نوع مقاله : مقاله پژوهشی
نویسندگان
گروه زمینشناسی، دانشکده علوم پایه، دانشگاه آزاد اسلامی، واحد شیراز، ایران
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
Introduction
As the oldest member of the Shemshak group, the Nayband formation has been widely outcropped in Central Iran. It has been important to geologists for a long time due to its significant coal deposits. Accordingly, the geological data about the Nayband Formation is relatively extent that geological studies have been carried out on almost all known outcrops of this formation. Considering that the deposits belonging to the Upper Triassic in the subcontinental blocks of Central Iran are different according to the performance of the previous Cimmerian event due to having separate characteristics, and due to the heterogeneous activities of the blocks of Central Iran, the mentioned deposits have various depositional environments (Seyed-Emami et al, 2004). Nayband Formation is sandwiched between carbonate sediment of Shotori Formation (below) and clastic sediment of Ab Haji Formation. Actually, the underalid formation (dolomite of Shotori Formation) is assigned to Mid Triassic, whereas the Ab Haji Formation (sandstone) is referred to Lias. The lower contact of the Nayband Formation is marked by red bed as disconformarble contact while the upper lithostratigraphic contact with Ab Haji is gradational type. In general, the Central Iran Microcontinent is consisted of several blocks such as: Yazd, Posht Badam, Tabas, Kalmard and Lut (Aghanabati, 2004), while there are several records of tectonic evolution of the Central Iran Microcontinent (Kargaranbafghi et al, 2015; Chu et al, 2021; Abbaspour et al, 2024). With regard to active tectonic of the Central Iran as well as Cimmerian event the foraminiferal biostratigraphy is significant for Upper Triassic deposits (Nayband Formation). Some records of paleoenvironment and paleontological of the Nayband Formation were provided (Zamani et al, 2023; Shepherd et al, 2016; Bayetgoll and Daraei, 2017). It is necessary to note that the both litho-bio stratigraphic limits of the Upper Triassic/Lower Jurassic (Lias) is so significant not only in view of biostratigraphy but also in the field of micropaleontological features (Boudagher-Fadel, 2008). With regard to the previous studies (Zamaniyan et al, 2018; Mannani and Sherafat, 2021; Amirhassankhani et al, 2023; Hashemi Yazdi et al, 2023; Hadadi and Afghah, 2024), the foraminiferal biostratigraphy of the Upper Triassic deposits (Nayband Formation) is so significant for regional biostratigraphic correlation. For this reason, the studies of different sections of the Nayband Formation in other blocks of the Central Iran subcontinent can be important, and in the present study, its biostratigraphy has been studied in the Cheshmeh Gaz section located in the southern areas of the Posht Badam Block. However, recieving a foraminiferal biozonation which is affected by early Cimmerian event is the main goal of the present study. It should emphasized the mentioned event affected the paleoecological condition on foraminiferal environment. Moreover, compare of lithofacies and biostratigraphic data can clarify the mechanism of early Cimmerian effect in the Posht Badam Block.
Materials and Methods
In this research, to study the biostratigraphy of the Nayband Formation, the Cheshmeh Gaz stratigraphic section was selected and sampled in the Posht badam block. In order to biostratigraphy of the Nayband Formation, the lithostratigraphic contacts have been studied in detail. The thickness of the studied section is 180 meters. Based on the field characteristics of different layers, rock samples with the abbreviation Shm were numbered and sampled, and microscopic thin sections were prepared and studied. The sampling of the studied section has been conducted by lithofacies change with different sample interval. Since the Nayband Formation is composed of clastic and carbonates sediments, the foraminiferal contents are recognized just in carbonate sediments. According to Adams and Bourgeois, 1967, Donatomilanes, 1988, Loeblich and Tappan, 1988, Kobayashi et al. 2006, Boudagher -Fadel, 2008, Gale, 2012, Gale et al. (2016, 2020), the index foraminifers are identified and biozonation described.
Results and Discussion
So far, no official biozonation has been provided for the deposits of the Nayband Formation in the Upper Triassic age range. But due to features such as abundance, high diversity, as well as a significant number of the first appearance and the last appearance of foraminiferal fauna in the mentioned age range, in terms of biostratigraphy, the mentioned fauna can be part of the very important paleobiotic elements of the Nayband Formation. Based on biostratigraphic studies, it is possible to understand the differences in different regions and identify specific fossil communities of each region. Also, in the studied area, because the lithological changes of Late Triassic deposits are directly related to the sedimentation and climatic conditions, and therefore the distribution of biological populations and existing biocenoses have undergone obvious changes to paleo-ecological developments. However, the foraminiferal association of the studied section represents the shallow water environment of the Nayband Formation. As mentioned before, the foraminiferal contents are recognizable in the carbonate units of the Nayband Formation. Generally, 25 species from 24 genera were identified and their age ranges were studied and finally the age was determined based on this. Foraminifera of the Nayband Formation in the studied section of Cheshmeh Gaz have a low abundance, but they have a significant diversity. Families such as Permodiscidae, Turrispirilinidae, Ammodiscidae and Frondicularidae have the highest abundance in the studied section in this research. Based on stratigraphic distribution of identified foraminifers, seven biozones were established.
Conclusion
Based on the biostratigraphic studies, there are seven biozones including Ammodiscus parapriscus Interval Zone, Angulodiscus communis Interval Zone, Nodosaria nitidana Interval Zone, Turrispirllina minima Interval Zone and Trochammina almtalensis Concurrence Rang Zone, Triassina hantkeni Concurrence Rang Zone, Aulotortus bronimanni Concurrence Rang Zone have been identified in the Upper Triassic deposits. Based on the identified biozones, the age range of the studied sequences is suggested to be late Norian-latest Rhaetian, which is equivalent to the Howz-e Khan and Qadir members of the Nayband Formation in central Iran. It is necessary to note that the identified foraminiferal biozones as well as vertical change in lithofacies of the Upper Triassic deposits (Nayband Formation) represent the change in sedimentary regime as well as foraminiferal composition which is assigned to early Cimmerian event. In order to detail understanding of Upper Triassic foraminiferal distribution, more investigation stratigraphic sections are required in the Central Iran Microcontinent.
کلیدواژهها [English]
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