Journal of Stratigraphy and Sedimentology Researches (Jun 2024)
Biostratigraphy and palaeoecology of Ostracodes of the Members c1 to c4 and e of the Qom Formation in the South of Shurab, Southeast of Qom
Abstract
Abstract: In order to study the Qom Formation, the ostracodes of members c1 to c4 and e of this formation were studied in the south Shurab section. The studied strata in this section have a significant abundance and diversity of ostracods. Changes in the abundance and diversity of species in the ostracodes of the studied members show that Member c2 was deposited in a shallower environment compared to the middle and upper part and the base of Member c3. The high abundance of psychrophilic species at the base of Member c4 and the presence of thermophilic forms at the top of Member c4 indicate that the basin was deeper at the time of deposition of the base Member c4 sediments than at its top. Also, the visible changes in the ostracods collection in marly Member e indicate frequent fluctuations in the depth of the environment during the deposition of this member in the studied area. In general, it can be inferred that the environment of members c1 to c4 and e of the Qom Formation was a warm with moderate salinity sea water. In the biostratigraphic studies, 42 species belonging to 36 genera of ostracoda and four biozones have been identified. Based on the biostratigraphic studies, the Chattian age was determined for the sediments of the top of Member b and c1, the Aquitanian for the sediments of members c2 to c4, and the Burdigalian for the sediments of Member e of the Qom Formation in the Shurab section was determined. Keywords: Qom Formation, Ostracoda, South of Shurab, Biostratigraphy, Palaeoecology Introduction The Qom Formation is a significant geological unit due to its hydrocarbon resources, making it a focus of geological studies. This formation in its extensive area in central Iran, within the Sanandaj-Sirjan and Urumieh-Dokhtar zones exhibits remarkable changes both laterally and vertically, making it a subject of geological interest both in terms of surface outcrops and subsurface sections. Diverse fossil contents and significant facies changes have prompted stratigraphic researchers to investigate the Qom Formation based on its available fossil contents, in order to understand more about this formation, and also solve existing problems and ambiguities , and complete its study chain. In this regard, one of the fossil groups with significant diversity and abundance in the Qom Formation is the ostracods, which has received less attention and few studies have been conducted on it. The significance of studying the Qom Formation from one side and the importance of ostracodes as a useful tool in paleontological studies, particularly for determining the age of different parts of the Qom Formation and identifying its palaeoenvironment, on the other hand, has led this research to deal with biostratigraphic and palaeoecological studies of members c1 to c4 and e of the Qom Formation in the south Shurab section, located southeast of Qom based on ostracod findings in the area. Material & Methods In this study, research was conducted in two phases: fieldwork and laboratory work (sample preparation and ostracod identification using a stereomicroscope). A total of 255 samples were collected and examined from members c1 to c4 and e of the Shurab section. Sample preparation by washing method included the use of 35, 70, 80, and 120 mesh sieves. Separation of ostracods from the sediments was performed using a stereomicroscope, which led to the identification of 2685 ostracodes, including 42 species belonging to 36 genera from 12 families and two suborders. After identifying the ostracodes, their range chart was drawn, and then, - the ages of members c1 to c4 and e of the Qom Formation in the Shurab section were determined along with their biostratigraphic zones.. In the palaeoecology study, counting and statistical analysis of ostracod species was done so that the number of species present in each sample was identified separately and the percentage of abundance of each species in relation to the total identified ostracodes was calculated. Finally, the palaeoenvironment of the studied members of the Qom Formation in the south of the Shurab section was predicted based on ostracodes. Discussion of Results & Conclusions The Qom Formation with a thickness of 544 meters is outcropped in the Shurab stratigraphic section located in the southeast of Qom. In this section, members c1, c2, c3, c4, and e respectively with thicknesses of 30, 113.5, 13 and 19 meters, and Member e with a thickness of 49.8 meters were subjected to biostratigraphic and palaeoecological studies. Based on the identification of 2685 ostracodes obtained from 255 samples from the mentioned members, while recognizing 42 species belonging to 36 genera from two suborders in the Podocopida order, the following 4 biozones are identified and introduced. assemblage zone Cyamocytheridea reversa -4- Cytherella vulgate assemblage zone Cytheridea Josephina-3- Neonecidea gerda 2- Quadrocytheris symmetric- Actinocythereis rosefieldensis assemblage zone 1-Cytheretta (Flexus) trifurcate - Cytherella obesa assemblage zone Among the mentioned biozones, biozone 1 belongs to the top of Member b, biozones 2 and 3 belong to members c2 to c4, and biozone 4 belongs to Member e. In Member c1, due to the presence of dense limestone facies and the impossibility of separating ostracodes, the biozones were not introduced. On the basis of the identified biozones and the ostracodes available there, the Aquitanian age for members c2 to c4, and the Burdigalian for Member e is suggested. For sediments at the top of Member b, based on its existing ostracodes, and for Member c1 based on its stratigraphic position and regional studies, the age of the Chattian was determined. Palaeoecological studies of the existing ostracodes in the upper sediments of the members b to e, significant changes in abundance and diversity are seen under the influence of changes in depth, temperature and salinity. These changes show that the environment of the formation of the above-mentioned members had significant fluctuations in depth, so that the presence of thermophilic genera belonging to shallow areas, such as Actinocythereis, at the top of Member b and the base of Member c2. The genera belonging to deeper areas, such as Cytheropteron, in the middle to the end of Member c2 and the marl base of Member c3, indicate the changes in depth from the shallow depth at the top of Member b and the base of Member c2 to a greater depth in the middle part of c2 to the base of c3. This deepening is also evident at the base of Member c4, so that psychrophilic genera such as Cytherella, Krithe and Paracypris have a significant abundance in this part, while towards the top of Member c4, the abundance of psychrophilic genera decreases and thermophilic genera belonging to shallower environments, such as Pontocypris, Actinocythereis, Propontocypris, increase and generally show a shallowing towards the top of this member. In Member e, the changes in the abundance and diversity of ostracodes indicate fluctuations in the water depth at the time of the formation of this member, so that at the beginning of Member e, the abundance of thermophilic species indicates the establishment of a shallow environment in the continuation of the evaporative environment of Member d. But this gradually upwards with the increase of the abundance of psychrophilic genera such as Krithe, Cytherella, and Paracypris, and the environment has became deeper. Similarly, in the second half of Member e, as the the depth decreased, thermophilic species initially increased, and finally as the depth increased, psychrophilic species became more abundant. The ostracodes identified in the studied members show that the most abundant during the deposition of the mentioned succession belonged to thermophilic species. Also, the salinity studies show that the most abundant of species in the aforementioned members belong to the species related to the marine environment with normal salinity.
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