The role of fossils in the microfacies studies and recognizing the sequence boundaries of the Qom Formation: A case study in Ghasr-e-Bahram Section, northwest part of Siahkuh, south Garmsar

Abstract

Abstract Sequence stratigraphy is a branch of stratigraphy that can enable us to correlate deposits with considerable accuracy. This research aims to investigate the role of fossils in the recognition of system tracts and sequence boundaries along succession using microfacies features. To achieve this aim, we studied the Qom Formation at the Qasr-e-Bahram section located in the south of Garmsar. This formation is mainly composed of argillaceous limestones, limestones, and marls, overlies the Lower Red Formation, and lies under the Upper Red Formation disconformably. Based on paleontological studies, the suggested age is the Late Aquitanian to Burdigalian (Early Miocene). The microfacies analysis indicates a variety of facies and environments (lagoon, carbonate barrier, and open marine). The significant presence of red algal with other reef-forming elements within microfacies as well as the presence of turbidite facies probably implies a shelf carbonate platform. According to geological field and laboratory studies, deposits of the Qom Formation include four third-order depositional sequences, and the presence and abundance of fossils such as foraminifera, algae, bryozoans, and corals were useful tools in identifying the systems tracts and their boundaries. Keywords: Qom Formation, Early Miocene, Shelf, Sequence biostratigraphy, South Garmsar. Introduction Sequence stratigraphy is one of the most interesting areas in geological research which is a useful tool for correlation deposits in stratigraphic studies on a local to global scale. Many researchers have used sequence stratigraphy in order to understand the Qom sedimentary basin in central Iran. Although many studies have been based on the characteristics of sedimentary facies, most researchers believe that fossils are more sensitive indicators for determining past sedimentary environments than non-skeletal particles and they help to recognize depositional sequences and sequence boundaries (Brett 1995; Emery and Myers 1996; Armentrout 1996; Fürsich and Pandy 2003; Armstrong and Brasier 2005). In Iran, very few sequence stratigraphic studies focused on the role and importance of fossils (for example Vaziri-Moghaddam et al. 2006; Reuter et al. 2007, Taheri et al. 2008, 2010, Daneshian et al. 2008; Daneshian et al. 2017a, Daneshian et al. 2017b). This research aims to study the sequence stratigraphy and recognize the depositional sequences with special attention on the role of fossils, especially foraminifera, and it has been tried to use them to detect the system tracts, sequence boundaries, and maximum flooding surfaces. Material & Methods The Qom Formation in the Ghasr-e-Bahram section with coordinates of 34°, 45ʹ, 2ʹʹ N and 52°, 5ʹ to 52°, 8ʹ, 6ʹʹ E and 359 meters thick is mainly composed of argillaceous limestones, limestones, and marls and includes members c-3 to f of the type area. This rock unit overlies the Lower Red Formation and lies under the Upper Red Formation disconformably. A detailed study of these microfossils was carried out by Daneshian and Derakhshani (2008), and the age of Late Aquitanian and Burdigalian was determined. In the present study, the fossil assemblages were re-examined according to Daneshian and Ramezani Dana (2019), the deposits belong comparable to the Elphidium sp.14 interval zone with Late Aquitanian age and Borelis melo curdica total range zone to Burdigalian age. The study section was compared with the type area of rock unit, and adjacent areas such as the Deh Namak in the northeast of Garmsar, Attari and Garmab sections in the northeast and southeast of Semnan and Aftar section in the northwest of Semnan for understanding the expansion of the deposits in the region. The carbonate rock specimens were named according to Dunham (1962) and Embry and Klovan (1971), and microfacies determination was done based on Flügel (2010). Hunt and Tucker (1992, 1995), Catuneanu (2002, 2006), and Emery and Meyers (1996) were used to study depositional sequences. Planktonic foraminifers, hyaline and porcelaneous benthic foraminifers, echinoids, red algae, corals and bryozoans were evaluated and the trend of vertical changes of each allochem was drawn on the stratigraphic column of Ghasr-e-Bahram section as well. Discussion of Results & Conclusions The study of 191 samples collected from the Qom Formation in the Ghasr-e-Bahram section, which was previously considered by Daneshian and Derakhshani (2008), showed that the age of this rock unit is Late Aquitanian to Burdigalian (Early Miocene) based on the benthic and planktonic foraminifera. The examination results were not changed by the assigned age. The boundary between Aquitanian and Burdigalian is determined based on the first occurrence of Borelis melo curdica, and the age of the Late Aquitanian is based on the first occurrence of Elphidium sp.14 (Adams and Bourgeois 1967, Daneshian and Ramezani Dana 2019). The correlation of the study section with the type area shows that the Qom Formation is somewhat similar to the type area in terms of lithology, but shows a remarkable decrease in thickness. Microfacies analysis of the Qom Formation in the Ghasr-e-Bahram section and comparison to standard microfacies of Flugel (2010) led us to recognize 12 carbonate microfacies. They are classified mainly into three facies associations, including lagoon, carbonate barrier and open marine. The significant presence of red algal microfacies and other reef-forming allochems and also the presence of turbiditic facies, probably indicate the formation of these deposits in a shelf carbonate platform. According to the geological field and laboratory studies, four third-order depositional sequences were recognized. The presence and abundance of fossils such as foraminifera, algae, bryozoans and corals are a useful tool in identifying the systems tracts and their boundaries. Our examination in the first depositional sequence shows that the abundance of bryozoan as stenohaline organisms can be defined as remarkable evidence for maximum flooding surface (mfs). Then, the trend of fossils assemblages to higher levels in the section indicates a rise of lagoonal foraminifers, and the sequence boundary on top was described by the most abundant ostracodes in microfacies. In the second depositional sequence, we observed an increasing percentage of benthic foraminifera with hyaline tests such as Amphistegina, Asterigerina, Heterostegina, Reussella, and broken test of planktic foraminifera such as Globigerina, Globorotalia, Globigerinoides which they indicate to deepening depositional environment. In this depositional sequence, mfs are defined based on the highest percentage of echinoid debris. Again, the trend of foraminiferal assemblages upward shows a lagoonal environment, and the sequence boundary was characterized by pelloidal microfacies containing porcelaneous foraminifera and ostracodes. In the third depositional sequence, we detected an increase of benthic foraminifers with a hyaline test (e.g., Amphistegina, Asterigerina, Heterostegina, Planorbulina, Heterolepa and planktonic foraminifers such as Globigerina and Globorotalia) which implies a deepening trend. The highest percentage of echinoid debris with hyaline benthic foraminifers specified mfs of the third depositional sequence. Upper sequence boundary of this depositional sequence is determined by the highest percentage of lagoonal foraminifera. The presence of lagoonal foraminifera (e.g., Borelis, Archaias, Spiroloculina, Triloculina, Quinqueloqulina, Pyrgo) versus stenohaline organisms such as echinoids and bryozoans indicate shallower environment. The last depositional sequence, a deepening trend upward, is characterized by the rise of the percentage of hyaline benthic foraminifers and echinoid debris which abundance of the latter shows the mfs. Then, the presence and increase of porcelaneous foraminifera including Archaias, Borelis and miliolids such as Quinqueloqulina, Pyrgo, Triloculina, and Spiroloculina indicate the shallowing trend. Comparison of the identified depositional sequences with global sea level curves shows that the sequence boundaries during the Burdigalian can be consistent with the global sea level changes. The global sea level changes are more important than local and regional tectonics in this region.

Keywords

• qom formation
• early miocene
• shelf
• sequence biostratigraphy
• south garmsar