Wuchiapingian and early Changhsingian ammonoid biostratigraphy in northwestern Iran

Abstract

Abstract The Permian–Triassic sedimentary succession in the Julfa region is lithostratigraphically composed of limestones and shales of the Julfa Formation, the Zal Member shales, and the Paratirolites Limestone of the Ali-Bashi Formation, as well as shales and carbonate beds of the Elikah Formation. The Lower Julfa Beds are rich in benthic organisms such as brachiopods and corals, while the Upper Julfa Beds and Ali-Bashi Formation contain pelagic assemblages including ammonoids, fishes, and conodonts. These rocks have already been studied from different palaeontological and geochemical perspectives, but most have focused on the strata near the Permian–Triassic boundary. In the present study, ammonoids from older intervals around the Wuchiapingin–Changhsingian boundary in the four stratigraphic sections Aras Valley, Ali-Bashi 1, Ali-Bashi 4, and Zal are outlined. Fourteen genera and 22 species of ammonoids were identified and assigned to five successive biozones. The Araxocers latissimum Zone in the Lower Julfa Beds and the Vedioceras ventrosulcatum Zone in the Upper Julfa Beds document the Wuchiapingian. Following upwards, the Iranites transcaucasius-Phisonites triangulus Zone, Dzhulfites nodosus Zone and Shevyrevites shevyrevi Zone in the Zal Member confirm an early Changhsingian age. This follows the previously presented ages based on conodonts. Keywords: Permian, Julfa, Ammonoid, Wuchiapingian, Changhsingian. Introduction The northwestern region of Iran along the Permian was a part of the Cimmerian blocks, separated from the Gondwana margin, and migrated northward parallel to the opening of the Neo-Tethys Ocean. At the Permian–Triassic boundary, this region was located near the equator, surrounded by the Neo-Tethys in the south and Paleo-Tethys in the north (Stampfli and Borel 2002; Kent and Muttoni 2020). Northwestern Iran contains valuable Lopingian successions and holds evidence of the largest extinction event in Earth's history at the end of Permian. The classical stratigraphic sections in the Caucasus and Julfa have been of interest to geologists since the 19th century. (e.g., Abich 1878; Rieben 1934; Stepanov et al. 1969; Kozur 2007; Richoz et al. 2010; Ghaderi et al. 2014a, b; Korn et al. 2016; Gliwa et al. 2020). The Permian–Triassic sequences of Julfa and Ali-Bashi mountains have been studied by Stepanov et al. (1969). They have categorized the whole succession into eight major rock units, including Genishik Beds (A), Khachik Beds (B), Lower Julfa beds (C), Upper Julfa beds (D), Permian-Triassic Transition Beds (E), Paratirolites Limestone (F), Lower Elika Formation (G) and Upper Elika Formation (H). After Stepanov et al. (1969), Teichert et al. (1973) also reviewed the Ali-Bashi Mountains region, especially the units E and F in Stepanov et al. (1969) during four parallel stratigraphic sections (sections 1 to 4). They have combined E and F units and introduced a new formation called Ali-Bashi Formation. In the following years, the rock sequences in the Ali-Bashi Mountains have been of great importance for studying molluscan fossils and conodonts. Based on conodonts, brachiopods, and ammonoids, the biostratigraphic information and chemical stratigraphy of these stratigraphic sections have been studied in various papers over the last two decades (e.g., Kozur 2007; Shen and Mei 2010; Ghaderi et al. 2014a, b; Schobben et al. 2015, 2017; Korn et al. 2016) and have greatly improved our knowledge about this lesser-known area in Central Tethys. However, there are still deficiencies in some aspects. In the present paper, ammonoid fauna of the Wuchiapingian–Changhsingian boundary in four different stratigraphic sections of Aras valley, Ali-Bashi 1 and 4, and Zal has been identified, and the biostratigraphy of the sections is presented accordingly. Material & Methods 142 different ammonoid specimens were taken in situ from the Julfa and Zal Beds, of which 84 belong to the Aras Valley section, 11 belong to Ali-Bashi 1, 18 belong to Ali-Bashi 4, and 29 belong to the Zal. Morphological characteristics of the specimens such as conch geometry and measurements of conch diameter, whorl height and width, apertural height, umbilical width, shape of the venter, arrangement and the shape of flanks, umbilical margin and the umbilical wall, shell ornaments such as growth lines, ribs, constrictions, and suture line were investigated according to Korn (2010) method. Cross-sections and suture lines of most of the ammonoids were drawn in Corel Draw 2019 software. Systematic paleontological studies have also been performed using various references (e.g., Ruzhencev and Shevyrev 1965; Zhao et al. 1978; Bando 1979; Kotlyar et al. 1983; Korn 2003; Leonova 2011; Ghaderi et al. 2014a; Korn et al. 2016; Korn and Ghaderi 2019). The ammonoid specimens discussed in this study are stored in the museum of the Geology Department of the Ferdowsi University of Mashhad, and some in the repository of the Museum fur Naturkundeh in Berlin, Germany. Discussion of Results & Conclusions The Lopingian ammonoid assemblages of the Julfa area have a different distribution in different parts of the sections in terms of abundance and species diversity; most of them are of late Changhsingian age in these successions (Ghaderi et al. 2014a; Korn et al. 2016). Older specimens of Lopingian have less variety and abundance. The fauna in this study includes 14 genera and 22 species of ammonoids of Julfa Beds and Ali-Bashi Formation Zal Member. Fourteen genera and 18 species have been identified in the Aras Valley section, eight genera and eight species in Ali-Bashi 1, 9 genera and 11 species in Ali-Bashi 4, and eight genera and 10 species in the Zal section. Based on the Korn and Ghaderi (2019) for the Aras Valley section and what has been obtained in the present study, the classic ammonoid biostratigraphy proposed for the Wuchiapingian–Changhsingian boundary interval in the Transcaucasia (Ruzhencev and Shevyrev 1965) revised and used for the Julfa region with some modifications. The following biozones are described here and presented in ascending order: Araxoceras latissimum Range Zone: Due to the partial outcrop of the Lower Julfa Beds in the Aras Valley section and the absence of their basal parts, the constituent interval of this biozone in the mentioned section is incomplete, and its thickness is small compared to other sections. The ammonoid assemblage accompanying this biozone in different sections includes Pseudogastrioceras relicuum, Araxoceras insolens, Prototoceras discoidale, Vescotoceras sp. and Araxoceras truncatum, which well confirms the age of early Wuchiapingin for this biozone. Vedioceras ventrosulcatum Range Zone: This biozone has extended into the Upper Julfa Beds; however, the zonal maker species Vedioceras ventrosulcatum was not observed in the studied sections in this study. Korn and Ghaderi (2019) have described other species of the genus Vedioceras, such as Vedioceras fusiforme, as a respectable alternative to the species Vedioceras ventrosulcatum and the definition of this biozone. The ammonoid fauna associated with this biozone in different sections, including Pseudogastrioceras relicuum, Pseudotoceras armenorum, Dzhulfoceras sp., and Vedioceras sp. Iranites transcaucasius - Phisonites triangulus Assemblage Zone: Korn et al. (2019) in the Aras Valley section and the authors of the present study in the other three sections, Ali-Bashi 1, 4, and Zal introduce the Iranites transcaucasius - Phisonites triangulus Assemblage Zone at the Changhsingianin base. This biozone begins with the unveiling of the Zal Member in the lowest part of the Ali-Bashi Formation. Disappearance of Dzhulfoceras and Vedioceras and the emergence of the first Iranites are the most distinctive feature of this biozone. Phisonites triangulus is also present in the platy marly limestone near the member’s base, associated with Xenodiscus dorashamensis and Pseudogastrioceras relicuum. With the onset of this biozone in the basal part of the Zal Member, there is a significant reduction in benthic versus pelagic fauna, indicating a deepening of the basin, minimization of benthos organisms such as brachiopods, and the predominance of pelagic animals like ammonoids, conodonts, and fishes. Dzhulfites nodosus Range Zone: This range zone begins with the appearance of Dzhulfites as well as the newly introduced genus Araxoceltites (Korn et al. 2019) and is located approximately in the middle of the Zal Member. The best record of this biozone in northwestern Iran is related to the Aras Valley section, where Araxoceltites sanestapanus, Dzhulfites nodosus, and Dzhulfites spinosus have been found together at -9.5 meters below the extinction horizon of the section. The ammonoids Araxoceltites laterocostatus, Araxoceltites cristatus, and Pseudogastrioceras relicuum are also present as the accompanying fauna in this biozone. Shevyrevites shevyrevi Interval Zone: This biozone with a very limited stratigraphic range at the top of the Zal Member is just located below the Paratirolites limestone and begins with the appearance of Shevyrevites shevyrevi. Arexoceltites cristatus is one of the most common species of this biozone, which is found alongside Shevyrevites and rare species belonging to Dzhulfites. The other ammonoids identified in this biozone in different sections include Araxoceltites laterocostatus, Araxoceltites sanestepanus, Dzhulfites nodosus, Dzhulfites spinosus, Pseudogastrioceras relicuum, and Shevyrevites nodosus. It should be noted that all Lopingian ammonoids found in the Julfa region, except for Pseudogastrioceras and Timorites, which are belonging to Goniatitida, are ceratitic types. The predominance of ceratitids in the studied ammonoid assemblages indicates the effect of habitat on end Guadalupian extinction. So that nectobenthic and benthopelagic taxa with less lateral compaction, living in shallower tropics, are more damaged and more severely deformed. In contrast, those with high lateral compression have the most preservation. This confirms the selective effect of extinction on the mentioned fauna, indicating in unfavorable conditions, the ceratitids migrated to deeper areas due to their ability to live in the deep-water column and even survived from the end Permian great extinction event

Keywords

• permian
• julfa
• ammonoid
• wuchiapingian
• changhsingian