Bio and sequence stratigraphy for Middle-Late Eocene sediments in Sanandaj-Sirjan zone with carbon and oxygen isotopes studies

Document Type : علمی -پژوهشی


Department of Geology, University of Erlangen, Nuremberg, Germany


IntroductionSanandaj-Sirjan zone is located in the southwestern margin of central Iran, which lies on the immediate northeast boundary of the main thrust of the Zagros Zone. Rock and structural features of Sanandaj - Sirjan zone represent a deep through or middle block rift in the Precambrian shield of Iran and Saudi Arabia.The northern boundary of this zone is separated by the Urmieh-Dokhtar Volcanic Belt and the Sirjan, Marvdasht, Gavkhuni, and Mighan Desert embayment and its southern boundary is the Zagros thrust Fault.The sedimentation regime and structure of the Sanandaj-Sirjan zone are similar to those of central Iran. In addition, all of the major Mesozoic and Tertiary discontinuities present in central and northern Iran are also known in this zone, which some of them are absent in Zagros.Material and methods   Eocene sedimentary deposits in two sections of Sanandaj-Sirjan zone in Soh area of Kashan with a total thickness of 171 m and in Zefreh area of Isfahan with a total thickness of 40 m were among the areas that were identified as suitable for this study.For paleontological, microfacies, and sequence stratigraphy studies, 42 samples from the Soh section and 58 samples from the Zefreh section were taken for thin section preparation. For the study of microphysics, Dunham's (1962), Embrio Clown's (1971), Wilson's (1975) and Flügel's (2004) methods were used.To select samples with low alteration, a number of micritic sections were selected for study by Cathode Luminescence (CL) method. In this study, it has been attempted to analyze stable carbon and oxygen isotopes in sediments and to find information on the paleontological temperature and diagenesis process in the area.The abundance, diversity and high degree of evolution of large foraminifera have made these foraminifers very useful tools in biostratigraphy to determine the age of sediments deposited in shallow marine platforms; therefore, for the time range from the early Paleocene to the Eocene-Oligocene boundary, 20 Shallow Benthic Zones have been identified with the SBZ abbreviation. Basically an SBZ region is characterized by stratigraphic distribution of several taxa belonging to alveolinids, nummulitids, and orthophragminids.Results and DiscussionThe Eocene sedimentary facies in the studied sections consists of 8 facies belts that are from sea to coast: Marly Maudstone facies (F1), Bioclastic (nummulitids-discocyclinide) wackestone-packstone facies (F2), Bioclastic (small rotaliids-miliolid) peloidal wackestone-packstone facies (F3), Alveolina-Coskinolina-Dictyoconus wackestone-packstone facies (F4), Conglomerate facies (F5), Sandstone facies (sandstone) (F6), Siltstone facies (F7) and Red Mudstone facies (F8). The sedimentary model is considered suitable for Eocene sediments in Soh, and Zefreh areas are open shelf zones. Based on the identified microfacies and the distribution of organisms, this open plateau is divided into three parts: outer, middle, and inner. The sedimentary model considered for the Jahrum sequence in the Zafreh and the Soh deep-to-shallow zones is the outer shelf environment, including the Marly Mudstone which belong to the very deep shelf area. ConclusionThe bioclastic (nummulitid-discocyclinid) wackestone-packstone facies also belong to the middle shelf area. The bioclastic (small rotaloiides - miliolids) peloidal wackestone-packstone facies and Alveolina-Caskinolina-Dictyoconus-wackestone-packstone facies belong to the inner shelf. Coastal facies also include conglomerate, sandstone, siltstone, and red mudstone.Finding suitable specimens in the Sanandaj-Sirjan zone was difficult because of the more severe diagenesis than the Zagros specimens.Since even these samples were altered by very low degrees of diagenesis, the resulting temperature could not reflect the paleo-temperature of the sedimentary environment, Therefore, to calculate the initial temperature of diagenesis, the lightest oxygen isotope (18O) in (PDB) -9.34‰ for the Soh cross section and (PDB) - 9.9‰, was used for calculating the initial diagenesis temperature for the Zefreh cross section. Accordingly, the temperature obtained for the Soh is 66 ° C and 64 ° C for the Zefreh. This high temperature in the Sanandaj-Sirjan zone not only aggravated the diagenesis process but also caused severe discoloration in many of the foraminifers' samples in this zone.Sequence stratigraphic studies resulted in three sequences: sedimentary sequence 1 (DS1), sedimentary sequence 2 (DS2) and sedimentary sequence 3 (DS3), respectively.