Calcareous nannofossils at the upper part of Gurpi Formation and the lower part of Pabdeh Formation at Arkavaz section, south east Ilam
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Research/Original Article (دارای رتبه معتبر)
Abstract:

In this study the Paleocene sediments have been investigated at the upper part of Gurpi Formation and the lower part of Pabdeh Formation at Arkavaz section in southeast of Ilam. The upper part of the Gurpi Formation is mainly consists of grey shale and the lower part of the Pabdeh Formation consists of purple shale. As a result of this study, 22 species belong to 17 genera of calcareous nannofossils were detected. Nine bioevents were recorded and based on these bioevents, the Ellipsolithus macellus Zone (NP4/CNP6–CNP7) and Fasciculithus tympaniformis Zone (NP5/CNP7–CNP8) are recognized at the uppermost part of the Gurpi Formation. Subsequently, Heliolithus kleinpellii Zone (NP6/CNP8) and Discoaster mohleri Zone (NP7) / Heliolithus riedelii Zone (NP8) (NP7/8 combined Zone–CNP9/10), are identified at the base of the Pabdeh Formation, respectively. As a result of this study and based on the identified calcareous nannofossil biozones, the age of the uppermost part of the Gurpi Formation is Late Danian–Selandian and the lower part of the Pabdeh Formation is Selandian–Thanetian, and based on CNP8 the boundary between these two formations is continuous. The identified biozones in this study were compared with other parts of the Zagros Basin.     

Introduction

One of the most extensive Cretaceous and Palaeogene deposits in Zagros is the marine sediments of the Gurpi and Pabdeh formations, which were first identified in the Zagros Basin on the basis of stratigraphy and paleontology. Type section of the Gurpi Formation is studied in Lali oilfield located in the NE of Masjed soleiman. Also, the term Pabdeh Formation has been introduced for the argillaceous limestone, marly limestone and shale succession exposed at the Tang-e Pabdeh, from the southeastern part of Pabdeh Mountain, located in north of the Lali oilfield. The most important thing to do in these formations is to determine the exact boundary by calcareous nannofossils. At the Arkavaz section, the upper part of Gurpi Formation (with 41.5 m thick) and the lower part of Pabdeh Formation (with 64.9 m thick) consists of 106.4 m thick was selected which mainly consists of gray shale and purple shale, respectively.  

Material and Methods I

n this study 47 samples from the Gurpi and Pabdeh formations have been studied. Samples were prepared following a standard smear slide method (Bown and Young 1998). All slides were studied under polarized light microscope at ×1000 magnification. Calcareous nannofossil nomenclature follows the taxonomic schemes of Perch-Nielsen (1985). 

Discussion of Results and Conclusions

Calcareous nannofossils recorded in the Mesozoic and Cenozoic strata are believed to be an appropriate means for biostratigraphic studies. As a result of this study, 17 genera and 23 species of calcareous nannofossils have been identified. Abbreviations used in this study are the NP (Nannofossil Paleogene) and CNP (Calcareous Nannofossil Paleocene). The nannofossil zonation used in the present study is based on the Nannoplankton zonation of Martini (1971) and Agnini et al. (2014), respectively. As a result of this study, 23 species belong to 17 genera of calcareous nannofossils were detected. Nine bio events were recorded and based on these bio events, the Ellipsolithus macellus Zone (NP4/CNP6–CNP7) and Fasciculithus tympaniformis Zone (NP5/CNP7–CNP8) are recognized at the uppermost part of the Gurpi Formation. Subsequently, Heliolithus kleinpellii Zone (NP6/CNP8) and Discoaster mohleri Zone (NP7)/Heliolithus riedelii Zone (NP8) (NP7/8 combined Zone–CNP9/10), are identified at the base of the Pabdeh Formation, respectively. Two biozones of the zonation of Martini (1971) are recognized at the upper part of Gurpi Formation as follows: (1) Ellipsolithus macellus Zone (NP4/CNP6–CNP7): The first nannofossil unit recorded in this study is the NP4 zone. This biozone is recorded from the FO Ellipsolithus macellus to the FO of Fasciculithus tympaniformis. The age of this zone is late Late Danian–Early Selandian. (2) Fasciculithus tympaniformis Zone (equivalent to NP5/ CNP7–CNP8): The second nannofossil unit recorded at the upper part of Gurpi Formation is the NP5 zone. This bio zone is recorded from the FO Fasciculithus tympaniformis to the FO of Heliolithus kleinpellii. The age of this zone is Selandian. Also, two biozones of the zonation of Martini (1971) are recognized at the lower part of Pabdeh Formation as follows: (1) Heliolithus kleinpellii Zone (equivalent to NP6/CNP8): The next nannofossil unit recorded in this study is the CC19 zone. This bio zone is recorded from the FO of Heliolithus kleinpellii to the FO of Discoaster mohleri. The age of this zone is Late Selandian–Early Thanetian. (2) Discoaster mohleri Zone (NP7) / Heliolithus riedelii Zone (NP8) or (NP7/8 combined Zone–CNP9/10): The last nannofossil unit recorded at the lower part of Pabdeh Formation is the NP7/8 combined zone. This biozone is recorded from the FO of Discoaster mohleri to the FO of Heliolithus riedelii. The age of this zone is Thanetian. In this study the Paleocene sediments have been investigated at the upper part of Gurpi Formation and the lower part of Pabdeh Formation at Arkavaz section in southeast of Ilam. The upper part of the Gurpi Formation is mainly consists of grey shale and the lower part of the Pabdeh Formation consists of purple shale. The detail study based on calcareous nannofossils, enables the subdivision of the studied deposits into four bio zones. As a result of this study and based on the identified calcareous nannofossil bio zones, the age of the uppermost part of the Gurpi Formation is Late Danian–Selandian and the lower part of the Pabdeh Formation is Selandian–Thanetian, and CNP8 Zone is located in the boundary between two formations. The biozones identified in this study were compared with sections from other regions of Zagros Basin.

Language:
Persian
Published:
Stratigraphy and Sedimentology Researches, Volume:35 Issue: 3, 2019
Pages:
23 to 36
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