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Conodonts from the Niur Formation (Silurian) of the Derenjal Mountains, Central Iran

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Conodonts from the Niur Formation (Silurian) of the Derenjal Mountains, Central Iran See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/239735578 Conodonts from the Niur Formation (Silurian) of the Derenjal Mountains, Central Iran Article in Geological Magazine · July 2013 DOI: 10.1017/S001675681200088X CITATIONS READS 15 315 3 authors: Peep Mannik C. Giles Miller Tallinn University of Technology Natural History Museum, London 101 PUBLICATIONS 2,383 CITATIONS 81 PUBLICATIONS 586 CITATIONS SEE PROFILE SEE PROFILE Vachik Hairapetian Islamic Azad University Isfahan (Khorasgan) Branch 77 PUBLICATIONS 587 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: TERSANE: Temperature-Related Stresses as a Unifying Principle in Ancient Extinctions View project Studies on Silurian conodonts from eastern Qinling and the related strata View project All content following this page was uploaded by C. Giles Miller on 27 May 2014. The user has requested enhancement of the downloaded file. Geol. Mag. 150 (4), 2013, pp. 639–650. c Cambridge University Press 2013 639 doi:10.1017/S001675681200088X Conodonts from the Niur Formation (Silurian) of the Derenjal Mountains, Central Iran ∗ P. MÄNNIK †, C. G. MILLER‡ & V. HAIRAPETIAN§ ∗ Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia ‡Department of Palaeontology, Natural History Museum, London SW7 5BD, UK §Department of Geology, Khorasgan (Esfahan) Branch, Islamic Azad University, PO Box 81595−158, Esfahan, Iran (Received 5 October 2011; accepted 25 September 2012; first published online 24 January 2013) Abstract – A Llandovery to Ludlow age for the Niur Formation in the Derenjal Mountains (east-central Iran) is proposed based on new conodont data and previous work on other fossils. The uppermost part of the studied section yielded no diagnostic conodonts but may be Pridoli in age. Some intervals can be dated more precisely: Unit 11 (at least its upper part) is middle Telychian in age and corresponds to the Pterospathodus amorphognathoides lennarti Zone; the lowermost part of Unit 16 is earliest Ludlow in age and corresponds to the Kockelella crassa Zone; the uppermost Unit 16 is late Ludlow (Ludfordian) in age and corresponds to the Ozarkodina snajdri Interval Zone. The Llandovery–Wenlock boundary lies between units 12 and 13 based on sedimentological evidence. The precise location of the Wenlock– Ludlow boundary in the section is not clear but lies below Unit 16. Present-day Iran was located far away from Baltica and Laurentia, on the other side of the Rheic Ocean. This ocean does not seem to have been a major migration barrier for most organisms including the conodonts. Keywords: stratigraphy, palaeogeography, conodonts, Niur Formation, Silurian, Iran. 1. Introduction weight varied between 0.5 and 3 kg and the number of specimens per productive sample from 2 to 361. The Lower Palaeozoic of Iran including the Silurian, is All samples were dissolved in buffered 10 % acetic well exposed in several regions: in the north along the acid and the residues washed through a 15.6 μm sieve. southern coast of the Caspian Sea (Alborz Mountains All residues were picked completely; heavy liquid and the Kope Dagh zone), in east-central Iran (Kerman– separation techniques were not applied. Illustrated spe- Saghand and Tabas regions; Fig. 1) and in the Zagros cimens were gold-palladium coated and photographed Basin (Gahkum and Faraghan mountains). The section using a Phillips XL-30 scanning electron microscope at studied here is located about 65 km northwest of the Natural History Museum, London. All material is Tabas, on the eastern side of the Dahaneh-e-Kolut deposited at the Department of Palaeontology, Natural Gorge in the Derenjal Mountains (Figs 1, 2). Here, History Museum (prefix NHMUK PM X). Where approximately 551 m of the Silurian Niur Formation conodont apparatus structure and therefore biological is exposed (Fig. 3). Silurian conodonts have been positioning of elements at generic level is proven reported from the Derenjal Mountains by Ruttner, from natural assemblages, the P ,P,M,S notation Nabavi & Hajian (1968) and Hamedi et al. (1997). 1 2 0-X outlined by Purnell, Donoghue & Aldridge (2000) is However, both publications provide lists of taxa and followed. When there is no evidence for homology no illustrations. The only known illustrations of Niur between different element types, the traditional Pa, Pb, Formation conodonts are from the Huk section located M, Sa, Sb, Sc notation has been followed. in NE Iran (Weddige, 1984). New samples from a section in the Derenjal Mountains have been studied for conodonts. The aims of this paper are to give a brief lithological description of the section, to update 3. Geographical and geological setting conodont identifications and associated ages, and to The Niur Formation was formally established by briefly discuss the palaeogeographic affinities of the Ruttner, Nabavi & Hajian (1968). In the studied fauna. section, the Niur Formation is underlain by fossiliferous limestone of the Shirgesht Formation of Early to Middle 2. Materials and methods Ordovician age and overlain by the siliciclastic Padeha Formation of presumed Early Devonian age. The type Forty-two samples were collected by V. Hairapetian section of the Niur Formation is located in the vicinity during fieldwork in 2004 and 2007. Only calcareous of Niur village in the Ozbak-Kuh Mountains, c.90km intervals of the section were sampled and processed, northeast of the section discussed in this paper. The with 17 samples yielding conodonts (Fig. 3). Sample section studied here in the Derenjal Mountains was referred to as the ‘reference section’ of the Niur † Author for correspondence: [email protected] Formation by Ruttner, Nabavi & Hajian (1968). There http://journals.cambridge.org Downloaded: 11 Jun 2013 640 P. MÄNNIK AND OTHERS 3.a. Hill A (units 1–4: 135.70 m) Base of the section. Lowermost part of the northern slope of Hill A; co-ordinates: 34◦ 05 8.3 N and 56◦ 4814.0 E; altitude: 1072 m. Description of the section starts from the base of a bed of olivine basalt, from the level tentatively correlated with the Ordovician–Silurian boundary. Unit 1. 12.80 m. Altered dark green massive olivine basalt. Unit 2. 23.50 m. Brown to grey, medium- to thin- bedded bioclastic packstone and grainstone with a bed of brown re-crystallized limestone at the top. Corals, brachiopods, ostracods and trilobites occur. Conodont samples (all levels are measured from the base of Unit 2; Fig. 3): S1, 3.20 m; S2, 4.10 m; S3, 4.40 m; S4, 7.10 m; S5, 8.40 m; S6, 9.10 m; S7, 10.70 m; S8, 16.10 m; S9, 16.80 m; S10, 17.70 m; S11, 21.00 m. Unit 3. 56.40 m. Altered dark green massive olivine basalt. TopofUnit3. Co-ordinates 34◦ 05 07.0 N and 56◦ 48 13.9 E; altitude 1077 m. The measured section continues about 600 m to the east of Hill A. The upper boundary of Unit 3 was traced to this point. Location of the base of Unit 4.34◦ 05 12.4 N and 56◦ 48 39.6 E; altitude 1071 m. Unit 4. 43.00 m. Intercalation of siltstone and silty shale. The rock is reddish brown and yields brachiopods, corals and rare trilobite fragments. At the base of the unit lies a bed of sandy limestone. Sample S11/1 comes from a level 35.80 m above the base of the unit. The upper boundary of Unit 4 (the top of Hill A) is probably faulted. 3.b. Hill B (units 5–12: 219.30 m) Figure 1. Location of the study area (Derenjal Mountains) The section continues on Hill B, which is to the south of in east-central Iran (a), and of the sampled section of the Hill A. The valley separating these two hills is covered Niur Formation in the Derenjal Mountains (b). (Modified from Hairapetian et al. 2008 with permission from Acta by alluvium. Palaeontologica Polonica). Base of the strata exposed on Hill B. Co-ordinates 34◦ 5 2.9 N and 56◦ 48 14.0 E; altitude 1040 m. Top of the section. Co-ordinates 34◦ 04 56.1 N and 56◦ 48 14.8 E; altitude 1082 m. are considerable lithological differences between these Unit 5. 63.00 m. White medium- to thin-bedded two sections. The sandstone members (units 5, 8, 10, sandstone (quartz arenite) with cross-bedding and fine 14) recognized in the section in the Derenjal Mountains lamination in some intervals. An interval of brown (Fig. 3) are missing in the type section of the Niur sandstone occurs in the middle part of the unit. Formation in the Ozbak-Kuh Mountains. The faulted Unit 6. 6.70 m. Brown thin-bedded sandy limestone. Ordovician–Silurian contact is overlain by dolostones Silicified brachiopods (mostly unstudied spiriferids) followed mainly by coral-bearing limestones higher in are quite common. Conodont sample: S12, 6.50 m the type section (Ruttner, Nabavi & Hajian, 1968). above the base of the unit. However, contrary to the original description of the Unit 7. 33.80 m. Grey to brown, medium- to thin- section in the Derenjal Mountains by Ruttner, Nabavi & bedded bioclastic packstone and grainstone rich in Hajian (1968), the lower contact of the Niur Formation brachiopods, corals, bryozoans, orthoconic nautiloids is also faulted (Bruton, Wright & Hamedi, 2004; and tentaculites. The uppermost beds of the unit are Ghobadi Pour et al. 2006). represented by sandy limestone. Conodont samples In the Derenjal Mountains, the Silurian is exposed (all levels are measured from the base of Unit 7): S13, on three major hills: A, B and C (Figs 2, 3). A general 13.00 m; S14, 16.30 m; S15, 19.40 m; S16, 28.50 m; description of the section is provided below.
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