University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 39 2 2023 06 22 Palaeoesedimentary environment, sequence stratigraphy and geochemistry of the Taleh Zang Formation in Ritt Anticline, southeastern Lorestan Basin Palaeoesedimentary environment, sequence stratigraphy and geochemistry of the Taleh Zang Formation in Ritt Anticline, southeastern Lorestan Basin 81 98 28004 10.22108/jssr.2023.138826.1267 FA Iraj Maghfouri Moghaddam Associate Professor, Department of Geology, Faculty of Sciences, Lorestan University, Lorestan, Iran 0000-0002-0116-1768 Iraj Sedaghatnia PhD student, Bu Ali Sina University, Hamedan, Iran; Central Laboratory ,Lorestan University, Khorram Abad, Iran Journal Article 2023 08 29 The carbonate deposits of the Taleh Zang Formation are exposed extensively in the southeastern to northern Lorestan zone, SW Iran. The thickness of the shallow water carbonate Taleh Zang Formation in Ritt Anticline is 84.5 meters. It overlies on top of the turbidity Amiran Formation and it is overlain by the clastic-dominated Kashkan Formation. Petrographic examinations revealed seven facies deposited in tidal flat, lagoon, shoal and open marine. Detailed analysis of sedimentary facies indicates that in the Late Paleocene interval, the depositional system of the Taleh Zangi Formation was a ramp carbonate platform. Changes in the depositional facies and cycle stacking patterns indicate one transgressive-regressive sea-level cycle from the bottom to the top of the section, which is equivalent to the last eustatic sea level rise in the Late Paleocene (Thanetian). This depositional sequence is separated by type 2 and 1 sequence boundaries at its lower and upper boundaries, respectively. Elemental geochemical evidence indicates a closed to weakly open digenetic system, with low water-rock interaction for carbonates of the Taleh Zang Formation. The relatively open digenesis system in this formation could be due to the influence of meteoric fluids in the erosional discontinuity at the border between the two formations, the Taleh Zang and Kashkan formations. Keywords: Taleh Zang Formation, Geochemistry, Sedimentary environment, Sedimentary sequence, Lorestan zone Introduction The use of Paleogene biozones (Shallow benthic zone = SBZ) of the Mediterranean basin (introduced by Serra Kilel et al. 1998) in the biostratigraphic studies of the Taleh Zang Formation started several years ago (Bagherpour and Vaziri 2012). Using these biozones, the deposits of Taleh Zang Formation were separated into stages and even substages, which made it possible to compare their palaeoecological conditions and palaeoenvironment with global events (e.g. in South Lorestan by Zohdi and Adabi, 2008; in the south of Kermanshah by Shalavand et al. 2020). Recent studies show that the facies changes of the Taleh Zang Formation are affected by two important factors, one is global changes especially the evolution of the Tethys carbonate platforms (Scheibner and Speizer 2008) and the other is the location of the section of the Tele Zang Formation in the Lorestan Zone. In a way, the age of this formation becomes younger from the north-east to the south-west of the Lorestan region (Jafarizadeh et al. 2023), which is the result of drastic changes in the biological contents and palaeoecological conditions of this formation in different regions. Therefore, it seems that the study of the Taleh Zang Formation in different anticlines of the Lorestan zone plays a significant role in clarifying the evolution of this zone. In the current research, the sedimentary environment and sequence stratigraphy of this formation have been investigated from the perspective of sedimentology and elemental geochemistry of the Taleh Zang formation in Ritt Anticline. The study of the biostratigraphy of the Tele Zang Formation in this anticline has been done by (Zakerzadeh et al. 2023), whose results are the basis for determining the age of the Taleh Zang Formation in the present study. Material & Methods A total of 60 samples were collected. Thin sections were prepared from the collected samples and subsequently analyzed using an optical microscope. Facies description was based on field observation and microfacies characteristics including skeletal and non-skeletal components, depositional texture, and grain size. The microfacies classification follows (Flügel 1010) and (Wilson 1975). The study area is located in Kerki Village about 63 southeast of Khorram Abad city. The section was measured in detail at 33°04′1″ N and 48°14′23″E. Twenty samples of 50 grams were selected and the powdered. The samples were subjected to elemental analysis in the Central Laboratory of Lorestan University. In the present study, elemental geochemical methods (use of major and minor elements present in carbonate samples) and petrographic studies were used to determine the sedimentary environment and the depositional sequence of the Taleh Zang Formation. Discussion of Results & Conclusions Based on biogenic composition, textural and lithological characteristics, seven facies were identified in the carbonate succession of the Taleh Zang Formation. These facies are related to four sub-environments including tidal flat, lagoon, shoal and open marine, of the carbonate ramp. The facies of the tidal zone (MF1) include dolomicrites (microcrystalline dolomites). The absence of skeletal particles and a small amount of detrital particles are the characteristics of this sub-environment. The microfacies of the lagoon environment (MF2-5) include sandy wackestone to packstone; bioclastic benthic foraminifera pelloid wackestone; bioclastic benthic foraminifera–green algae wackestone to packstone; bioclastic pelloid packstone. The facies of the shoal consists of Coral boundstone (MF6). The open marine facies represented by bioclastic coral-corallinacean floatstone (MF7). According to the field and laboratory evidence, the Taleh Zang Formation in this section consists of a third-order depositional sequence with a thickness of 84.5 meters. This depositional sequence is mostly carbonate and consists of limestones and to a lesser content dolomitic limestone. The lower boundary of this depositional sequence with the Amiran Formation is sequence boundary (SB) type 2. The upper boundary of this sequence was defined by the clastic of the Kashkan Formation that showed disconformity (SB1). Considering the discontinuity between the Taleh Zang and the Keshkan formations and the closed to slightly open digenesis system of the studied deposits, the strata of the high strand system tracts (HST) facies are slightly affected by meteoric digenesis fluids, and their Sr content is slightly higher than that of the transgressive system tracts (TST) facies. It may be due to the partial dissolution of aragonite shells in the environment, while the Mn values of these sediments are higher than those of the TST facies. The change process of Sr element to Na and Mn versus Sr indicates their primary aragonite mineralogy. Comparison of Sr/Ca values versus Mn and Mg elements indicates a closed to slightly open digenesis system with low water-rock interaction for the carbonates of Taleh Zang Formation. The relatively open digenesis system in this formation can be due to the influence of meteoric fluids, which is affected by the erosional discontinuity at the boundary of the Taleh Zang and Kashkan formations The carbonate deposits of the Taleh Zang Formation are exposed extensively in the southeastern to northern Lorestan zone, SW Iran. The thickness of the shallow water carbonate Taleh Zang Formation in Ritt Anticline is 84.5 meters. It overlies on top of the turbidity Amiran Formation and it is overlain by the clastic-dominated Kashkan Formation. Petrographic examinations revealed seven facies deposited in tidal flat, lagoon, shoal and open marine. Detailed analysis of sedimentary facies indicates that in the Late Paleocene interval, the depositional system of the Taleh Zangi Formation was a ramp carbonate platform. Changes in the depositional facies and cycle stacking patterns indicate one transgressive-regressive sea-level cycle from the bottom to the top of the section, which is equivalent to the last eustatic sea level rise in the Late Paleocene (Thanetian). This depositional sequence is separated by type 2 and 1 sequence boundaries at its lower and upper boundaries, respectively. Elemental geochemical evidence indicates a closed to weakly open digenetic system, with low water-rock interaction for carbonates of the Taleh Zang Formation. The relatively open digenesis system in this formation could be due to the influence of meteoric fluids in the erosional discontinuity at the border between the two formations, the Taleh Zang and Kashkan formations. Keywords: Taleh Zang Formation, Geochemistry, Sedimentary environment, Sedimentary sequence, Lorestan zone Introduction The use of Paleogene biozones (Shallow benthic zone = SBZ) of the Mediterranean basin (introduced by Serra Kilel et al. 1998) in the biostratigraphic studies of the Taleh Zang Formation started several years ago (Bagherpour and Vaziri 2012). Using these biozones, the deposits of Taleh Zang Formation were separated into stages and even substages, which made it possible to compare their palaeoecological conditions and palaeoenvironment with global events (e.g. in South Lorestan by Zohdi and Adabi, 2008; in the south of Kermanshah by Shalavand et al. 2020). Recent studies show that the facies changes of the Taleh Zang Formation are affected by two important factors, one is global changes especially the evolution of the Tethys carbonate platforms (Scheibner and Speizer 2008) and the other is the location of the section of the Tele Zang Formation in the Lorestan Zone. In a way, the age of this formation becomes younger from the north-east to the south-west of the Lorestan region (Jafarizadeh et al. 2023), which is the result of drastic changes in the biological contents and palaeoecological conditions of this formation in different regions. Therefore, it seems that the study of the Taleh Zang Formation in different anticlines of the Lorestan zone plays a significant role in clarifying the evolution of this zone. In the current research, the sedimentary environment and sequence stratigraphy of this formation have been investigated from the perspective of sedimentology and elemental geochemistry of the Taleh Zang formation in Ritt Anticline. The study of the biostratigraphy of the Tele Zang Formation in this anticline has been done by (Zakerzadeh et al. 2023), whose results are the basis for determining the age of the Taleh Zang Formation in the present study. Material & Methods A total of 60 samples were collected. Thin sections were prepared from the collected samples and subsequently analyzed using an optical microscope. Facies description was based on field observation and microfacies characteristics including skeletal and non-skeletal components, depositional texture, and grain size. The microfacies classification follows (Flügel 1010) and (Wilson 1975). The study area is located in Kerki Village about 63 southeast of Khorram Abad city. The section was measured in detail at 33°04′1″ N and 48°14′23″E. Twenty samples of 50 grams were selected and the powdered. The samples were subjected to elemental analysis in the Central Laboratory of Lorestan University. In the present study, elemental geochemical methods (use of major and minor elements present in carbonate samples) and petrographic studies were used to determine the sedimentary environment and the depositional sequence of the Taleh Zang Formation. Discussion of Results & Conclusions Based on biogenic composition, textural and lithological characteristics, seven facies were identified in the carbonate succession of the Taleh Zang Formation. These facies are related to four sub-environments including tidal flat, lagoon, shoal and open marine, of the carbonate ramp. The facies of the tidal zone (MF1) include dolomicrites (microcrystalline dolomites). The absence of skeletal particles and a small amount of detrital particles are the characteristics of this sub-environment. The microfacies of the lagoon environment (MF2-5) include sandy wackestone to packstone; bioclastic benthic foraminifera pelloid wackestone; bioclastic benthic foraminifera–green algae wackestone to packstone; bioclastic pelloid packstone. The facies of the shoal consists of Coral boundstone (MF6). The open marine facies represented by bioclastic coral-corallinacean floatstone (MF7). According to the field and laboratory evidence, the Taleh Zang Formation in this section consists of a third-order depositional sequence with a thickness of 84.5 meters. This depositional sequence is mostly carbonate and consists of limestones and to a lesser content dolomitic limestone. The lower boundary of this depositional sequence with the Amiran Formation is sequence boundary (SB) type 2. The upper boundary of this sequence was defined by the clastic of the Kashkan Formation that showed disconformity (SB1). Considering the discontinuity between the Taleh Zang and the Keshkan formations and the closed to slightly open digenesis system of the studied deposits, the strata of the high strand system tracts (HST) facies are slightly affected by meteoric digenesis fluids, and their Sr content is slightly higher than that of the transgressive system tracts (TST) facies. It may be due to the partial dissolution of aragonite shells in the environment, while the Mn values of these sediments are higher than those of the TST facies. The change process of Sr element to Na and Mn versus Sr indicates their primary aragonite mineralogy. Comparison of Sr/Ca values versus Mn and Mg elements indicates a closed to slightly open digenesis system with low water-rock interaction for the carbonates of Taleh Zang Formation. The relatively open digenesis system in this formation can be due to the influence of meteoric fluids, which is affected by the erosional discontinuity at the boundary of the Taleh Zang and Kashkan formations

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