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Terminal Cambrian and Early Ordovician (Tremadocian) Conodonts from Eastern Alborz, North-Central Iran

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Terminal Cambrian and Early Ordovician (Tremadocian) Conodonts from Eastern Alborz, North-Central Iran Alcheringa: An Australasian Journal of Palaeontology ISSN: 0311-5518 (Print) 1752-0754 (Online) Journal homepage: http://www.tandfonline.com/loi/talc20 Terminal Cambrian and Early Ordovician (Tremadocian) conodonts from Eastern Alborz, north-central Iran Hadi Jahangir, Mansoureh Ghobadi Pour, Alireza Ashuri & Arash Amini To cite this article: Hadi Jahangir, Mansoureh Ghobadi Pour, Alireza Ashuri & Arash Amini (2015): Terminal Cambrian and Early Ordovician (Tremadocian) conodonts from Eastern Alborz, north-central Iran, Alcheringa: An Australasian Journal of Palaeontology, DOI: 10.1080/03115518.2016.1118298 To link to this article: http://dx.doi.org/10.1080/03115518.2016.1118298 Published online: 14 Dec 2015. Submit your article to this journal Article views: 2 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=talc20 Download by: [Gazi University] Date: 17 December 2015, At: 08:12 Terminal Cambrian and Early Ordovician (Tremadocian) conodonts from Eastern Alborz, north-central Iran HADI JAHANGIR, MANSOUREH GHOBADI POUR*, ALIREZA ASHURI and ARASH AMINI HADI JAHANGIR,MANSOUREH GHOBADI POUR,ALIREZA ASHURI &ARASH AMINI, XX.XX.2015. Terminal Cambrian and Early Ordovician (Tremadocian) conodonts from Eastern Alborz, north-central Iran. Alcheringa ##, ###-###. ISSN 0311-5518. Uppermost Cambrian (Furongian) and Lower Ordovician (Tremadocian) deposits of eastern Alborz in northern Iran contain several successive low- to moderate-diversity conodont associations including 13 genera and 19 species of euconodonts, paraconodonts and protoconodonts, which define six biozones: 1, the Proconodontus muelleri;2,Eoconodontus notchpeakensis;3,Cordylodus andresi;4,Cordylodus proavus;5,Paltodus deltifer; and 6, Paroistodus proteus zones. With the exception of Cordylodus andresi, which is otherwise known from Baltoscandia and from the Oaxaquia terrane (Mexico), all index-taxa are geographically widespread, allowing long-range correlation within the Cold Domain or the North Atlantic Province, and in particular with Baltica. Invasion of euconodonts in the Alborz region, defined by the first occurrence of Proconodontus muelleri, coincides closely with a steady rise in sea level and termination of carbonate sedimentation, whereas the transition from the Proconodontus muelleri to Eoconodontus notchpeakensis zones occurs during a highstand interval unlike in Laurentian sequences. The interval corresponding to the Cordylodus andresi and Cordylodus proavus zones, and the transition from the Paltodus deltifer to Paroistodus proteus zones coincided with unstable sea levels and the formation of shoal complexes. The lower boundary of the Floian Stage can be provisionally placed slightly below the first documented occurrence of Acodus sp. cf. A. kechikaensis, somewhat below the second unit of andesitic lava flows in the Simeh-Kuh section. Hadi Jahangir [[email protected]] and Alireza Ashuri [[email protected]] Department of Geology, Faculty of Sciences, Ferdowsi University, Azadi Square, Mashhad 91775-1436, Iran; Mansoureh Ghobadi Pour* [[email protected]; [email protected]] and Arash Amini [[email protected]], Department of Geology, Faculty of Sciences, Golestan University, Gorgan 49138-15739, Iran. *Also affiliated with Department of Geology, National Museum of Wales, Cathays Park, Cardiff CF10 3NP, UK. Received 13.7.2015; revised 18.10.2015; accepted 6.11.2015. Key words: Terminal Cambrian, Ordovician, Tremadocian, conodonts, Alborz, Iran. CAMBRIAN (Furongian) conodonts have been known elements from the Deh-Molla section (Tables 1–3). from the Alborz region of northern Iran for more than However, the assemblages are diverse taxonomically 40 years; however, they have not been the focus of and include 13 genera and 20 species (Fig. 2). research since the pioneering publication by Müller The conodont fauna of the Simeh-Kuh section, (1973). Hence, the application of conodonts to the which represents the most complete Furongian to Lower lower Palaeozoic biostratigraphy of the region has been Ordovician sedimentary succession in the Alborz Downloaded by [Gazi University] at 08:12 17 December 2015 limited. Apart from a few preliminary reports on their region, was chosen as the major target of our study. presence in Lower to Middle Ordovician successions of The Furongian succession in Deh-Molla is important the eastern Alborz Mountains (Ghobadi Pour 2006, because, in addition to the Proconodontus muelleri, Ghobadi Pour et al. 2011a, b), Ordovician conodonts Eoconodontus notchpeakensis and Cordylodus andresi were practically unknown from the region. Only zones, it also preserves the Cordylodus proavus Zone recently has more comprehensive information on the (Fig. 2). Furongian conodont distributional data from late Tremadocian conodonts from the Deh-Molla section the Mila-Kuh section derive mainly from the paper by been published (Jahangir et al. 2014). This study aims Müller (1973). Our important supplementary informa- to analyse the taxonomic composition and stratigraphic tion supports the proposed conodont zonation for the distribution of terminal Cambrian (Furongian) and Early Furongian to Lower Ordovician (Tremadocian) succes- Ordovician (Tremadocian) conodont assemblages from sion of the Alborz region. the Simeh-Kuh, Mila-Kuh and Deh-Molla sections in Traditionally, the middle Cambrian to Ordovician the eastern Alborz Mountains, Semnan Province, Iran succession in the Alborz Mountains has been subdi- (Fig. 1). The conodont yield from the studied succes- vided into the middle to upper Cambrian Mila Forma- sions is fairly low; only 334 conodont elements were tion with the type section at Mila-Kuh introduced by recovered from the Simeh-Kuh section and 109 Stöcklin et al.(1964), and the Ordovician Lashkarak Formation with its type section in Alam-Kuh introduced by Gansser & Huber (1962); however, the relationships © 2015 Association of Australasian Palaeontologists http://dx.doi.org/10.1080/03115518.2016.1118298 between these two lithostratigraphic units have 2 HADI JAHANGIR et al. ALCHERINGA Fig. 1. Geographic setting of the uppermost Cambrian to Lower Ordovician (Tremadocian) conodont-bearing localities in the Alborz Mountains of Iran. remained unresolved, and the position of the Cambrian– Material and methods fi Ordovician boundary has not been de ned. As recently The studied samples were dissolved in dilute (10–15%), demonstrated by Ghobadi Pour et al.(2011c), the buffered acetic acid and after treatment, the residue was fi Lashkarak Formation, as originally de ned by Gansser handpicked without using heavy liquid separation. & Huber (1962), can be applied to the Middle and Almost all the identified taxa are figured; the specimens Upper Ordovician deposits of the Alborz Mountains. Its were photographed using a LEO 1450VP scanning elec- lower boundary coincides with the regional disconfor- tron microscope (SEM) at the University of Mashhad. mity at the base of the Darriwilian, which is traceable However, for some euconodonts, the outline of the Downloaded by [Gazi University] at 08:12 17 December 2015 across the Alborz region. basal cavity is diagnostic and thus can not be seen or The validity of the Mila Formation was questioned shown on figures from a SEM. For such specimens, by Geyer et al.(2014), because it is homonymous with photographs using transmitted light were taken using a ‘ ’ the Mila Formation introduced one year earlier by Canon EOS60D digital camera mounted on an Olympus Assereto (1963). The latter unit, as originally desig- binocular microscope. nated, also includes Devonian deposits. Instead, Geyer Thirty-seven limestone samples, each about 1.5 kg, et al.(2014) proposed to assign the Furongian part of were collected from the Simeh-Kuh and Deh-Molla sec- the Cambrian succession in the Alborz Mountains to tions. With a few exceptions, the source rocks are fi the newly de ned Deh-Molla Formation with its type mainly tempestites, or in some cases, shell beds in a section at Shahmirzad. However, the position of the siliciclastic succession. The conodont specimens are upper boundary of this lithostratigraphic unit is also usually complete or only slightly fragmented, but they uncertain and, according to our observations, the strati- are very delicate and fragile; thus many were recovered graphic interval corresponding to Mila Formation mem- broken during the course of preparation. The specimens ber 5 in the Mila-Kuh section is absent at Shahmirzad. are glossy black, dark-brown to light-brown, but Therefore, pending a general revision of the early translucent, when observed in transmitted light. Figured Palaeozoic lithostratigraphy of the Alborz region, we specimens are housed in the Azad University, avoid assignment of the Cambrian (Furongian) and Khorosgan Branch, Esfahan, Iran, under accession Lower Ordovician succession exposed at Simeh-Kuh to numbers AEU/L 5000–5099. any existing formal lithostratigraphic unit. ALCHERINGA CAMBRIAN–ORDOVICIAN CONODONTS FROM IRAN 3 Fig. 2. Stratigraphic columns of the Cambrian–Ordovician boundary beds at Mila-Kuh, Simeh-Kuh and Deh-Molla showing the levels of fossil assemblages and stratigraphic ranges of selected conodont, trilobite and brachiopod species. Simeh-Kuh section (2006) and revised herein (Fig. 2). It rests on a thick The Simeh-Kuh section is located in the eastern Alborz unit of light-grey bioclastic limestones including bra- Range, northeastern Iran, about 13 km northwest of
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