Global regression evidence in the Middle Miocene strata in the Zanjan area

Abstract

AbstractThe study of the effect of global regression on the Middle Miocene (upper parts of the Qom Formation and Lower parts of the Upper Red Formation) strata has been carried out on two stratigraphic sections in the Zanjan area, northwest Iran. Lithological and microfacies analysis shows that these carbonate strata gradually turn to clastic (sandstone and conglomerate) and, in some regions, turn to evaporate deposits. Microfacies studies also confirm this sedimentary facies changes. Thus, the pelagic wack/packstone microfacies gradually change to sandy bioclast pelagic wack/packstone and finally, at the top of the section changes to the continental sandstone. Facies and lithological changes confirm the shoreline regression that can be correlated with the eustatic sea-level curve. Evaporitic strata in some parts of this area show small restricted basins because of this event. The eustatic sea-level curve in the Langhian and Serravallian ages shows global sea-level fall that can be correlated with the global oxygen isotope changes and glaciation events at this time. Therefore, we can conclude that the Middle Miocene strata have been affected by this global regression in the Zanjan area. Keywords: Regression, Miocene, Microfacies, Zanjan. IntroductionThe Qom Formation is cropped out in central, north and northwest Iran (Daneshain and Dana 2019). The Burdigalian (Daneshian and Saiedi Mehr 2005; Daneshian et al. 2009; Noroozpour 2020; Rabbani and Zohdi 2021) and the Aquitanian–Burdigalian (Daneshian et al. 2010) ages have been proposed for these successions from northwest of Iran. The extinction of Borelis melo curdica confirms a the Burdigalian age in this formation (Rabbani et al. 2022). The Middle Miocene regression is one of the most important global sea-level fluctuations in the world (Haq et al. 1987; Thomas 2008). Some glaciation evidence in the South pole at this time can confirm this global regression (Lewis et al. 2014; Halberstadt et al. 2021). This study tries to find the effect of this global regression in the Zanjan area of northwest Iran. Material & Methods Tow stratigraphic outcrop sections have been measured, sampled and studied in detail in the southwest (Ghachoghay) and Northwest (Aghbolagh) of Zanjan. Thin section of 48 samples from the Aghbolagh section and 42 samples from the Ghamchoghay section have been studied by polarized microscope at the Geology of Department, University of Zanjan. All microfacies have been identified using standard limestone classification (Dunham, 1962; Embry and Klovan, 1971) and correlated with standard microfacies (Flugel 2010) for the depositional environmental interpretations. Discussion of Results & ConclusionsBased on the stratigraphic range of two index fossils (Borelis melo curdica and Praeorbulina glomerosa), we can propose the Burdigalian–Langhian (Middle Miocene) age for the upper parts of the Qom Formation strata that are composed of limestones and sandy limestones. Facies studies led to the identification of two microfacies and two facies as follows:Pelagic wack/packstone: this microfacies can be seen in Q39 and Q40 (Aghbolagh), and S37, S38 and S39 (Ghamchoghay) samples. The high abundance of planktonic foraminifera fossils (Globigerina praebulloides) in wackestone texture is the most important feature in these microfacies. These microfacies can be correlated with SMF3 related to the toe of the slope and distal part of the carbonate platform.Sandy bioclast pelagic wack/packstone: This facies is observed in the Q43, Q46, and Q47 samples of the Aghbolagh section, and S40, S41 and S42 samples of the Ghamchoghay section. Benthic foraminifera, bioclast and a low abundance of planktonic foraminifera and sand grains can be seen within the wack/packstone texture of these samples. This microfacies can be correlated with SMF4 related to the slope area of the carbonate platform. Other studies confirm carbonate shelf as a sedimentary environment for these strata (Rabbani et al. 2020; Rabbani and Zohdi 2021). The sand grains originated from terrestrial to the marine environment by submarine channels between bioclastic reefs. Thus, we propose an open carbonate shelf as a sedimentary environment for the Qom Formation in this area. Some sedimentary structures such as asymmetrical ripple marks and cross-bedding in these strata can confirm unidirectional flow in the submarine channels in the slope area.3 and 4- Clastic and evaporitic facies: The uppermost layers of the Qom Formation have been followed by the lowermost parts of the Upper Red Formation in this area that contains sandstones and conglomerates. An evaporative unit in the Ghamchoghay section can be considered the lower part of the Upper Red Formation. This unit also has been reported from the Andabad area in the south of the Aghbolagh section.The result shows a very good correlation with the eustatic curve in the Middle Miocene age (Haq et al. 1987) that confirms the connection of the Neo-Tethys Ocean with open marine (Reuter et al. 2009). This regression can be correlated with the global ice house in the Middle Miocene that was reported from different parts of the continents on Earth (Passchier et al. 2013; Griener et al. 2015; Bassant et al. 2017; Raitzsch et al. 2021)

Keywords

• regression
• miocene
• microfacies
• zanjan