Conodonts from the Niur Formation (Silurian) of the ...

c Cambridge University Press 2013 Geol. Mag. 150 (4 ), 2013, pp. 639–650. doi:10.1017/S001675681200088X

639

Conodonts from the Niur Formation (Silurian) of the Derenjal Mountains, Central Iran P. M Ä N N I K ∗ †, C . G . M I L L E R ‡ & V. H A I R A P E T I A N §

∗ 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

The Lower Palaeozoic of Iran including the Silurian, is well exposed in several regions: in the north along the southern coast of the Caspian Sea (Alborz Mountains and the Kope Dagh zone), in east-central Iran (Kerman– Saghand and Tabas regions; Fig. 1) and in the Zagros Basin (Gahkum and Faraghan mountains). The section studied here is located about 65 km northwest of Tabas, on the eastern side of the Dahaneh-e-Kolut Gorge in the Derenjal Mountains (Figs 1, 2). Here, approximately 551 m of the Silurian Niur Formation is exposed (Fig. 3). Silurian conodonts have been reported from the Derenjal Mountains by Ruttner, Nabavi & Hajian (1968) and Hamedi et al. (1997). However, both publications provide lists of taxa and no illustrations. The only known illustrations of Niur Formation conodonts are from the Huk section located 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 conodont identifications and associated ages, and to briefly discuss the palaeogeographic affinities of the fauna. 2. Materials and methods

Forty-two samples were collected by V. Hairapetian during fieldwork in 2004 and 2007. Only calcareous intervals of the section were sampled and processed, with 17 samples yielding conodonts (Fig. 3). Sample † Author for correspondence: [email protected]

http://journals.cambridge.org

Downloaded: 11 Jun 2013

weight varied between 0.5 and 3 kg and the number of specimens per productive sample from 2 to 361. All samples were dissolved in buffered 10 % acetic acid and the residues washed through a 15.6 μm sieve. All residues were picked completely; heavy liquid separation techniques were not applied. Illustrated specimens were gold-palladium coated and photographed using a Phillips XL-30 scanning electron microscope at the Natural History Museum, London. All material is deposited at the Department of Palaeontology, Natural History Museum (prefix NHMUK PM X). Where conodont apparatus structure and therefore biological positioning of elements at generic level is proven from natural assemblages, the P1 , P2 , M, S0-X notation outlined by Purnell, Donoghue & Aldridge (2000) is followed. When there is no evidence for homology between different element types, the traditional Pa, Pb, M, Sa, Sb, Sc notation has been followed.

3. Geographical and geological setting

The Niur Formation was formally established by Ruttner, Nabavi & Hajian (1968). In the studied section, the Niur Formation is underlain by fossiliferous limestone of the Shirgesht Formation of Early to Middle Ordovician age and overlain by the siliciclastic Padeha Formation of presumed Early Devonian age. The type section of the Niur Formation is located in the vicinity of Niur village in the Ozbak-Kuh Mountains, c. 90 km northeast of the section discussed in this paper. The section studied here in the Derenjal Mountains was referred to as the ‘reference section’ of the Niur Formation by Ruttner, Nabavi & Hajian (1968). There

640

P. M Ä N N I K A N D O T H E R S 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◦ 48 1 4.0 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 thinbedded 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. Top of Unit 3. 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) in east-central Iran (a), and of the sampled section of the Niur Formation in the Derenjal Mountains (b). (Modified from Hairapetian et al. 2008 with permission from Acta Palaeontologica Polonica).

are considerable lithological differences between these two sections. The sandstone members (units 5, 8, 10, 14) recognized in the section in the Derenjal Mountains (Fig. 3) are missing in the type section of the Niur Formation in the Ozbak-Kuh Mountains. The faulted Ordovician–Silurian contact is overlain by dolostones followed mainly by coral-bearing limestones higher in the type section (Ruttner, Nabavi & Hajian, 1968). However, contrary to the original description of the section in the Derenjal Mountains by Ruttner, Nabavi & Hajian (1968), the lower contact of the Niur Formation is also faulted (Bruton, Wright & Hamedi, 2004; Ghobadi Pour et al. 2006). In the Derenjal Mountains, the Silurian is exposed on three major hills: A, B and C (Figs 2, 3). A general description of the section is provided below.



The section continues on Hill B, which is to the south of Hill A. The valley separating these two hills is covered by alluvium. 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. Unit 5. 63.00 m. White medium- to thin-bedded sandstone (quartz arenite) with cross-bedding and fine lamination in some intervals. An interval of brown sandstone occurs in the middle part of the unit. Unit 6. 6.70 m. Brown thin-bedded sandy limestone. Silicified brachiopods (mostly unstudied spiriferids) are quite common. Conodont sample: S12, 6.50 m above the base of the unit. Unit 7. 33.80 m. Grey to brown, medium- to thinbedded bioclastic packstone and grainstone rich in brachiopods, corals, bryozoans, orthoconic nautiloids and tentaculites. The uppermost beds of the unit are represented by sandy limestone. Conodont samples (all levels are measured from the base of Unit 7): S13, 13.00 m; S14, 16.30 m; S15, 19.40 m; S16, 28.50 m; S17, 31.00 m. Macrofossils Mesoleptostrophia

Conodonts from the Niur Formation

641

Figure 2. (Colour online) Field photographs of the section of the Niur Formation, Derenjal Mountains, east-central Iran. (a) Upper part (units 19 and 20, Hill C) of the section and the boundary between the Niur (Silurian) and the overlying Padeha (Devonian) formations; dashed line indicates base of the lowermost white quartzitic bed of the Padeha Formation. (b) Upper part of Hill B (units 10 to 12) and lower part of Hill C (units 13 to 16), view from the north. (c) Hill B, units 5 to 10, view from the west. (d) General view of the basal part of the section (Hill A, units 2 and 3).



642

P. M Ä N N I K A N D O T H E R S

Figure 3. Studied section and distribution of conodonts. From left to right: systems, formations; hills where a particular interval of the section was studied; described units; lithological log (arrows below and above the log indicate that the section continues in both directions); locations and numbers of samples processed for conodonts (numbers in bold – samples that yielded conodonts); number of specimens in a sample; distribution of conodonts; conodont zones (after Cramer et al. 2011b) and general stratigraphy (Stage, Series, System). Grey boxes in the column of conodont zonation indicate zones (part of) which were recognized in the studied section. Abbreviations: Pt. – Pterospathodus; a. –amorphognathoides; p. – pennatus; K. – Kockelella; Oz. – Ozarkodina; s. – sagitta; o. – ortus; v. – variabilis; Anc. – Ancoradella; Pol. – Polygnathoides; S. Z. – Superzone; I. Z. – Interval Zone; Gorst. – Gorstian.




643

(Mesoleptostrophia) sp., Isorthis (Ovalella) inflata Popov, Modzalevskaya & Ghobadi Pour, Dalejina? rashidii Popov, Modzalevskaya & Ghobadi Pour, Stegocornu denisae Popov, Modzalevskaya & Ghobadi Pour, Rhyidorhachis? sp., Stegorhynchus? sp., Hercotrema sp. and Striispirifer? ocissimus Popov, Modzalevskaya & Ghobadi Pour were identified from sample S14 (Hairapetian et al. 2012). Unit 8. 14.20 m. White medium- to thin-bedded allochemic sandstone. At some levels poorly preserved brachiopods occur. Unit 9. 7.80 m. Intercalation of brown medium- to thin-bedded sandy limestone and dolomitic limestone. Rare fragments of brachiopods have been found. Unit 10. 77.70 m. White medium- to thin-bedded allochemic sandstone. At some levels poorly preserved brachiopods occur. Unit 11. 11.70 m. Grey thin-bedded bioclastic mudstone, wackestone and grainstone with brachiopods and corals. At the top of the unit, a ∼ 0.15 m thick bed of yellow biodetrital limestone with relatively coarse lithoclasts occurs. Conodont samples (both levels are measured from the base of Unit 11): S18, 7.80 m; S19, 10.70 m. Unit 12. 4.40 m. Brown to red sandy limestone.

Unit 17. 1.50 m. Grey to yellow thin-bedded fossiliferous marlstone. Conodont sample S29 is from 0.15 m above the base of the unit. Unit 18. 20.50 m. Grey thin- to medium-bedded dolomitic bioclastic packstone. The sample S30 comes from 12.10 m above the base of the unit. Unit 19. 26.20 m. Grey thin- to medium-bedded limestone with crinoids, brachiopods and corals. Conodont samples (all levels are measured from the base of Unit 19): S31, 1.30 m; S32, 8.80 m; S33, 10.10 m; S34, 12.10 m; S35, 19.70 m. Sample S31 contains unidentified spiriferids and rhynchonellids; a bed 3 m above sample S31 yields some unidentified rhynchonellids, atrypids and spiriferids; in sample S35 brachiopods Lyssatrypa sp. and also some athyridides were found. Unit 20. 35.10 m. Brown medium-bedded more or less calcareous dolostone with some sand in the uppermost beds. Some poorly preserved brachiopods and corals(?) occur at the base of the unit. Sample S36 was taken at 13.10 m above the base of the unit. In the upper part of the section, the boundary between the Niur Formation and the overlying white quartzite of the Padeha Formation is conformable and transitional.

3.c. Hill C (units 13–20: 196.30 m)

4. Age of the Niur Formation

The section continues without a gap in the succession to Hill C. Base of the section on Hill C. Co-ordinates 34◦ 04 49.8 N and 56◦ 48 6.4 E; altitude 1078 m. Top of section. Co-ordinates 34◦ 04 44.6 N and ◦ 56 48 7.3 E; altitude 1069 m. Unit 13. 34.20 m. Highly fossiliferous bioclastic dark grey thin-bedded argillaceous wackestone and packstone. Conodont samples (all levels are measured from the base of Unit 13): S20, 1.30 m; S21, 4.90 m; S22, 18.10 m. Unit 14. 7.00 m. Brown medium- to thin-bedded partly calcareous (dolomitic?) sandstone. Unit 15. 40.80 m. Grey medium- to thin-bedded argillaceous bioclastic limestone (mudstone, wackestone, packstone and grainstone) with brachiopods and corals. Conodont samples (all levels are measured from the base of Unit 15): S22/1, 4.15 m; S22/2, 8.20 m; S22/3, 9.00 m; S23, 13.80 m; S23/1, 30.20 m; S23/2, 34.90 m; S23/3, 36.70 m; S24, 40.80 m. Samples S22 and S23 contain brachiopods Isorthis sp. sensu lato and some rhynchonellids. Unit 16. 31.00 m. Grey thin- to medium-bedded bioclastic packstone with brachiopods and dolostone interbeds. Two beds rich in corals, possibly with algae and bryozoans, occur in intervals 6.00–6.20 m and 11.30–11.45 m above the base of the unit. Additionally, a 0.20 m thick bed of coral limestone was observed in the interval 7.90–8.10 m above the base. Conodont samples (all levels are measured from the base of Unit 16): S25, 3.10 m; S26, 5.00 m; S27, 14.30 m; S28, 29.20 m.

4.a. Conodonts



Based on conodonts (identified by O. H. Walliser), but also on some other faunas from the Derenjal Mountains, a Llandovery to (probably) Pridoli age was suggested for the Niur Formation by Ruttner, Nabavi & Hajian (1968). These conclusions were later supported by R. J. Aldridge (in Hamedi et al. 1997). Weddige (1984) discussed conodonts from the Niur Formation exposed in the Huk locality in the Ozbak-Kuh Mountains and in a section in the Binalud Mountains near Mashad in NE Iran. 4.a.1. Llandovery–Wenlock

One sample studied by Ruttner, Nabavi & Hajian (1968; W1119 from bed no. 12, most probably equivalent to our Unit 7) yielded Hadrognathus staurognathoides s.f. Walliser. Other taxa identified were Lonchodina sp. s.f., Neoprioniodus sp. s.f. and Ozarkodina sp. s.f. Without restudying these specimens, no valuable stratigraphical information can be obtained. However, a late Valentinian ( = approximately Llandovery) to early Wenlock age suggested by Ruttner, Nabavi & Hajian (1968) for this level agrees, in general, with our data. The conodont assemblage from Unit 7 (our samples S15–S17; Fig. 3) includes two undescribed taxa, Ozarkodina sp. nov. (aff. Ozarkodina sp. C Mabillard & Aldridge) (Fig. 4o) and Ozarkodina sp. nov. A (Fig. 4f), along with Distomodus staurognathoides, Wurmiella ex gr. excavata and some poorly preserved elements of Panderodus and Oulodus?. D. staurognathoides

644


Figure 4. Selected conodonts from the Niur Formation. Scale bar corresponds to 0.1 mm. (a) Wurmiella ex. gr. excavata (Branson & Mehl); NHMUK PM X 3592, lateral view of P1 element, sample S15. (b–d) Wurmiella excavata; (b) NHMUK PM X 3591, lateral view of P1 element, sample S20; (c) NHMUK PM X 3593, posterior view of S1–2 element, sample S21; (d) NHMUK PM X 3594, lateral view of P2 element, sample S21. (e) Distomodus staurognathoides (Walliser); NHMUK PM X 3275, upper view of Pa element,




645

indicates that the Unit 7 limestone cannot be older than middle Aeronian and corresponds to the D. staurognathoides Zone (Loydell, Nestor & Männik, 2010). W. ex gr. excavata in this unit (Fig. 4a) is morphologically quite similar to W. excavata puskuensis (Männik), an older form of the W. excavata lineage. W. e. puskuensis is known to occur in strata of late Rhuddanian and Aeronian age (Männik, 1994). If the Wurmiella specimens in samples S12 and S15 from Unit 7 are really W. e. puskuensis, the unit (at least the lower and middle parts of it; Fig. 3) is middle to late Aeronian in age. Ozarkodina sp. C Mabillard & Aldridge has been described from the Telychian of the Marloes Bay section, SW Dyfed, Wales (Mabillard & Aldridge, 1983). However, our specimens of Ozarkodina sp. nov. (aff. Ozarkodina sp. C Mabillard & Aldridge), although in general morphologically similar to those of Ozarkodina sp. C Mabillard & Aldridge, evidently represent another taxon which could be an older representative of the same lineage (see taxonomic discussion in Section 7 below). Hence, the conodont data suggest that the Aeronian–Telychian boundary most probably lies within Unit 7 (Fig. 3). The next samples, S18 and S19, come from the upper part of Unit 11. Sample S19 yields the richest and most abundant fauna (361 specimens) in the studied section (Fig. 3). The fauna recovered includes Pterospathodus amorphognathoides lennarti (Fig. 4i–k), D. staurognathoides, Pseudooneotodus tricornis (Fig. 4q), Ozarkodina aff. waugoolaensis (Fig. 4l), Aspelundia? sp., Wurmiella? sp., Oulodus? spp. and some poorly known or undescribed taxa: Ozarkodina sp. nov. (aff. Ozarkodina sp. A Mabillard & Aldridge) (Fig. 4h) and gen. et sp. nov. A (aff. I.? sandersi Mabillard & Aldridge) (Fig. 4r–s). Gen. et sp. nov. A (aff. I.? sandersi Mabillard & Aldridge) first appears in sample S18 (together with Ozarkodina aff. waugoolaensis) but is particularly common in S19. The occurrence of Pt. a. lennarti in S19 indicates that this sample comes from the Pt. a. lennarti Zone and is middle Telychian in age (Männik, 2007a; Fig. 3). As the samples from just above Unit 12, from the lowermost and middle parts of Unit 13, yield only W. excavata excavata (Fig. 4b–d), a taxon characteristic of Sheinwoodian and younger strata, it seems most probable that the Llandovery–Wenlock boundary in the

section should be placed between samples S19 and S20. Lack of any other taxa in the lowermost part of Unit 13 indicates that S20 comes from a level above the last datum of the Ireviken Event ( = the level of extinction of D. staurognathoides; Jeppsson, 1998), i.e. well above the Llandovery–Wenlock boundary (Männik, 2007b). In this paper, the Llandovery–Wenlock boundary is tentatively drawn at the boundary between units 12 and 13, at a level where sandy limestone of Unit 12 is replaced by argillaceous limestone of Unit 13. Conodonts currently available do not allow precise dating of the strata just above this level. As a result, the base of Unit 13 up to the upper part of Unit 15 is tentatively correlated with the Wenlock in this paper (Fig. 3). 4.a.2. Wenlock–Ludlow

The conodont sample studied by Ruttner, Nabavi & Hajian (1968; W1155 from bed 28) probably corresponds to a level within our Unit 19. Spathognathodus steinhornensis subsp. indet. s.f. Walliser, Ozarkodina denckmanni s.f. Ziegler, Lonchodina greilingi s.f. Walliser, Oz. media s.f. Walliser, Trichonodella inconstans s.f. Walliser, Neoprioniodus primus s.f. (Branson & Mehl), S. primus s.f. (Branson & Mehl) and Lonchodina sp. A s.f. were identified in this sample by O. H. Walliser. Based on this assemblage, a late Ludlow to earliest Devonian age was suggested, with a proviso that it was probably latest Ludlow (Ludfordian) in age. In modern terminology, the assemblage probably included Oz. confluens (Branson & Mehl), W. excavata and Oulodus? sp. Assignment of elements identified as S. steinhornensis subsp. indet. s.f. to any known apparatus is more complicated. It is most probably a representative of the Oz. steinhornensis s.l. ‘group’ appearing in most known successions in the late Ludfordian. More material needs to be found and studied to decide if it belongs to Zieglerodina or to Genus W eosteinhornensis (Walliser) sensu Murphy, Valenzuela-Ríos & Carls (2004). All samples processed by us from Unit 19 were barren. The uppermost conodont in our collection Zieglerodina? sp. (Figs 3, 4p) comes from sample

sample S12. (f) Ozarkodina sp. nov. A; NHMUK PM X 3595, lateral view of P1 element, sample S15. (g) Ozarkodina cf. roopaensis Viira; NHMUK PM X 3271 lateral view of P1 element, sample S26. (h) Ozarkodina sp. nov. (aff. Ozarkodina sp. A Mabillard & Aldridge); NHMUK PM X 3596, lateral view of P1 element, sample S19. (i–k) Pterospathodus amorphognathoides lennarti Männik; (i) NHMUK PM X 3597, lateral view of Pb2 element; (j) NHMUK PM X 3598, upper view of Pa element; (k) NHMUK PM X 3599, lateral view of Pb1 element. All specimens from sample S19. (l) Ozarkodina aff. waugoolaensis Bischoff; NHMUK PM X 3600, lateral view of P1 element, sample S19. (m) Ozarkodina ex gr. snajdri (Walliser); NHMUK PM X 3268, upper (m1) and lower (m2) views of P1 element, sample S28. (n) Ozarkodina bohemica bohemica (Walliser); NHMUK PM X 3273, lateral (n1) and lower (n2) views of P1 element, sample S24. (o) Ozarkodina sp. nov. (aff. Ozarkodina sp. C Mabillard & Aldridge); NHMUK PM X 3601, lateral view of P1 element, sample S12. (p) Zieglerodina? sp.; NHMUK PM X 3602, lateral (p1) and lower (p2) views of P1 element, sample S30. (q) Pseudooneotodus tricornis Drygant; NHMUK PM X 3277, upper view, sample S19. (r–s) gen. et sp. nov. A (aff. I.? sandersi Mabillard & Aldridge); (r) NHMUK PM X 3603, lateral (r1), lower (r2) and upper (r3) views of Pa element; (s) NHMUK PM X 3604, upper (s1) and lower (s2) views of ‘ambalodiform’ P? element. Both specimens from sample S19.



646


S30, from the middle part of Unit 18. Zieglerodina, which first appears in the uppermost Ludlow, is more characteristic of the Pridoli and particularly of the lowermost Devonian (Carls, Slavik & Valenzuela-Ríos, 2007). As a result, our material does not allow improved dating of the uppermost part of the studied section but it does not contradict the conclusions of Ruttner, Nabavi & Hajian (1968). Based on our collections, two other intervals in the studied section can be dated precisely. Oz. bohemica (Figs 3, 4n) has been identified from the lowermost part of Unit 16, and probably from the uppermost beds of Unit 15. At least two chronological subspecies are known in the Oz. bohemica lineage: Oz. b. longa Jeppsson characteristic of the upper Wenlock (Homerian) and Oz. b. bohemica occurring in the lowermost Ludlow (in the lowermost Gorstian; Jeppsson & Aldridge, 2000; Calner & Jeppsson, 2003; Jeppsson, Eriksson & Calner, 2006). The P1 element of Oz. b. longa is relatively long and low, whereas that of Oz. b. bohemica is shorter and higher. However, variation in these dimensions is only distinct in larger collections, particularly in those containing many specimens of Oz. b. longa. As a result, when only a small number of specimens are available, it is often difficult to tell which subspecies is present. Only two well-preserved P1 elements from samples S24 and S25 (Fig. 3) have been recovered from the Niur Formation. However, as only short and high specimens with large basal cavities (Fig. 4n) occur, it seems most probable that our specimens belong to Oz. b. bohemica. In this case, the lowermost part of Unit 16 is earliest Ludlow (earliest Gorstian) in age and correlates with a level in the Kockelella crassa Zone (Fig. 3). Specimens of Oz. bohemica from samples S23/1 and S23/2 are too poorly preserved to allow subspecies identification. In sample S26, in the next productive sample above the level yielding the uppermost Oz. b. bohemica, six poorly preserved specimens of robust Ozarkodina were found (Figs 3, 4g). Morphologically they seem to be closest to Oz. roopaensis, which occurs in the Paadla Stage in the Baltic (Gorstian to lower Ludfordian) corresponding to an interval from the K. crassa Zone to the Oz. snajdri Interval Zone (Viira, 1994; Cramer et al. 2011a). Two samples (S28 and S29) from the topmost beds of Unit 19 yield 29 specimens of Oz. ex gr. snajdri (Figs 3, 4m), a taxon characteristic of the middle to late Ludlow (Corradini & Serpagli, 1999; Viira, 1999). From the data presented above, it is evident that Unit 16 in the studied section corresponds to an interval from the lower(most) Gorstian to the middle Ludfordian. Tentatively, the Wenlock–Ludlow boundary in the section is placed at the boundary between units 15 and 16 (Fig. 3). However, as units 13, 14 and 15 cannot be reliably dated, the boundary could be at a lower level in the section. Rugose corals from units 13 and 15 suggest a Ludlow age for these strata (Section 4.b). Based on currently available data, it is not possible to locate the Ludlow–Pridoli boundary in the section.



4.b. Other faunas

Dating of the Niur Formation based on other fossils is consistent with the results of the conodont studies. Rugose and tabulate corals, stromatoporoids, brachiopods and ostracods are common in the studied section. Flügel & Saleh (1970) studied the diverse but mostly endemic rugose faunas in detail. From the outcrops on Hill A (units 2 and 4) and the middle part of Hill B (Unit 7), they identified Grewingkia alternata Saleh, Paliphyllum (Paliphyllum) oblongaecystosum Saleh, Schlotheimophyllum sp., Streptelasma shirgeshtensis Saleh, S. ruttneri Saleh and Tryplasma sp., suggesting a Llandovery age for these strata (Flügel & Saleh, 1970). Characteristic Ludlow taxa were reported on Hill C from Unit 19 (beds 27–30 in Ruttner, Nabavi & Hajian, 1968), Cystiphyllum (Holmophyllum) pauciseptatum Flügel & Saleh, and from units 13 and 15 (beds 24 and 26), Gyalophyllum (Coronoruga) sp. From Hill A, units 2 and 4, Hubmann (1991a) described tabulate corals of Llandovery age, including Eocatenipora nicholsoni Kiaer, Catenipora obliqua Fischer-Benzon, C. micropora Whitfield, C. gotlandica Yabe, C. cf. louisvillensis Stumm, C. cf. jarviki Stasinska, C. khorasanensis Hubmann and Halysites labyrinthicus Goldfuss. The Llandovery stromatoporoid Ecclimadictyon pseudofastigiatum (Riabinin) described by Flügel (1969) most probably comes from the lower part of Unit 4 exposed in an outcrop on the western side of the Dahaneh-e-Kolut Gorge. Another stromatoporoid, Clathrodictyon pustulatum Yavorsky, was identified by him from units 13 and 15 (beds 24 and 26). Ostracods Pachydomella wolfei Copeland, Steusloffina cuneata (Steusloff), Arcuaria? triangulata Neckaja and Punctobeecherella punctata Copeland in association with pentamerid brachiopods from Unit 2 suggest a mid Aeronian age for this unit (Hairapetian et al. 2011). The Stegocornu brachiopod association including the key rhynchonellid taxon Stegocornu denisae Popov, Modzalevskaya & Ghobadi Pour was collected from sample S14 (Unit 7) (Hairapetian et al. 2012). The dispersion of Stegocornu association brachiopods in Central Iran, Kope-Dagh and Afghanistan represents a significant Aeronian regional event (Hairapetian et al. 2012).

5. Palaeogeographic affinities of faunas

Based on the palaeogeographic reconstruction of Cocks & Torsvik (2002, fig. 8) for the late-Early Silurian – Late Silurian, the Iranian terranes, including Central Iran (Lut), Alborz, Sanand and Zagros terranes, were located between palaeolatitudes 15◦ and 30◦ S, and formed part of Middle Eastern periGondwanan/Gondwanan terranes (Fig. 5). East of modern Central Iran, a vast shallow-water shelf with a dominant carbonate-siliciclastic sedimentation regime developed in the Silurian. During the Llandovery, the Zagros and probably Sanand terranes (located SW


647

Figure 5. Palaeogeographic position of Iranian terranes during the Early Silurian (Llandovery). Base map from Cocks & Torsvik (2002), with modifications. Abbreviations: Al. – Alborz; Sa. – Sanand; Za. – Zagros. (Modified from Hairapetian et al. 2008 with permission from Acta Palaeontologica Polonica).

of the Central Iran terrane) were attached to the northern margin of Gondwana. These regions have a similar sedimentological and biological history to the Arabian domain (Lüning et al. 2000; Rickards, Wright & Hamedi, 2000; Wendt et al. 2002, 2005; Ruban, Al-Husseini & Iwasaki, 2007; Ghavidel-Syooki & Winchester-Seeto, 2004; Ghavidel-Syooki et al. 2011). Despite the peri-Gondwanan setting of Central Iran (according to the palaeogeographic reconstructions of Cocks & Torsvik, 2002), and its considerable distance from the low-latitude Laurentia and Baltica in the Silurian (Fig. 5), ostracods from the lower Niur Formation (from Unit 2) have strong Laurentian affinities. Taxa include Pachydomella wolfei, Steusloffina cuneata, Arcuaria? triangulata, Punctobeecherella punctata and several species of the genera Elliptocyprites, Punctaparchites, Aechmina, Dicranella, Lomatopisthia and Ovornina (Hairapetian et al. 2011). The ostracod fauna has some similarities to Baltic Palaeocontinent Late Ordovician – Early Silurian faunas. Common taxa include A.? triangulata, S. cuneata and several species of Bairdiocypris, Bulbosclerites and Aechmina (Neckaja, 1958; Meidla, 1996). Some taxa (S. cuneata, Bairdiocypris attenuatus Melnikova & Michailova), characteristic of the lower Niur Formation, have also been reported from Uzbekistan (Melnikova & Michailova, 1999). Assemblages of halysitid corals, particularly representatives of the genus Catenipora from the Niur Formation, also suggest palaeobiogeographic affinities with faunas known from the Laurussian terranes (Hubmann, 1991a,b; Flügel & Hubmann, 1993). Palaeogeographical distribution data of Niur Formation conodonts do not contradict any of the palaeo-



geographical conclusions based on other fossil groups. Poorly known conodont taxa in the Llandovery part of the studied section means that limited conclusions can be drawn at these levels. Conodonts occurring in units 7 and 11, such as Ozarkodina sp. nov. (aff. Ozarkodina sp. C Mabillard & Aldridge) (Fig. 4o), Ozarkodina sp. nov. (aff. Ozarkodina sp. A Mabillard & Aldridge) (Fig. 4h) and gen. et sp. nov. A (aff. I.? sandersi Mabillard & Aldridge) (Fig. 4r–s) seem to be restricted to this geographical region. However, their possible closest relatives are known from Avalonia on the other side of the Rheic Ocean in the Marloes Bay section, SW Dyfed, Wales (Mabillard & Aldridge, 1983). Many of the other conodont taxa found in the Llandovery part of the studied section (e.g. D. staurognathoides, Pseudooneotodus tricornis) are cosmopolitan. The distribution of Pt. a. lennarti is less well known, but based on data available, this taxon (as Pt. amorphognathoides in general) is also cosmopolitan (Männik, 1998). Conodonts are rare in strata of Wenlock and younger age in the studied section, but all taxa recognized are known away from Gondwana (Fig. 3). The data above suggest that for many taxa the Rheic Ocean separating Gondwana from Laurussia was not a major barrier. Conodonts from the Tafilalt region of Morocco have also supported this hypothesis (Männik, Loydell & Lubeseder, 2011). Tafilalt conodonts from the lowermost Tamaghrout Formation (lowermost Wenlock), from the strata corresponding to the Pt. pennatus procerus Superzone sensu Jeppsson (1997) are identical to Baltica palaeocontinent conodonts from the same interval. The description of the typical Baltic thelodont Loganellia cf. grossi Fredholm in units 16 and 19 in the studied Niur Formation section also suggests that there were migration paths between Gondwana and Laurussia (Hairapetian, Blom & Miller, 2008).

6. Conclusions

(1) Conodonts and other fossil groups suggest a Llandovery to Ludlow age for the Niur Formation. However, currently available faunas do not allow dating of the strata above Unit 16. (2) Unit 7 is either of Aeronian or Telychian in age. (3) Unit 11 (at least its upper part) is of middle Telychian in age and corresponds to the Pt. a. lennarti Zone. (4) The Llandovery–Wenlock boundary lies between samples S19 and S20 and is tentatively drawn as corresponding to the boundary between units 12 and 13. (5) The lowermost part of Unit 16 is of earliest Ludlow in age and corresponds to the Kockelella crassa Zone. (6) The Wenlock–Ludlow boundary lies below Unit 16 and is tentatively drawn at the boundary between units 15 and 16. However, as units 13, 14 and 15

648


cannot be reliably dated, it cannot be excluded that the boundary lies at a lower level in the section. (7) The uppermost Unit 16 is of late Ludlow (Ludfordian) age and corresponds to the Oz. snajdri Interval Zone. (8) Although the latest palaeogeographical reconstruction shows that present-day Iran was located far away from Baltica and Laurentia, on the other side of the Rheic Ocean, this seems not to have been a major migration barrier for many different organisms including the conodonts. 7. Comments on some taxa

Brief comments on some taxa in open nomenclature are given. A description of conodonts from the Derenjal section will be presented in another paper; key taxa are illustrated in Figure 4. Genus Ozarkodina Branson & Mehl, 1933 Ozarkodina sp. nov. (aff. Ozarkodina sp. C Mabillard & Aldridge) Figure 4o Remarks. P1 elements of the apparatus identified here as Ozarkodina sp. nov. (aff. Ozarkodina sp. C Mabillard & Aldridge) bear some similarity to those described by Mabillard & Aldridge (1983, pl. 3, figs 9, 10) as Ozarkodina sp. C. However, our specimens differ by having shorter and higher blades and less prominent cusps. Ozarkodina sp. nov. (aff. Ozarkodina sp. A Mabillard & Aldridge) Figure 4h Remarks. P1 elements of Ozarkodina sp. nov. (aff. Ozarkodina sp. A Mabillard & Aldridge) resemble Ozarkodina sp. A of Mabillard & Aldridge (1983, pl. 3, figs 7, 8) in general configuration only. Both taxa have short P1 elements with a high ventral blade and denticles on the dorsal blade that rapidly decrease in height from the cusp. Morphological differences are evident between these two taxa. According to Mabillard & Aldridge (1983), the Pa ( = P1 ) element of Ozarkodina sp. A has 3–4 short, erect, broad denticles of subequal size on its ventral blade whereas our specimens have relatively small denticles with one considerably larger denticle in the ventralmost part of the blade. The cusp of Ozarkodina sp. A is broad and distinct whereas our specimens have cusps that do not differ in size from the other denticles. The P1 element basal cavity of Ozarkodina sp. nov. (aff. Ozarkodina sp. A Mabillard & Aldridge) is shorter but more strongly flared than in Ozarkodina sp. A. gen. et sp. nov. A (aff. I.? sandersi Mabillard & Aldridge) Figure 4r–s Remarks. According to the original description, the apparatus of I.? sandersi contains a straight thin-



walled broad-based P element with icriodelliform denticulation on its anterior process, an incurved M element with prominent anticusp and denticulated posterior process, a sagittodontiform ‘S’ element, and symmetrical to asymmetrical delicate Sa–Sb elements (Mabillard & Aldridge, 1983, p. 33). A similar suite of elements can be recognized in our gen. et sp. nov. A apparatus, along with an ‘ambalodiform’(?) element of distinct morphology (Fig. 4s). This additional element has quite well-developed ‘icriodelliform’ denticulation on its processes. This element is missing in the 247 specimen collection of I.? sandersi described by Mabillard & Aldridge (1983). The apparatus of I.? sandersi is evidently icriodelliform whereas that of gen. et sp. nov. A (aff. I.? sandersi Mabillard & Aldridge) represents another, more complicated apparatus type. Acknowledgements. V. Hairapetian was assisted by Drs Leonid E. Popov and Mansoureh Ghobadi Pour in the field. Constructive reviews were provided by two anonymous referees. The study of P. Männik was supported by the Estonian Science Foundation (grant no. 8907). His visit to the NHM in March 2010, and the use of the SEM to study conodonts, was financed by the SYNTHESYS Project GBTAF-294.

References BRANSON, E. B. & MEHL, M. G. 1933. Conodonts from the Bainbridge Formation (Silurian) of Missouri. University of Missouri Studies 8, 39–52. BRUTON, D. L., WRIGHT, A. J. & HAMEDI, M. A. 2004. Ordovician trilobites of Iran. Palaeontographica A 271, 111–49. CALNER, M. & JEPPSSON, L. 2003. Carbonate platform evolution and conodont stratigraphy during the middle Silurian Mulde Event, Gotland, Sweden. Geological Magazine 140, 173–203. CARLS, P., SLAVÍK, L. & VALENZUELA-RÍOS, J. I. 2007. Revisions of conodont biostratigraphy across the Silurian– Devonian boundary. Bulletin of Geosciences 82, 145– 64. COCKS, L. R. M. & TORSVIK, T. H. 2002. Earth geography from 500 to 400 million years ago: a faunal and palaeomagnetic review. Journal of the Geological Society, London 159, 631–44. CORRADINI, C. & SERPAGLI, E. 1999. A Silurian conodont biozonation from late Llandovery to end Pˇridoli in Sardinia (Italy). Bollettino della Società Paleontologica Italiana 37, 255–73. CRAMER, B. D., DAVIES, J. R., RAY, D. C., THOMAS, A. T. & CHERNS, L. 2011a. Siluria revisited: an introduction. In Siluria Revisited: A Field Guide. International Subcommission on Silurian Stratigraphy, Field Meeting 2011 (ed. D. C. Ray), pp. 7–28. International Subcommission on Silurian Stratigraphy. CRAMER, B. D., BRETT, C. E., MELCHIN, J. M., MÄNNIK, P., KLEFFNER, M. A., MCLAUGHLIN, P. I., LOYDELL, D. K., MUNNECKE, A., JEPPSSON, L., CORRADINI, C., BRUNTON, F. R. & SALTZMAN, M. R. 2011b. Revised correlation of Silurian Provincial Series of North America with global and regional chronostratigraphic units and δ 13 Ccarb chemostratigraphy. Lethaia 44, 185– 202.


649

FLÜGEL, E. 1969. Stromatoporen aus dem Silur des östlichen Iran. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte 1969/4, 209–19. FLÜGEL, H. W. & HUBMANN, B. 1993. Paläontologie und Plattentektonik am Beispiel proto- und paläotethyder Korallenfaunen. Jahrbuch der Geologischen Bundesanstalt 136, 27–37. FLÜGEL, H. W. & SALEH, H. 1970. Die paläozoischen Korallenfauna Ost-irans. 1 Rugose Korallen der Niur Formation (Silur). Jahrbuch der Geologischen Bundesanstalt 113, 267–302. GHAVIDEL-SYOOKI, M., ÁLVARO, J. J., POPOV, L., GHOBADI POUR, M., EHSANI, M. H. & SUYARKOVA, A. 2011. Stratigraphic evidence for the Hirnantian (latest Ordovician) glaciation in the Zagros Mountains, Iran. Palaeogeography, Palaeoclimatology, Palaeoecology 307, 1–16. GHAVIDEL-SYOOKI, M. & WINCHESTER-SEETO, T. 2004. Chitinozoan biostratigraphy and palaeogeography of Lower Silurian strata (Sarchahan Formation) in the Zagros Basin of southern Iran. In Palaeontological and Micropalaeontological Studies in Honour of Geoffrey Playford (eds J. R. Laurie & B. C. Foster), pp. 161–82. Memoirs of the Association of Australasian Palaeontologists 29. GHOBADI POUR, M., WILLIAMS, M., VANNIER, J., MEIDLA, T. & POPOV, L. E. 2006. Ordovician records from east central Iran. Acta Palaeontologica Polonica 51, 551–60. HAIRAPETIAN, V., BLOM, H. & MILLER, C. G. 2008. Silurian thelodonts from the Niur Formation, central Iran. Acta Palaeontologica Polonica 53, 85–95. HAIRAPETIAN, V., GHOBADI POUR, M., POPOV, L. & MODZALEVSKAYA, T. L. 2012. Stegocornu and associated brachiopods from the Silurian (Llandovery) of Central Iran. Estonian Journal of Earth Sciences 61, 82–104. HAIRAPETIAN, V., MOHIBULLAH, M., TILLEY, L. J., WILLIAMS, M., MILLER, C. G., AFZAL, J., GHOBADI POUR, M. & HASSAN HEJAZI, S. 2011. Early Silurian carbonate platform ostracods from Iran: a peri-Gondwanan fauna with strong Laurentian affinities. Gondwana Research 20, 645–53. HAMEDI, M. A., WRIGHT, A. J., ALDRIDGE, R. J., BOUCOT, A. J., BRUTON, D. L., CHATTERTON, B. D. E., JONES, P., NICOLL, R. S., RICKARDS, R. B. & ROSS, J. R. P. 1997. Cambrian to Silurian of East−Central Iran: new biostratigraphic and biogeographic data. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte 7, 412–24. HUBMANN, B. 1991a. Halysitidae aus dem tiefen Silur E-Irans (Niur-Formation). Jahrbuch der Geologischen Bundesanstalt 134, 711–33. HUBMANN, B. 1991b. Silurian Catenipora Lamarck – a guide to ancient latitudinal and faunal relationships. Anzeiger der Österreichischen Akademie der Wissenschaften, Mathematisch-Naturwissenschaftliche Klasse 128, 113–20. JEPPSSON, L. 1997. A new latest Telychian, Sheinwoodian and Early Homerian (Early Silurian) Standard Conodont Zonation. Transactions of the Royal Society of Edinburgh: Earth Sciences 88, 91–114. JEPPSSON, L. 1998. Silurian oceanic events: summary of general characteristics. In Silurian Cycles: Linkages of Dynamic Stratigraphy with Atmospheric, Oceanic and Tectonic Changes (eds E. Landing & M. E. Johnson), pp. 239–57. New York State Museum Bulletin 491. JEPPSSON, L. & ALDRIDGE, R. J. 2000. Ludlow (late Silurian) oceanic episodes and events. Journal of the Geological Society, London 157, 1137–48.



JEPPSSON, L., ERIKSSON, M. E. & CALNER, M. 2006. A latest Llandovery to latest Ludlow high-resolution biostratigraphy based on the Silurian of Gotland – a summary. GFF 128, 109–14. LOYDELL, D. K., NESTOR, V. & MÄNNIK, P. 2010. Integrated biostratigraphy of the lower Silurian of the Kolka54 core, Latvia. Geological Magazine 147, 253– 80. LÜNING, S., CRAIG, J., LOYDELL, D. K., ŠTORCH, P. & FITCHES, B. 2000. Lower Silurian ‘hot shales’ in North Africa and Arabia: regional distribution and depositional model. Earth-Science Reviews 49, 121–200. MABILLARD, J. E. & ALDRIDGE, R. J. 1983. Conodonts from the Coralliferous Group (Silurian) of Marloes Bay, South-West Dyfed, Wales. Geologica et Palaeontologica 17, 29–43. MÄNNIK, P. 1994. Conodonts from the Pusku Quarry, lower Llandovery, Estonia. Proceedings of Estonian Academy of Sciences, Geology 43, 183–91. MÄNNIK, P. 1998. Evolution and taxonomy of the Silurian conodont Pterospathodus. Palaeontology 41, 1001– 50. MÄNNIK, P. 2007a. An updated Telychian (Late Llandovery, Silurian) conodont zonation based on Baltic faunas. Lethaia 40, 45–60. MÄNNIK, P. 2007b. Some comments on the Telychian–early Sheinwoodian conodont faunas, events and stratigraphy. Acta Palaeontologica Sinica 46 (Suppl.), 305–10. MÄNNIK, P., LOYDELL, D. & LUBESEDER, S. 2011. Sheinwoodian (Silurian) conodonts and graptolites from NE Anti-Atlas, Morocco. Lethaia 44, 410–16. MEIDLA, T. 1996. Late Ordovician ostracodes of Estonia. Fossilia Baltica 2, 1–222. MELNIKOVA, L. M. & MICHAILOVA, E. D. 1999. Late Ashgill– Early Llandovery ostracodes from the Zeravshan– Gissar mountainous region (Shakhriomon-II reference section): Platycopida, Metacopida and Podocopida. Paleontological Journal 33, 392–402. MURPHY, M. A., VALENZUELA-RÍOS, J. I. & CARLS, P. 2004. On classification of Pˇridoli (Silurian)–Lochkovian (Devonian) Spathognathontidae (conodonts). University of California, Riverside Campus Museum Contribution 6, 1–25. NECKAJA, A. I. 1958. New species and genera of ostracodes of the Ordovician and Silurian of the Russian platform. Trudy VNIGRI 115, 349–73 [in Russian]. PURNELL, M. A., DONOGHUE, P. C. J. & ALDRIDGE, R. J. 2000. Orientation and anatomical notation in conodonts. Journal of Paleontology 74, 113–22. RICKARDS, R. B., WRIGHT, A. J. & HAMEDI, M. A. 2000. Late Ordovician and Early Silurian graptolites from southern Iran. Records of the Western Australian Museum, Supplement 58, 103–22. RUBAN, D. A., AL-HUSSEINI, M. I. & IWASAKI, Y. 2007. Review of Middle East Paleozoic plate tectonics. GeoArabia 12, 35–56. RUTTNER, A., NABAVI, M. H. & HAJIAN, J. 1968. Geology of the Shirgesht area (Tabas area, East Iran). Geological Survey of Iran, Reports 4, 1–133. VIIRA, V. 1994. A new upper Silurian conodont species from Estonia. Proceedings of Estonian Academy of Sciences, Geology 43, 32–7. VIIRA, V. 1999. Late Silurian conodont biostratigraphy in the northern East Baltic. Bollettino della Società Paleontologica Italiana 37, 299–310. WEDDIGE, K. 1984. Zur stratigraphy und paläontologie des Devons und Carbons von NE-Iran. Senckenbergiana Lethaea 65, 179–224.

650


WENDT, J., KAUFMANN, B., BELKA, Z., FARSAN, N. & KARIMI BAVANDPUR, A. 2002. Devonian/Lower Carboniferous stratigraphy, facies patterns and palaeogeography of Iran. Part I. Southeastern Iran. Acta Geologica Polonica 52, 129–68.



WENDT, J., KAUFMANN, B., BELKA, Z., FARSAN, N. & KARIMI BAVANDPUR, A. 2005. Devonian/Lower Carboniferous stratigraphy, facies patterns and palaeogeography of Iran. Part II. Northern and central Iran. Acta Geologica Polonica 55, 31–97.