Microfacies, depositional environment and carbonate association of the Asmari Formation in Tang-e-Gojestan
AbstractThe Asmari Formation was logged and sampled in the Tang-e-Gojestan stratigraphic section (Eshgar Anticline, Izeh Zone) to study microfacies, depositional environment and carbonate associations. The Asmari Formation with 298 m-thickness mainly consists of thick-bedded/massive cliff-forming li...
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University of Isfahan
2025-03-01
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| author | Yadolah Ezampanah Mohammad Goodarzi |
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| description | AbstractThe Asmari Formation was logged and sampled in the Tang-e-Gojestan stratigraphic section (Eshgar Anticline, Izeh Zone) to study microfacies, depositional environment and carbonate associations. The Asmari Formation with 298 m-thickness mainly consists of thick-bedded/massive cliff-forming limestones (the lower to middle parts), passes upward into thin to medium-bedded limestones with shale intercalations. The Asmari Formation rests conformably on the Pabdeh Formation, whereas the upper contact is covered. Petrographic studies of 239 thin sections enabled the recognition of nine microfacies belongingto the outer, middle (distal and proximal parts) and inner ramp (semi-restricted and restricted lagoons and tidal flat). The absence of syndepositional deformation and slumped structures, gradual changes in microfacies, and the absence of a distinct bar and/or buildup, all suggest a homoclinal carbonate ramp platform for the Asmari Formation in the section studied. Based on identified allochems, the carbonate associations are represented by heterozoan types, including nannofor and forealgal which are characteristic of Cenozoic ramps.Keywords: Izeh Zone, Benthic foraminifera, Asmari Formation, Early Miocene, Zagros Basin. IntroductionCenozoic carbonate platform evolution resulted from the interplay of several factors, including local and global climate change, tectonic activity, eustatic sea-level fluctuations, and changes in dominant carbonate-producing organisms (Höntzsch et al. 2013). Following the extinction of rudist reef-building at the end of the Cretaceous, large benthic foraminifera (Nummulites, Assilina and Operculina, Discocyclina and Alveolinids) became dominant carbonate producers on the shallow carbonate platforms of the Tethys–Arabian belt, particularly in the inner to middle platform areas (Beavington-Penney and Racey 2004). The Oligo–Miocene carbonate platform deposits are globally widespread (Pomar et al. 2014). In southwestern Iran's Zagros foreland basin, an Oligo–Miocene intra-shelf basin developed, characterized by clastic sediments of the Eocene–early Miocene Razak Formation, deep-water shales and marls of the Paleocene through Oligocene Pabdeh Formation, and shallow-water platform carbonates of the Oligocene–early Miocene Asmari Formation (van Buchem et al. 2010; Allahkarampour Dill et al. 2018 and references therein). The Asmari Formation, a major oil reservoir in the Zagros Basin and the Middle East is notable for its extensive and accessible stratigraphic outcrops and rich fossil content (including benthic foraminifera), making it a focus of significant geological interest (Adams and Bourgeois 1967; Vaziri-Moghaddam et al. 2006; Ehrenberg et al. 2007; Laursen et al. 2009; van Buchem et al. 2010; Sadeghi et al. 2011; Shabafroz et al. 2015, 2020; Noorian et al. 2022). In this research, the Tang-e-Gojestan section was studied to investigate the microfacies and determine the sedimentary environments of the Asmari Formation. Material & MethodsThe studied section was selected in the easternmost part of the Izeh Zone (Fars subzone), 50 km NW of Nurabad Mamasani city (coordinates: 30°29′37.95′′ N, 51°16′34.37′′ E). The section was systematically sampled every 1 to 1.5 m. For the purpose of this study, we investigated 239 thin sections through the entire Asmari Formation and also the topmost horizons of the Pabdeh Formation. The petrographic characterization of the carbonate facies and depositional environments is based on Dunham (1962), Embry and Klovan (1975) and Flügel (2010). Discussion of Results & ConclusionsThe Asmari Formation (298 m-thick) in the studied section could be lithostratigraphically divided into the following three units: the first unit consists of medium- to thick-bedded limestones interbedded with shale beds, the second unit is predominantly represented by massive and cliff-forming limestones, and the third unit consists of thin to medium-bedded limestones interbedded with shale beds. Field observations and laboratory-obtained data indicate that the lower boundary of the Asmari Formation with the Pabdeh Formation is conformable. The upper contact of the formation with the Gachsaran is covered and could not be detected. Whereas in the adjacent outcrops in the Eshgar Anticline, the Gachsaran Formation overlies the Asmari Formation (Zare et al. 2019). The skeletal grains mostly comprise larger benthic foraminifera (specially Nummulitidae and Lepidocyclinidae), echinoids, gastropods, bivalve, planktonic foraminifera, bryozoa, red algae, Ditrupa and rare coral fragments. Petrographic studies of the studied interval led to the recognition of nine microfacies including MF1: planktonic foraminifera wackestone, MF2: planktonic and larger benthic foraminifera bioclast wackestone, MF3: larger hyaline benthic foraminifera wackestone/floatstone, MF4: lenticular hyaline foraminifera bioclast wackestone/packstone, MF5: porcelaneous and hyaline foraminifera wackestone, MF6: Porcelaneous foraminifera wackestone, MF7: bioclast wackestone/mudstone, MF8: dolomudstone/dolostone, MF9: stromatolite bindstone. According to introduced microfacies and identified biota the Asmari Formation was deposited on a ramp carbonate platform (the inner, middle and outer ramp). The absence of slumped structures, gradual microfacies changes, and the lack of a distinct bar and/or buildups, all suggest a homoclinal carbonate ramp. The Paleocene–Eocene Thermal Maximum (PETM), which occurred 56 million years ago, had profound effects on biota (Röhl et al. 2007). This event was associated with rising sea-level, changes in ocean chemistry, increased atmospheric CO2, the highest temperatures recorded in the last 100 million years, and the prevalence of carbonate platform deposits rich in large benthic foraminifera (Zachos et al. 2001, 2003, 2008; Scheibner and Speijer 2008; Tosquella et al. 2022). Increased atmospheric CO2, in turn, led to ocean acidification, and rising sea-surface temperatures caused coral bleaching (the expulsion of zooxanthellae algae), resulting in reduced coral reef growth or even complete absence of these reef types (Zachos et al. 2005, White and Schiebout 2008; Scheibner and Speijer 2008; Payros et al. 2010). Consequently, most neritic carbonate platforms in the early Cenozoic developed as ramps due to the absence of reef-building organisms (Beavington-Penney et al. 2005; Tosquella et al. 2022). Based on the identified allochems, the carbonate associations in the studied section are limited to heterozoan types (nannofor and forealgal associations) that are widespread throughout the Cenozoic ramps. |
| format | Article |
| id | doaj-art-52932a513b0e46139e8367e6f249bbb3 |
| institution | DOAJ |
| issn | 2008-7888 2423-8007 |
| language | English |
| publishDate | 2025-03-01 |
| publisher | University of Isfahan |
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| series | Journal of Stratigraphy and Sedimentology Researches |
| spelling | doaj-art-52932a513b0e46139e8367e6f249bbb32025-08-20T02:56:54ZengUniversity of IsfahanJournal of Stratigraphy and Sedimentology Researches2008-78882423-80072025-03-01411597810.22108/jssr.2024.142242.129229055Microfacies, depositional environment and carbonate association of the Asmari Formation in Tang-e-GojestanYadolah Ezampanah0Mohammad Goodarzi1Assistant Professor, Department of Geology Faculty of Science, Bu-Ali Sina University, Hamadan, IranPhD student, Department of Geology Faculty of Science, Lorestan University, Khoram Abad, IranAbstractThe Asmari Formation was logged and sampled in the Tang-e-Gojestan stratigraphic section (Eshgar Anticline, Izeh Zone) to study microfacies, depositional environment and carbonate associations. The Asmari Formation with 298 m-thickness mainly consists of thick-bedded/massive cliff-forming limestones (the lower to middle parts), passes upward into thin to medium-bedded limestones with shale intercalations. The Asmari Formation rests conformably on the Pabdeh Formation, whereas the upper contact is covered. Petrographic studies of 239 thin sections enabled the recognition of nine microfacies belongingto the outer, middle (distal and proximal parts) and inner ramp (semi-restricted and restricted lagoons and tidal flat). The absence of syndepositional deformation and slumped structures, gradual changes in microfacies, and the absence of a distinct bar and/or buildup, all suggest a homoclinal carbonate ramp platform for the Asmari Formation in the section studied. Based on identified allochems, the carbonate associations are represented by heterozoan types, including nannofor and forealgal which are characteristic of Cenozoic ramps.Keywords: Izeh Zone, Benthic foraminifera, Asmari Formation, Early Miocene, Zagros Basin. IntroductionCenozoic carbonate platform evolution resulted from the interplay of several factors, including local and global climate change, tectonic activity, eustatic sea-level fluctuations, and changes in dominant carbonate-producing organisms (Höntzsch et al. 2013). Following the extinction of rudist reef-building at the end of the Cretaceous, large benthic foraminifera (Nummulites, Assilina and Operculina, Discocyclina and Alveolinids) became dominant carbonate producers on the shallow carbonate platforms of the Tethys–Arabian belt, particularly in the inner to middle platform areas (Beavington-Penney and Racey 2004). The Oligo–Miocene carbonate platform deposits are globally widespread (Pomar et al. 2014). In southwestern Iran's Zagros foreland basin, an Oligo–Miocene intra-shelf basin developed, characterized by clastic sediments of the Eocene–early Miocene Razak Formation, deep-water shales and marls of the Paleocene through Oligocene Pabdeh Formation, and shallow-water platform carbonates of the Oligocene–early Miocene Asmari Formation (van Buchem et al. 2010; Allahkarampour Dill et al. 2018 and references therein). The Asmari Formation, a major oil reservoir in the Zagros Basin and the Middle East is notable for its extensive and accessible stratigraphic outcrops and rich fossil content (including benthic foraminifera), making it a focus of significant geological interest (Adams and Bourgeois 1967; Vaziri-Moghaddam et al. 2006; Ehrenberg et al. 2007; Laursen et al. 2009; van Buchem et al. 2010; Sadeghi et al. 2011; Shabafroz et al. 2015, 2020; Noorian et al. 2022). In this research, the Tang-e-Gojestan section was studied to investigate the microfacies and determine the sedimentary environments of the Asmari Formation. Material & MethodsThe studied section was selected in the easternmost part of the Izeh Zone (Fars subzone), 50 km NW of Nurabad Mamasani city (coordinates: 30°29′37.95′′ N, 51°16′34.37′′ E). The section was systematically sampled every 1 to 1.5 m. For the purpose of this study, we investigated 239 thin sections through the entire Asmari Formation and also the topmost horizons of the Pabdeh Formation. The petrographic characterization of the carbonate facies and depositional environments is based on Dunham (1962), Embry and Klovan (1975) and Flügel (2010). Discussion of Results & ConclusionsThe Asmari Formation (298 m-thick) in the studied section could be lithostratigraphically divided into the following three units: the first unit consists of medium- to thick-bedded limestones interbedded with shale beds, the second unit is predominantly represented by massive and cliff-forming limestones, and the third unit consists of thin to medium-bedded limestones interbedded with shale beds. Field observations and laboratory-obtained data indicate that the lower boundary of the Asmari Formation with the Pabdeh Formation is conformable. The upper contact of the formation with the Gachsaran is covered and could not be detected. Whereas in the adjacent outcrops in the Eshgar Anticline, the Gachsaran Formation overlies the Asmari Formation (Zare et al. 2019). The skeletal grains mostly comprise larger benthic foraminifera (specially Nummulitidae and Lepidocyclinidae), echinoids, gastropods, bivalve, planktonic foraminifera, bryozoa, red algae, Ditrupa and rare coral fragments. Petrographic studies of the studied interval led to the recognition of nine microfacies including MF1: planktonic foraminifera wackestone, MF2: planktonic and larger benthic foraminifera bioclast wackestone, MF3: larger hyaline benthic foraminifera wackestone/floatstone, MF4: lenticular hyaline foraminifera bioclast wackestone/packstone, MF5: porcelaneous and hyaline foraminifera wackestone, MF6: Porcelaneous foraminifera wackestone, MF7: bioclast wackestone/mudstone, MF8: dolomudstone/dolostone, MF9: stromatolite bindstone. According to introduced microfacies and identified biota the Asmari Formation was deposited on a ramp carbonate platform (the inner, middle and outer ramp). The absence of slumped structures, gradual microfacies changes, and the lack of a distinct bar and/or buildups, all suggest a homoclinal carbonate ramp. The Paleocene–Eocene Thermal Maximum (PETM), which occurred 56 million years ago, had profound effects on biota (Röhl et al. 2007). This event was associated with rising sea-level, changes in ocean chemistry, increased atmospheric CO2, the highest temperatures recorded in the last 100 million years, and the prevalence of carbonate platform deposits rich in large benthic foraminifera (Zachos et al. 2001, 2003, 2008; Scheibner and Speijer 2008; Tosquella et al. 2022). Increased atmospheric CO2, in turn, led to ocean acidification, and rising sea-surface temperatures caused coral bleaching (the expulsion of zooxanthellae algae), resulting in reduced coral reef growth or even complete absence of these reef types (Zachos et al. 2005, White and Schiebout 2008; Scheibner and Speijer 2008; Payros et al. 2010). Consequently, most neritic carbonate platforms in the early Cenozoic developed as ramps due to the absence of reef-building organisms (Beavington-Penney et al. 2005; Tosquella et al. 2022). Based on the identified allochems, the carbonate associations in the studied section are limited to heterozoan types (nannofor and forealgal associations) that are widespread throughout the Cenozoic ramps.https://jssr.ui.ac.ir/article_29055_826cd6a6eb363c6789baddda228eddc5.pdfizeh zonebenthic foraminiferaasmari formationearly miocenezagros basin |
| spellingShingle | Yadolah Ezampanah Mohammad Goodarzi Microfacies, depositional environment and carbonate association of the Asmari Formation in Tang-e-Gojestan Journal of Stratigraphy and Sedimentology Researches izeh zone benthic foraminifera asmari formation early miocene zagros basin |
| title | Microfacies, depositional environment and carbonate association of the Asmari Formation in Tang-e-Gojestan |
| title_full | Microfacies, depositional environment and carbonate association of the Asmari Formation in Tang-e-Gojestan |
| title_fullStr | Microfacies, depositional environment and carbonate association of the Asmari Formation in Tang-e-Gojestan |
| title_full_unstemmed | Microfacies, depositional environment and carbonate association of the Asmari Formation in Tang-e-Gojestan |
| title_short | Microfacies, depositional environment and carbonate association of the Asmari Formation in Tang-e-Gojestan |
| title_sort | microfacies depositional environment and carbonate association of the asmari formation in tang e gojestan |
| topic | izeh zone benthic foraminifera asmari formation early miocene zagros basin |
| url | https://jssr.ui.ac.ir/article_29055_826cd6a6eb363c6789baddda228eddc5.pdf |
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