Biological evolution of the carbonate platform of the Taleh Zang Formation in Kermanshah region

Abstract In this paper, the biological evolution of the carbonate platform of the Taleh Zang Formation with Paleocene age in the Kermanshah region has been investigated in two stages (time interval) concentrating on fossil debris (such as algae, corals, and benthic foraminifera). For this purpose, two suitable stratigraphic of this formation in the south (Kaboutar Bala section) and southwest (Barikeh section) of Kermanshah were selected, sampled, and studied. For a broader view, the results of this study have been compared with other parts of Tethys basin around the world. The depositional environment of the studied deposits of the Taleh Zang Formation is a carbonate ramp and is probably a form a low gradient ramp due to underdeveloped coral constructions. The gradient of this platform increases from the northwest (Barikeh section) to the southwest (Kaboutar Bala section). Four biofacies were recognized in the studied successions, which include green algae, coral, green algae and benthic foraminifera as well as benthic foraminifera and green algal biofacies. In the lower parts of the Taleh Zang Formation, the presence of algal and coral biofacies (patch reefs) indicates the first stage of carbonate biological evolution of this formation, in that such biofacies has been identified in the Paleocene sequences of other Tethys sectors. Throughout the time and in the upper parts of the Taleh Zang Formation, corals have disappeared and the abundance of green algae is greatly reduced. By the presence of benthic foraminifera with low amounts of green algae, the second stage of the biological evolution of the carbonate platform of the Taleh Zang Formation in the Kermanshah region is formed. In the Early Eocene in Kermanshah region and in the studied successions, due to the seawater regression, the production of carbonates in the Taleh Zang Formation stopped and the detrital red sediments of the Kashkan Formation replaced the carbonates of the Taleh Zang Formation. However, the carbonate production of this formation continued in other areas of the Zagros sedimentary basin (Lorestan province) until the Middle Eocene. Keywords: Biofacies, Carbonate platform biological evolution, Taleh Zang Formation, Paleocene, Kermanshah. Introduction During the Early Paleogene (Paleocene to Eocene), the major carbonate producers were algae, corals, and benthic foraminifera. At the end of the Early Paleogene due to the warming events such as the Paleocene–Eocene Thermal Maximum (PETM), the increase in temperatures in the oceans and CO2 in the atmosphere led to the decline of corals and the expansion of benthic foraminifera (Bernaola et al. 2007; Zamagni et al. 2012; Zhang et al. 2018, 2020). At this time, larger benthic foraminifera, which could tolerate higher temperatures, progressively became one of the main carbonate producers (Pujalte et al. 2003; Scheibner and Speijer 2008). During the Paleocene–Eocene, three biological evolutionary stages could be identified and separated in the carbonate platforms around the world (Scheibner and Speijer 2008; Zamagni et al. 2012; Martin et al. 2020). In stage one, Late Paleocene coralgal dominated platforms; in stage two, a transitional latest Paleocene platform stage with coralgal reefs dominating at middle palaeolatitudes and larger benthic foraminifera dominated platforms at low palaeolatitudes; and in stage three, Early Eocene larger foraminifera dominated platforms in all latitudes. One of the areas where Paleogene carbonates can be abundantly identified and studied is SW Iran, where during the Paleogene, the Taleh Zang Formation has unique outcrops in Kermanshah and Lorestan provinces. Algae, corals, and benthic foraminifera, which are the main producers of the carbonates, have a significant abundance in the Taleh Zang Formation in the Kermanshah region (Shalalvand 2019; Shalalvand et al. 2019 a). Previous studies on this formation have focused mainly on biostratigraphy in the Lorestan Province (Maghfouri Moghaddam et al. 2008; Rajabi et al. 2013; Shojaei Nejad et al. 2018) and in some cases determining the sedimentary environment (Zohdi and Adabi 2008), none of which mention the existence of algal and coral biofacies. In this study, besides identifying the algae and corals, the possible relationship between the biofacies changes of the Taleh Zang Formation is compared with the biological evolution of the Paleocene–Eocene carbonate platform exposed in other parts of the Tethys Basin. This study makes an attempt to cover an important gap in knowledge at middle latitudes (20° to 30°) located between the northern and southern Tethys platform. Materials and Methods After studying the 1/250000 map of Kermanshah (Braud 1978) and 1/250000 map of Ilam-Kuhdasht (Llewellyn 1974), the areas were survived and the outcrops (Barikeh and Kaboutar Bala outcrop sections) were selected as parts of the Zagros Paleocene deposits. Considering the aim of study, accurate sampling on these two stratigraphic sections was performed systematically at two-meter intervals. Finally, 130 hand specimens from Barikeh and 33 samples from Kaboutar Bala sections were collected for petrographic studies. Articles published by various researchers were used to recognize and study the calcareous algae (Elliott 1968; Deloffre and Radoicic 1978; Barattolo 1998; Genot and Granier 2011; Granier et al. 2013; Sobhi and Ahmed 2015; Bucur et al. 2016; Barani et al. 2017; Granier et al. 2017; Bucur et al. 2018; Barattolo et al. 2019). The identification of corals in this paper was also based on studies of different researchers (Turnsek and Kosir 2004; Bernecker and Weidlich 2005; Zamagni et al. 2012; Baron-Szabo 2018). Besides, the biofacies present in this study were identified based on field observations and compared with standard microfacies introduced by Flugel (2010). Discussion of Results and Conclusion In this paper, four biofacies of green algae, coral, green algae and benthic foraminifera as well as benthic foraminifera and green algae biofacies were identified. The abundant presence of benthic foraminifera and fragments of green algae led to the biofacies being considered as separate biofacies from the latter. According to the identified facies, the Taleh Zang Formation in the Kermanshah region was deposited in a shallow carbonate ramp during Paleocene. At this time, which is the first stage of evolution of the Paleogene carbonate platform, algae and corals are very widespread in the Taleh Zang Formation. In this stage, two separate biofacies have been identified, which include the green algae biofacies and the coral biofacies. These biofacies have also been identified in many Paleocene carbonate deposits of the Tethys Basin (Scheibner and Speijer 2008; Martin et al. 2020). During the Late Paleocene and in the Upper Thanetian, with the extinction of corals and most of the algae, the benthic foraminifera was very widespread. This is another stage (stage two) in the evolution of the carbonate platform of the Taleh Zang Formation. At this stage, two biofacies were identified and separated. These biofacies include the green algae and benthic foraminifera as well as benthic foraminifera and green algae biofacies. During the Early Eocene in Kermanshah region, in contrast to the other areas of the Zagros sedimentary Basin, the carbonate production of the Taleh Zang Formation disappears completely. At this time, Kashkan siliciclastic deposits including conglomerates and sandstones formed in a fluvial sedimentary environment (Rostami et al. 2019). In this paper, the obtained results are compared with other similar Tethyan sectors to build a broader view (Scheibner and Speijer 2008; Zamagni et al. 2012; Martin et al. 2020). The carbonate platforms in the West Carpathians (W-Slovakia), the Pyrenean platform (France and Spain), and the Ionian Isp lands (Greece) are located in mid-latitudes (30°–45° N). In these platforms, the major organisms in the first and second evolutionary stages are composed of coralgal reefs (Accordi et al. 1998; Baceta et al. 2005). In these areas, in the third stage of biological evolution, benthic foraminifera plays an important role, except in the Carpathian Mountains. In the Carpathian region at the end of the Paleocene, the whole reef complex was destroyed and no younger strata were recorded (Kazmer et al. 2003; Bucek and Kohler 2017). Carbonate platforms located in Tunisia, Western Morocco, Sirte Basin (Libya), and Zagros Basin in Kermanshah region with the presence of Taleh Zang Formation, are located at intermediate latitudes (20°–30° N). During the Paleocene, in some carbonate platforms (such as Morocco) of these areas, coral reefs and benthic foraminifera are absent. The absence of corals and benthic foraminifera makes this area different from other areas (Jorry et al. 2006; Scheibner and Speijer 2008; Martin et al. 2020). Carbonate platforms in Oman, Northwest India, and Northwest Somalia are also characterized by low latitudes (0°–20° N). Coral reefs indicate the first stage of evolution of these platforms. In the second evolutionary stage, coral reefs and benthic foraminifera are generally present. In the Early Eocene, the abundant presence of benthic foraminifera in all regions describes the third stage (Carbone et al. 1993; Sarkar 2015; Tomas et al. 2016). Overall, it can be concluded that only two biological evolutionary stages are present in the carbonate platform of the Taleh Zang Formation in the Kermanshah region. The first stage is quite similar to the platforms of the Carpathian, Pyrenean, Ionian Islands, Libya, Northwestern India, and Northwestern Somalia. The second stage of the biological evolution of the Taleh Zang Formation is also comparable to the platforms located in Libya and Oma