Microfacies, sedimentary environment and sequence stratigraphy of the Tarbur Formation in Semirom area (southwest of Isfahan)

IntroductionThe study area is located in Semirom sub zone (between Zagros and Sanandaj-Sirjan zones). Considering the main Zagros zonation, Navai and Mehdi Zadeh Tehrani (1986) used satellite data and tectonic studies, other than the two main structural zone of Zagros and Sanandaj-Sirjan, and introduced another subzone for Semirom. This sub zone, based on stratigraphy and folding, is different than Zagros zone. Therefore, different facies can be formed based on different movements of the independent blocks in the basement. These blocks with differential vertical movements have changed physical and chemical conditions of the sedimentary basins. In the current paper, a detailed microfacies, paleoenvironmental analyses, depositional models and sequence stratigraphic approaches are used to understand the relationship between paleoenvironmental parameters and change of sedimentary facies.
Material and methodsThe studied area, with geographical coordinate of 31° 22' 48" N and 51° 32' 01" E, is located about 5 km southwest of semirom. The thickness of the Tarbur Formation with Maaestrichtian age in the study area is about 462 meters. This formation consists mainly of carbonate and terrigenous rocks and has a gradual and conformable contact with underlying red shale unit (S2) while its upper contact with Kashkan Formation is marked by an erosional surface. The microscopic studies were performed on carbonate samples and shale samples were studied. Quantative and qualitative microscopic studies were conducted by polarizan microscope. A textural classification was obtained according to Dunham (1962) and Embery and Klovan (1975). Lateral and vertical changes of microfacies were studied and were compared with modern sedimentary environments. Finally, based on lithostratigraphy, sequence stratigraphy and vertical variations of microfaceis, environmental model are presented.
DiscussionAccording to the field observation, three litholigical units are recognized in the Tarbur Formation at the study area. The study of depositional textures, petrography and fossil content of 110 thin section led to recognition of six carbonate microfacies. The depositional setting includes orbitoidae bioclastic wackestone-packstone and rudist packstone (rudstone) of open marine, rudist boundestone of bar, miliolids loftusia orbitoidae rudist packstone and dicyclina/ dasycladacea/ loftusia miliolids bioclastic wackestone-pacstone of lagoon enviroments. Other than the above carbonate facies, two terrigenous facies (shallow (Lsh) and deep marine (Osh) are distinguished in the study area. The Tarbur Formation in the study area is deposited on a low-angle open shelf environment. Based on the sequence stratigraphy data, two sedimentary cycles with 3rd order sequences are recognized. These sequences are comparable to the global sequences.