Sedimentary environment, sequence stratigraphy, diagenesis and geochemistry of the carbonate Ruteh Formation in north of Mahabad section

The Ruteh Formation is the second sedimentary cycle in Alborz basin. The petrography and laboratory studies of these deposits led to the identification of 15 microfacies which were deposited in seven sub-environments including supratidal, intertidal, lagoon, shoal, open marine, middle ramp, and outer ramp. Since there were no turbidity deposits, the lack of marginal reef belt and the gradual changes of facies, it turned out that these sediments were deposited in a homoclinal carbonate ramp. The study of relative sea level changes was done according to vertical distribution of facies and their stacking patterns. These studies lead to identification of four depositional sequences. The petrography data and geochemical analyzes were used in order to identify the diagenetic processes and geochemical changes. The recognized diagenesis processes include micritization, cementation, compaction, stylolitization, silicification, ferrugenation and neomorphysm. These diagenetic processes took place in three diagenetic environments: marine-phreatic, meteoric-phreatic and burial. Geochemical analyzes confirms that the carbonate sediments of Ruteh Formation were originally composed of aragonite, which was eventually recrystallized to calcite during diagenesis. Plotting Sr/Ca versus Mn diagram approved that sediments was subjected to a meteoric diagenesis in a semi-closed to open diagenetic system. The results of this research can be used to provide a sedimentary model for these deposits which completes the palaeogeographical data for the Tethys Ocean.
 

The geological observations indicating a coherent Iranian-Gondwanaland continental landmass during the Late Precambrian to Permian are consistent with palaeomagnetic data (Berberian and King 1980). Palaeomagnetic evidences from the Upper Precambrian rocks and iron ores of Bafq area in central Iran (Becker et al. 1973), the Lower Paleozoic rocks of kuh-e-Gahkom and Surmeh of the Zagros (Burek and Furst 1975), the Cambrian purple sandstone of the salt range of Pakistan (McElhiny 1970), the Upper Devonian–Lower Carboniferous of the Alborz Mountain in north Iran (Geiroud Formation; Wensink et al. 1978) and from the Upper Precambrian, Ordovician and Permian rocks of central Iran (Soffel et al. 1975; Soffel and Forster 1977) show similar geomagnetic poles with those of Afro-Arabia. These data and the widespread similarity of Paleozoic sedimentary succession indicate that during the Late Precambrian and Paleozoic, Central Iran, the Alborz in northern Iran and the Zagros in south Iran were parts of Gondwana (Berberian and King 1981). In the Late Carboniferous–Early Permian, continental rifting separated these lands from Gondwana due to the expansion of Neo-Tethys Ocean. Due to the glaciers growth and Hercynian orogeny movements in that time (Berberian and King 1981; Lasemi 2000), the Early Permian sedimentary facies in Iran are mainly siliciclastic (Dorud Formation in Alborz–Azarbaijan zone). In the Late Permian, after deposition of the Droud Formation, glaciers regression, tectonic tension activities and increasing volume of mid ocean ridges led to the sea progress and carbonate platforms restoring in the southern margin of Paleo-Tethys (Lasemi 2000). The carbonate rocks of the Upper Permian Ruteh (Asserto 1963) and Nesen (Glaus 1964) formationsin the Alborz–Azarbaijan zone and the Jamal Formation in central Iran, record the development of these carbonate platforms. The Ruteh Formation studied here is exposed about 15 km north of Mahabad city. Since no studies have been carried out on the Ruteh Formation in this area so far, this research aims to identify the sedimentary environment, sequence stratigraphy, diagenesis and geochemistry of this formation.

After literature review and preliminary visits of the study area, for the most complete and appropriate succession in the field was selected. Then, its rock units were delineated by means of horizontal and vertical scrolling. Based on the geometric features of sedimentary units, stratigraphic planes, sedimentary structures and biotic and abiotic components, the existing sedimentary units were divided and separated into diverse facies. In order to determine the microfacies and sedimentary environment of the Ruteh Formation 101 rock samples were collected systematically, and by taking into account the changes in the facies. To analyze the microfacies, textures, depositional environments, the facies components, the ratio between its components and various diagenesis processes, the samples were sent to the laboratories, and a thin microscopic section was made from the collected rock samples. Thin sections were studied under polarizing microscopes. In order to determine the frequency of allochems, the comparison charts (Bacelle and Bosellini 1965) were used. The microfacies naming was done by means of Folk’s (1962) and Danham’s (1962) methods, environmental energy detection based on Flügel's method (2010), texture studies based on Tucker's (2002) method, diagenesis evidences, energy detection and sedimentary environments were done by means of Wilson’s (1975) and Flügel’s (2010) methods. To detect depositinal sequences, the models presented by Hunt and Tucker (1992) were also used. To study the early mineralogy of the Permian carbonate rocks in northern region of Mahabad, after thoroughly studying thin sections petrography, eight limestones samples with the highest micritic matrix and least alternation and lowest fossil contents were selected. Then they were subjected to elemental analysis using ICP-MS (Inductively Coupled Plasma Mass Spectrometry) and alkali melting methods in laboratory of Zarazma Company (Tehran), then geochemical values plotted in geochemical diagrams for early mineralogy determination.

With regard to the identified microfacies, the process of the basin deepening from the carbonate platform towards the deep basin, the absence of oncoids, pisoid and aggregate grains, which are specific to carbonate shelf and rarely found in carbonate ramps (Flügel 2010), also, lack of re-deposited carbonate facies, sediments related to gravity flow processes in sediments of the Ruteh Formation indicates sedimentation of these sediments in a low-slope and approximately uniform gradient in the basin floor slope a homoclinal ramp profile can be predicted for the deposits of the Ruteh Formation in this study area (Wilson 1975; Read 1985; Lee et al. 2001; Flugel 2010). Due to the low expansion of the reefs in the carbonate ramp compared to the rimmed carbonate platforms, the lack of widespread barrier reef and the presence of patch reef in the Ruteh Formation is another reason for this fact that the sedimentary environment of this formation is a homoclinal carbonate ramp (Bastami 2016). On the other hand, by reviewing of distributed facies in sub-environments and their constituents it can be concluded that Ruteh Formation depositional environment is a bioclastic carbonate ramp similar to that introduced by Kolodka (2012) for Dalan Formation in Fars province. Based on the vertical distribution of facies, relative sea level changes these studies lead to identification of four depositional sequences. Early marine diagenetic processes like micritization and marginal micrite cements well represente transgression of sea level. During the sea level regression (sequence boundaries), sediments have been affected by meteoric diagenetic processes such as granular calcite cement, blocky calcite cement, syntaxial cement, ferrugination and neomorphism. Also the evidences of deep burial diagenetic process observed by physical compaction, druzy calcite cement, stylolization, poikilotopic calcite cement and silisification processes. Geochemical analyzes confirms that the carbonate sediments of Ruteh Formation were originally composed of aragonite, which was eventually recrystallized to calcite during diagenesis. Plotting Sr/Ca vs. Mn diagram approved that sediments was subjected to a meteoric diagenesis in a semi-closed to open diagenetic system.