g. Estarkhi section), the Tirgan Formation includes in siliciclastic deposits with some marine marls, in that the detrital sediments was carried to the foreland basin as a result of erosion of high structures. In the same time, the western parts of the study area was covered by deposits belonging to a tidal-flat depositional setting, representing a deepening trend from the East towards the West of the area. During the Barremian (lower Barremian-upper Barremian), a major deepening trend has occurred for the both vertical and horizontal extend regionally in the study area. Therefore, the Estarkhi area was covered by tidal-flat deposits, and in the same time, the deepening trend was continued towards the west, led to the deposition of shoal and open marine facies in the Jozak and Zaw area. Finally, in the upper Barremian-lower Aptian, a stable condition was govern for the entire area, led to the deposition of orbitolina bearing limestones totally in the study area. Vertical distribution of facies belts shows that the eastern part of the study area (e.g. Estarkhi section), represents a major deepening upward trend during the upper Barremian-lower Aptian, which is comparable with general and global transgressive trends for the Barremian-Aptian interval. But, in the western part (e.g. Jozak and Zaw areas), the bathymetry was often stable, and somehow, shows a shallowing trend, that probably belongs to the role of basement faults and local tectonic activities, creating various accommodation space on the shallow platform setting in this part of the Kopet-Daghh Basin.