Biostratigraphy, microfacies analysis and depositional environment of the Bartonian–Priabonian limestones in Shan-Abad anticline (west of Rafsanjan)

Abstract

Abstract The Middle–Late Eocene carbonate deposits have developed in the Shan-Abad anticline as a thin belt among volcanic rocks. The nummulite banks are one of the most interesting deposits in this belt and composed of nummulite accumulations in the form of small, low-relief build-ups. Investigating three outcrops of these deposits were carried out at the eastern flank of Shan-Abad anticline in the present study. A high diversity of larger benthic foraminifera (LBF) and a small number of planktonic foraminifera are recognized, and the sedimentary environment is reconstructed.Based on LBF, three Tethyan foraminiferal biozones (SBZ 17–SBZ 19) spanning the Middle to Late Eocene interval are identified. The textural analysis and faunal assemblages reveal eleven microfacies types. These microfacies types (MFT) are deposited on a carbonate ramp model from tidal to open marine settings. Analysis of the Bartonian–Priabonian deposits based on facies distribution and paleoecology of the biotic components suggest deposition in the inner, middle and proximal outer ramp settings. The inner ramp includes proximal and distal parts. The proximal part of inner ramp is characterized by tidal flat, lagoon, and shoal belts. The distal part of inner ramp includes the nummulite banks. The open marine microfacies were deposited in mid to proximal outer ramp settings. Keywords:Bartonian, Priabonian, Microfacies, Foraminifera, Nummulite accumulations, Rafsanjan. Introduction During the Paleogene, LBFs formed the most common foraminifera groups in shallow marine sediments of different parts of the Tethyan region. Among them, the genus Nummulites hasa unique potential for the rock-forming. The Eocene nummulite banks are widespread along the Tethyan margins and make significant hydrocarbon reservoirs in many places, such as Tunisia (Racey et al. 2001) and Libya (Anketell and Mriheel 2000). In the last few decades, valuable LBF biostratigraphic schemes were conducted by Schaub (1981), Papazzoni and Sirotti (1995), and Serra-Kiel et al. (1998), especially in western Tethys. In Iran, the first comprehensive studies of Eocene LBF were done by Bozorgnia and Kalantari (1965), Rahaghi (1978, 1980, 1983) and Rahaghi and Schaub (1976) and continued by other researchers (e.g., Mosaddegh et al. 2017, Hadi and Vahidinia 2019 and Hadi et al. 2019 a, b). In western Rafsanjan city (Kerman Province, southeastern Iran), the Eocene successions were dominated by volcanic rocks, while sedimentary outcrops and nummulite accumulations of the Eocene deposits are scarce. Despite poor distribution, they are rich in LBFs, which led to the preparation of an excellent tool for paleontological and paleoenvironmental investigations. This study presents the biozonation, microfacies, and depositional environment of the Eocene successions in the Shan-Abad anticline (western Rafsanjan). Material & Methods For this study, three stratigraphic sections on the Eastern flank of Shan-Abad anticline were measured in the field, and their lithologies and sedimentary patterns were described. Shan-Abad 1 and 2 are located 20 km west of the city of Rafsanjan (close to Shan-Abad village) with a thickness of 19.2 m and 18 m, respectively. The Deh-Zahir section is 27.8 m thick is located near Deh-Zahir village (about 8 km south of the Shan-Abad sections). Over 70 samples were collected for the microfacies and micropaleontological analysis. LBF assemblages were investigated in thin sections in order to determine the age of the deposits, applying the Shallow Benthic Zones (SBZs) of Sera-Kiel et al. (1998). A total of 11 MFTs have been defined based on LBF assemblages and sedimentary textures. The textural classification follows the classifications of Embry and Klovan (1971) and Dunham (1962). For the microfacies analysis, the methods of Wilson (1975) and Flügel (2010) were followed. Discussion of Results & Conclusions The Middle–Late Eocene shallow marine successions of the Shan-Abad anticline in Rafsanjan contain stratigraphically important LBFs, which have been used for biostratigraphy and microfacies analysis. Three shallow-water Tethyan foraminiferal biozones (SBZ17–SBZ19) of Middle–Late Eocene are recognized. These biozones are recognized based on one or more fundamental biostratigraphic markers and their first and last occurrences. The co-occurrence of Nummulites incrassatus, N. ptukhiani, N. striatus, N. perforatus, N. cf. N. lyelli, N. malatyensis, Alveolina nuttalli, and A. fusiformis are considered to indicate an Early Bartonian (SBZ17) age. In the Deh-Zahir section, these species are accompanied by Morozovelloides coronatus and M. crassatus, showing this interval is comparable with the E12 biozone of Wade Biozonation (Wade et al.2011). The SBZ18 (Late Bartonian) is marked by the first appearance of Pellatispira madraszi, N. chavannesi, Heterostegina cf. reticulata tronensis, and Nummulites aff. biedai. These species are reported from the Late Middle Eocene from various parts of Tethyan regions such as Italy, Turkey, Armenia and India (Eames 1952; Cole 1970; Jones 1961; Banerji 1981; Serra-Kiel et al. 1998; Bassi 1998; Less et al. 2008; Özcan et al. 2010; Matsumaru and Sarma 2010; Less and Özcan 2012; Cotton et al. 2016). The SBZ 19 is defined by the first appearance of Nummulites aff. fabianii, which appears within SBZ 19 and ranges to SBZ 20. It has been documented from the Late Eocene (Priabonian) of many Tethyan and Indo–pacific regions such as Italy, Turkey, Libya, Pakistan, India and Tanzania (AbdulSamad 2000; Matsumaru and Sarma 2010; Cotton and Pearson 2011; Less et al. 2011; Less and Özcan 2012, Bukhari et al. 2016, Babar et al. 2018). Based on the lithological characteristics, sedimentary texture, and biological content of the studied successions, 11 MFTs were recognized from tidal to open marine belts. From the shoreline towards the sea, they are as follow: 1. Tidal flat microfacies belt: The MFTs of this microfacies belt are bioclastic microconglomerate (MFT1), lithoclastic dolomudstone (MFT2), lithoclast foraminifera bioclast wackestone (MFT3). 2. Lagoon microfacies belt: The bioclast perforate and imperforate foraminifera wackestone/packstone was deposited in lagoon environments. 3. Shoal microfacies belt: The imperforate foraminifera peloidal grainstone (MFT5), bivalve rudstone/floatstone (MFT6) and foraminifera bioclast lithoclast intraclast grainstone (MFT7) were deposited in high-energy shoal environments. 4. Nummulite bank microfacies belt: The MFTs of this microfacies belt include nummulithoclastic packstone and Nummulites rudstone/floatstone/packstone that contain high accumulations of Nummulites. 5. Open marine microfacies belt: Perforate foraminifera bioclast packstone/wackestone and nummulitid orthophragminid packstone/wackestone indicate deeper, low-energy and low light environments of this belt. These MFTs are developed on different parts of a carbonate ramp. The tidal flat, lagoon, and shoal MFTs have formed in the proximal inner ramp environments. Distal inner ramp settings are characterized by the nummulite bank microfacies. Open marine MTFs were deposited in the mid, to proximal outer ramp settings.

Keywords

• bartonian
• priabonian
• microfacies
• foraminifera
• nummulite accumulations
• rafsanjan