Palynostratigraphy of the Shemshak and Dalichai formations boundary, north of Semnan and its paleogeographic significance

Abstract

AbstractThe palynofloras of the Shemshak (upper part) and Dalichai (lower part) formations, Deh-Soufian, north of Semnan include 57 species of spores (34 genera), 25 species of pollen (13 genera), and 14 species of dinoflagellate cysts (8 genera). Based on the stratigraphic distribution of miospores, the Klukisporites variegatus-Striatella spp.-Contignisporites burgeri assemblage biozone (Middle Jurassic) is introduced. The co-occurrence in the palynofloras examined of Contignisporites burgeri and Striatella seebergensis allows for the Bajocian–Bathonian boundary to be located 75m below the boundary of Shemshak-Dalichai rock units. The presence in the strata below this chronostratigraphic horizon of Mendicodinium scabratum (Bajocian) supports this age assignment. The boundary of Shemshak and Dalichai (with basal sandstone) formations is discontinuous. While no significant change in the composition of miospores is discernible in palynofloras retrieved from the above rock units, the relative abundance of such terrestrial palynomorphs varies drastically. This might be attributed to side effects of the mid-Cimmerian Orogeny i.e., significant change in sedimentation rate resulting in change from the deltaic to marine depositional settings. It should be noted that 45 m below the boundary of the two rock units 24 miospore species exit whereas 11 different ones appear 20 m above that boundary. Notable correspondence of the Iranian Middle Jurassic palynofloras across the Alborz Ranges signifies to a rather significant coastal vegetation similarity of the time in the area. Whereas both Gondwanan and Eurasian miospore key species occur in the palynofloras examined those particularly endemic to the southern landmasses are missing from the palynofloras studied. Palynological evidence seemingly confirms the location of the Alborz Ranges on the northwestern margin of the Neotethys Ocean in the Middle Jurassic. The available paleontological data (ammonites and dinoflagellate cysts) apparently support this interpretation.Keywords: Palynostratigraphy, Paleogeography, Middle Jurassic, Shemshak and Dalichai formations. IntroductionJurassic strata widely distributed and exposed in the Alborz Ranges (Majidifard 2008) consist of deposits of two large sedimentary-tectonic cycles (Aghanabati 1975). Those of the first Upper Triassic–Lower Jurassic sedimentary cycle are known as the Shemshak Formation (Assereto 1966; Fürsich et al. 2009) or the so-called Shemshak Group whereas strata of the second sedimentary megacycle (Middle-Upper Jurassic) in east-central Iran are known as the Magu Group.Lithological variation, biological attributes, and depositional settings reflected by the Jurassic rocks in Iran indicate that the geologic changes were closely related to the tectonic events (Majidifard 2003). Much of the Shemshak Formation sedimentation in the Alborz Ranges took place on coastal plains in fluvial, lacustrine, or deltaic settings, paralic swamps, lakes, meandering, as well as, braided rivers, and shallow–deep shelfal environments (e.g., Seyed-Emami 2003; Seyed-Emami et al. 2008). The Upper Triassic and the lowermost Jurassic rocks are very similar, so in most cases, the two-time intervals cannot be differentiated (Majidifard 2003; Seyed-Emami 2003; Seyed-Emami et al. 2008). Due to its vast thickness in many areas (up to 4000 m), some have elevated the unit to group rank (e.g., Aghanabati 1998; Seyed-Emami 2003; Fürsich et al. 2009a). Consequently, the unit has been subdivided into various members/formations, and different lithostratigraphic schemes have been proposed (Nabavi and Seyed-Emami 1977; Nabavi 1980; Repin 1987; Aghanabati 1998; Seyed-Emami et al. 2020). It contains diverse biota such as plant fossils (e.g., Sadeghi and Hashemi 2021), and ammonites (e.g., Seyed-Emami et al. 2020). However, due to the poor preservation of palynomorphs, the Shemshak Formation has remained almost unstudied palynologically.The Magu Group, the second sedimentary megacycle consists primarily of fossiliferous marls and limestones indicating basin and platform depositional setting (Lasemi 1995). These accumulated from the end of the Mid-Cimmerian tectonic phase (Middle Jurassic) till the Late Jurassic and, in some places, continued during the Neocomian (Seyed-Emami 1975; Keshani 1988; Seyed-Emami and Alavi-Naini 1990; Majidifard 2003, 2008; Fürsich et al. 2009).The Dalichai Formation (Steiger 1966), the oldest rock unit of the Magu Group (Aghanabati 1998) comprises shale, light-grey to bluish-grey limestones with thin intercalations of marls (Majidifard 2003, 2008). It contains diverse biota such as ammonites, spores and pollen grains, dinoflagellates cysts, belemnites, bivalves, brachiopods, echinoids, sponges, bryozoans, and foraminifera (Stöcklin 1972; Alavi-Naini 1972; Sussli 1976; Nabavi and Seyed-Emami 1977; Alavi-Naini et al. 1982; Seyed-Emami et al. 1985, 1989, 1995, 1996; Wheeler and Sarjeant 1990; Schairer et al. 1991; Shahrabi 1994; Makvandi 2000; Tutunchi 2001; Ghasemi-Nejad and Khaki 2002; Shafeizad et al. 2002; Farisi Kermani 2003; Shafeizad and Seyed- Emami 2005; Alvani 2006; Shams 2007; Niknahad 2007; Ghasemi-Nejad et al. 2008, 2012; Hashemi Yazdi 2008, 2015; Majidifard 2008; Seyed-Emami et al. 2008; Sabbaghiyan 2009; Sajjadi et al. 2009; Vaziri et al. 2008, 2011; Dehbozorgi 2013; Dehbozorgi et al. 2013; Mafi et al. 2013; Navidi Izad 2013; Dietze et al. 2014; Skupien et al. 2015). Study of ammonites indicates a late Bajocian to Oxfordian age for the Dalichai Formation (Lorenz 1964, Sussli 1976; Nabavi and Seyed-Emami 1977; Seyed-Emami et al. 1985, 1995, 2013; Schairer et al. 1991; Majidifard 2003, Seyed-Emami and Schairer 2010, 2011a, b). According to paleontological and lithological evidence the Dalichai Formation was deposited in a marine environment (Lorenz 1964, Steiger 1966, Wheeler and Sarjeant 1990, Zarei 2017). Furthermore, ammonite data shows close relationships to those from epicontinental seas fringing the northern margin of the western Tethys and an intermediate position between the Subboreal and Submediterranean provinces (Seyed-Emami et al. 2013, 2018; Dietze et al. 2014; Seyed-Emami and Raoufan 2017).The aim of this study is to document the palynological content of the Shemshak and Dalichai formations boundary at Deh-Soufian, north of Semnan, to assess its stratigraphic and palaeogeographical significance and compare it to other Middle Jurassic records of Eurasia and Gondwana. Material & MethodsTwenty-one samples were collected from shale intervals at the Deh-Soufian stratigraphic section (N 35° 47' 44" and E 53° 40' 03"); nine samples from the uppermost of the Shemshak Formation and twelve samples base of the Dalichai Formation. Standard palynological procedures (e.g., Phipps and Playford 1984, Wood 1996) applied for extraction and concentration of the palynomorphs. Samples were first disaggregated into pea-sized pieces (1–2 mm in diameter) and treated with 30% HCl to eliminate carbonates. The residue was then washed to neutrality and the remaining inorganic matter dissolved in HF (ca. 40%); fluoride precipitants formed were subsequently removed using hot 30% HCl followed by washing to neutrality. The organic residue was sieved via 20 μm mesh. Heavy minerals and other remaining inorganic particles were then removed by the use of a heavy-liquid solution (ZnCl2) at appropriate specific gravity. The remaining residue was then sieved through 20 μm nylon sieve prior to mounting on permanent slides. At least three permanent strew slides per sample were prepared using Entellan as the mounting medium. Discussion of Results & ConclusionsThe palynofloras examined comprise 57 spore species (assigned to 34 genera), 25 pollen species (attributed to 13 genera), and 14 species of dinoflagellate cysts (allocated to eight genera). The vertical ranges of certain miospore species with known stratigraphic significance and/or persistence throughout the section investigated authorize the introduction of one informal local biozone as Klukisporites variegatus-Striatella spp.-Contignisporites burgeri assemblage biozone. The presence of multiple Striatella species, as well as, Contignisporites burger and Klukisporites variegatus collectively indicate a Middle Jurassic (Bajocian–Bathonian) age. The co-occurrence in the palynofloras examined of Contignisporites burgeri and Striatella seebergensis allows for the Bajocian–Bathonian boundary to be located 75 m below the boundary of Shemshak and Dalichai rock units. The presence in the strata below this chronostratigraphic horizon of Mendicodinium scabratum (Bajocian) supports this age assignment.The lithology of the Shemshak Formation includes sandstone and shale alternations, and the Dalichai Formation includes basal sandstone followed by limestone and shale alternations. The boundary between the two mentioned formations is discontinuous due to the presence of a sandstone layer at the base of the Dalichai Formation.No significant change in the diversity of miospore species is observed in the border of the mentioned rock units, but their relative abundance in this horizon is associated with a significant decrease. This might be considered a side effect of the mid-Cimmerian Orogeny, i.e., a significant change in sedimentation rate resulting difference in the sedimentation environment from the deltaic (Shemshak Formation) to the marine environment (Dalichai Formation).It should be noted that 24 miospore species disappear 45 meters below the border of the Shemshak and Dalichai formations, and 11 different miospore species appear 20 meters above this border. The comparative palynofloras found in the Iranian Middle Jurassic rocks of the Alborz Ranges recommend that there were likely comparative types of coastal plants in that area during that time. The key miospore species from both Gondwana and Eurasia are found in the examined samples, but the species that are unique to the southern landmasses (e.g. Aequitriradites norrisii, Anapiculatisporites dawsonensis, A. pristidentatus, and ….) are not present in the samples studied. The investigated palynomorphs indicate that throughout the Middle Jurassic, the Alborz Ranges were located alongside the northwestern edge of the Neotethys Ocean. The ammonites and dinoflagellate cysts apparently support this elucidation.

Keywords

• palynostratigraphy
• paleogeography
• middle jurassic
• shemshak and dalichai formations