Biostratigraphy of the Cretaceous deposits based on ammonites in the southwest of Qayen area (Qumenjan section)

• Main Authors: Seyed Naser Raisossadat, Abdolmajid Mosavinia, Ahmadreza khazaei, Shahram Asadi
• Format: Article
• Language: fas
• Published: University of Isfahan 2021-03-01
• Series: Journal of Stratigraphy and Sedimentology Researches
• Subjects: biostratigraphy; ammonite; albian; cenomanian; qayen; east iran
• Online Access: https://jssr.ui.ac.ir/article_25121_6edd83dd55afd249e0a730fbb408f2d9.pdf
• ISSN: 2008-7888; 2423-8007

Abstract Ammonite biostratigraphy could be used in age determination and correlation of rock units in different sedimentary basins. Qumenjan section with 125 meters thickness is mainly formed by grey sandy limestone with grey and thick-bedded limestone intercalations. The study area is located in the eastern margin of the Lut Block. For a stratigraphical study of Lower–Upper Cretaceous deposits in southwestern of Qayen area, a stratigraphic section was selected and measured. Eight ammonites, including Anisoceras, Calycoceras, Hypoturrilites, Hysteroceras, Mantelliceras, Puzosia, and Scaphites genera and thirteen representative species were identified. Based on the identified assemblage fossils and comparison with international biozones, Hysteroceras orbignyi subzone for Late Albian age, Mantelliceras mantelli biozone for the lower part of Early Cenomanian age, Mantelliceras dixoni biozone for the upper part of Early Cenomanian age, and Turrilites costatus biozone for Middle Cenomanian age is suggested. According to identified genera and species a Late Albian–Middle Cenomanian age is assigned for ammonite bearing of the section. The first occurrence of Mantelliceras mantelli is taken as the base of Cenomanian. Compression of the identified ammonites with other basins shows that the east Iran basin was a part of Tethys Realm during late Albian–middle Cenomanian. Keywords:Biostratigraphy, Ammonite, Albian, Cenomanian, Qayen, East Iran.   Introduction Ammonites are one of the best macrofossils for Cretaceous biostratigraphy. The Albian and Cenomanian ammonites of Kopet Dagh and Central Iran have been studied by numerous researchers such as Seyed-Emami and Aryai (1981), Seyed-Emami (1977), Seyed-Emami, (1982), Seyed-Emami et al. (1984), Immel and Seyed-Emami (1985), Seyed-Emami and Immel (1995, 1996), Immel et al. (1997), Mosavinia et al. (2007), Mosavinia and Wilmsen (2011), Mosavinia et al. (2014), and Wilmsen et al. (2015). The Cretaceous deposits have spread in the east of Iran and in Birjand and Qayen area. These deposits have different facies. The Cretaceous rocks include conglomerate and sandstone units in the base and then changed to limestones in Late Barremain to Aptian. During Albian–Cenomanian, the depth of the sedimentary basin increased and thin-bedded limestones, marly limestones, and marls were deposited. The Lower Cretaceous succession is composed of different rock units that are outcropped in the eastern margin of Lut Block. These deposits usually start with a basal conglomerate. “Orbitolinind Limestone” is an informal name that is used for a succession containing Orbitolina limestone and marl intercalations. Aptian–Cenomanian succession mostly is composed of detrital, marl, and carbonate beds most of which are biogenic sediments deposited in carbonate platforms. Upper Cretaceous deposits are mostly widespread in Maastrichtian (Stocklin et al. 1972; Fauvelet and Eftekharnezhad 1990). Flyschiod facies are also spread with Ophiolites in Upper Cretaceous (Aghanabati2004). One of the oldest studies  conducted in the east of Iran is published by Clap (1940). Several geological and paleontological studies have been done by several researchers such as Stocklin et al. (1972), Tirrul et al. (1983), Fauvelet and Eftekharnezhad (1990), Berthiaux et al. (1991), Raisossadat and Skelton (2005), Babazadeh et al. (2010), Khazaei et al. (2011), Raisossadat et al. (2016), Sharifi et al. (2016), Motamedalshariati et al. (2016), Raisossadat and Noori (2016), Latifi et al. (2018), Raisossadat et al. (2017) and Zarei et al. (2019). Despite the above-mentioned studies , there still seems the need  to do more studies in paleontological fields. Ammonites are known as useful fossil groups in age determination and are used as biostratigraphical and biozonation tool for the Cretaceous deposits. The aim of this study is to provide an ammonite biozonation and a short palaeobiogeographic discussion on the Cretaceous deposits of the study area.   Material & Methods Based on the geological map and field observation, a stratigraphical section was selected and measured in the southwest of Qayen and in south of the Qumenjan Village. The geographical coordinates of the base of the section are 58° 52' 11" E 33° 32' 19" N. Sixteen rock and ammonite samples were collected. Ammonites were cleaned, covered with magnesium vapor and photographed. Collected specimens were identified. The collected samples were stored in the Department of Geology, University of Birjand.   Discussion of Results & Conclusion The Qumenjan section with 125 meters thickness  starts with a conglomerated bed at the base, and then continues with sandy limestones. The major part including a succession of grey limestone and argillaceous limestone intercalations. In this study, the following ammonites are identified: Anisoceras plicatile, Calycoceras gentoni, Calycoceras (Newboldiceras) asiaticum, Calycoceras sp., Hypoturrilites sp < em>., Hysteroceras orbignyi, Mantelliceras dixoni, Mantelliceras saxbii, Mantelliceras mantelli, Mantelliceras cf. lymense, Puzosia (P.) sp., Scaphites sp.  Based on reported microfauna (Ahrari 2009; Moloudi 2014) and identified ammonites a Late Aptian to Middle Cenomanian age is suggested for the section. Based on the identified ammonites, the following biozones is suggested for the studied section: Hysteroceras orbignyi subzone: this subzone is introduced as an index subzone for the Late Albian. Range of this zone is from the first occurrence of Hysteroceras orbignyi to the first occurrence of Mantelliceras mantelli. The associate ammonite is Puzosia sp. The thickness of this subzone is 25 meters. Mantelliceras mantelli Interval Zone: This zone is reported from south of the UK, France, Spain, and Portugal and south of Africa. Mantelliceras mantelli is discussed as index zone of Early Cenomanian. The range of this zone is from the first occurrence of Mantelliceras mantelli to the first occurrence of Mantelliceras dixoni. The associate ammonites are Hyphoplites cf. costosus, Hypoturrilites cf. gravesianus, Scaphites sp. The thickness of this subzone is 19 meters. Mantelliceras dixoni Interval Zone: This zone is an index zone for lower part of Middle Cenomanian. However, this zone could confirm Middle Cenomanian. The range of this zone is from the first occurrence of Mantelliceras dixoni to the first occurrence of Turrilites costatus. The associate ammonite is Mantelliceras cf. lymense. The thickness of this subzone is 14 meters. Turrilites costatus Interval Zone: This zone is introduced by Hancock (1991). Alabushev (1995) introduced it as a subzone. Wilmsen et al. (2007) mentioned Anisoceras plicatile and Turrilites costatus as index fossils for Acanthoceras rhotomagense Zone. Kennedy and Gale (2015) reported Calycoceras gentoni from Middle Cenomanian. Acanthoceras rhotomagense is reported as the index zone of Middle Cenomanian. In the studied section A. rhotomagense has not been reported. Therefore, based on the occurrence of Turrilites aff. costatus Zone, Middle Cenomanian age is suggested for the upper part of the section. The associate assemblages  include Calycoceras (Newboldiceras) asiaticum, Calycoceras gentoni, Anisoceras plicatile and Calycoceras sp. Latitudinal and provincial control of ammonite faunas was common throughout the Mesozoic. Both the ranks (realm, province, etc.) and names have varied from author to author. Tectonic movements and sea-level changes (regressions and transgressions) caused seaways connecting basins to open marine or close and led to several distinct new basins or limitation of seaways and between marine basins. Thus, numerous sub-realms and provinces have been named. Three biogeographic realms  existed during the Mesozoic; Boreal Realm embraced the Arctic basins and the seas extending southward from there over parts of North America, Northern Europe, and Siberia. The Boreal seas were largely landlocked but linked by narrow seaways to the Tethyan Realm. The Tethyan Realm includes low latitude areas such as Africa, the Pacific, the Middle East, and America, and centered on the Paleo-Tethys Ocean. An Austral Realm has also been proposed for high southerly latitudes (Stevens 1973). To study the palaeobiogeographic conditions of Upper Albian–Lower Cenomanian in eastern Iran, the ammonite fauna of the studied section was compared with other reported ammonites from different areas. The reported ammonites from east of Iran show affinity with those reported from Kopet Dagh, Central Iran, Europe, and USA. The absence of Scholenbachia that is an index genus of the Boreal Realm and the presence of some genera such as Anisoceras, Scaphites, Hypoturrilites, and Puzosia could suggest that the study area is a part of Tethys Realm. However, the presence of Mantelliceras could show the effects of the influence of Boreal. The analysis of the ammonite stratigraphic distribution in the selected section in east of Iran allowed us to establish the faunal succession for the Upper Albian–Middle Cenomanian in the region. Based on identified assemblage, the following biozones are suggested. Hysetroceras orbignyi subzone is suggested as index zone of Late Albian, Mantelliceras mantelli Zone could confirm lower part of Early Cenomanian, Mantelliceras dixoni Zone is known as the upper part of Early Cenomanian and Turrilites costatus Zone is suggested as Middle Cenomanian. Compression of the identified ammonites with other basins shows that the east Iran basin was a part of Tethys Realm during Late Albian–Middle Cenomanian.