Journal of Stratigraphy and Sedimentology Researches (Dec 2020)
Investigating bio-events and time expansion of Calcareous nannofossils in the upper part of the Gurpi Formation and lower part of the Pabdeh Formation, northwestern Shiraz
Abstract
Abstract In this study, calcareous nannofossils are investigated in the upper part of the Gurpi Formation and lower part of the Pabdeh Formation at the Par-e Nobar section located in the northwest of Shiraz. The thickness of the studied section is about 41 m. The upper part of the Gurpi Formation is mainly composed of shales and pelagic limestones while the lower part of the Pabdeh Formation consists of shale facies. As a result, 32 species and 22 genera of calcareous nannofossils were detected. According to the first and last occurrence of index species and fossil assemblages, two bio-zones including Nephrolithus frequens Zone (CC26/ UC20dTp ) and Markalius inversus Zone (NP1) are recognized in the upper part of the Gurpi Formation, respectively. Fasciculithus tympaniformis Zone (NP5) and Heliolithus kleinpellii Zone (NP6) are reported for the first time from the lower part of the Pabdeh Formation, respectively. Based on the identified calcareous nannofossils zones, the age of the upper part of the Gurpi Formation is the latest Maastrichtian–early Danian, and the lower part of the Pabdeh Formation is the middle Paleocene/Selandian. Therefore, according to the nannofossil data, the boundary between the two formations is unconformable. Keywords: Paleogene, Bio-event, Zagros, Cretaceous, Calcareous nannofossil. Type text or a website address or translate a document. Showing translation for مقطع تیپ سازند در اطراف قم مطالعه شده است Translate instead مقطع تیپاین سازند در اطراف قم مطالعه شده است Introduction One of the most extensive Cretaceous and Cenozoic deposits are the Gurpi and Pabdeh formations in the Zagros Basin, which are studied in this research based on calcareous nannofossils. The type section of the Gurpi and Pabdeh formations is located in the southwestern Tang-e Pabdeh, North of Lali oilfield (Jams & Wynd, 1965). The important goals of studying the upper part of the Gurpi Formation and lower part of the Pabdeh Formation is the evaluationof boundaryand biostratigraphy. Material & Methods In this study, 33 samples from the upper part of the Gurpi Formation and lower part of the Pabdeh Formation have been studied. Samples were prepared following the standard smear slide method (Bown and Young 1998). All slides were studied under a polarized light microscope at × 1000 magnification. The nomenclature of calcareous nannofossils follows the taxonomic schemes of Perch-Nielsen (1985). Discussion of Results & Conclusions To study biostratigraphy of the succession based on calcareous nannofossils, the boundary of the Gurpi and Pabdeh formations in the Par-e Nobar section located in the northwest of Shiraz was selected. In this section, the upper part of the Gurpi Formation with 22.2 m thickness mainly consists of shales and argillaceous limestones and the lower part of the Pabdeh Formation with 18.8 m thickness is consists of shale facies. For introducing index species, calcareous nannofossil assemblages, and bio-zones, slides have been studied at the Gurpi/Pabdeh formations boundary which led to the recognition of 32 species and 22 genera of calcareous nannofossils. According to the first and last occurrence of index species and recording bio-events, two bio-zones include Nephrolithus frequens Zone (CC26/ UC20dTp ) and Markalius inversus Zone (NP1) has been recognizedin the upper part of the Gurpi Formation. The Fasciculithus tympaniformis Zone (NP5) and Heliolithus kleinpellii Zone (NP6) are reported for the first time from the lower part of the Pabdeh Formation in the studied section, respectively. The bio-zones identified in the upper part of the Gurpi Formation with a thickness of 22.2 m at the Cretaceous to Paleogene boundary are as follows: Nephrolithus frequens Zone (CC26) / (UC20bTP, UC20d Tp ): The first zone in the upper part of the Gurpi Formation is recorded from the FO to LO of Nephrolithus frequens. The age of this zone is late Late Maastrichtian. The thickness of this zone is 14.8 m. Markalius inversus Zone (NP1) / (CNP1): This zone spans the interval from the last occurrence of Cretaceous taxa to the FO of Cruciplacolithus tenuis. The age of this zone is early Paleocene (early Danian). The thickness of this zone is 7.4 m. Bio-zones introduced in the lower part of the Pabdeh Formation are as follows: These bio-zones in the studied section are 18.4 m thickness from the lower part of the Pabdeh Formation. Fasciculithus tympaniformis Zone (NP5): The first nannofossil unit recorded in the lower part of the Pabdeh Formation is theNP5. This zone spans the interval from the FO ofFasciculithus tympaniformisto the FO ofHeliolithus kleinpellii. The age of this zone is Paleocene (Selandian). The thickness of this zone was measured at about 7.4 m. Heliolithus kleinpellii Zone (NP6): The last bio-zone recorded is NP6. This zone is from the lower part of the Pabdeh Formation.This zone spans the interval from the FO of Heliolithus kleinpellii to the FO of Discoaster mohleri. The age of this zone is Paleocene (Selandian). The thickness of this zone was measured at about 11 m. Thus, based on the identified calcareous nannofossils zones, the age of the upper part of the Gurpi Formation is the latest Maastrichtian–early Danian, and thelower part of the Pabdeh Formation is the middle Paleocene/Selandian. Therefore, according to the nannofossil data, the boundary between the two formations is unconformable. In addition, the Cretaceous–Paleogene boundary is continuously at the top of the Gurpi Formation. This interval was recorded by a significant decrease in Cretaceous nannofossil species, along with an increase in Thoracosphaera operculata and the appearance of the Paleocene species. Also, the conditions of the basin that is part of the eastern Tethys were identified by determining the index species calcareous nannofossils. Therefore, the presence of these species confirms warm climate conditions during the sedimentation of mentioned deposits in this part of Zagros.
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