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A Summary of the Jurassic System in North and East-Central Iran N München, 31.12.2020

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A Summary of the Jurassic System in North and East-Central Iran N München, 31.12.2020 99 Zitteliana 94 A summary of the Jurassic System in North and East-Central Iran n München, 31.12.2020 1 2 3 n Manuscript: Kazem Seyed-Emami *, Markus Wilmsen & Franz T. Fürsich 4 5 6 received: with contributions by Mahmoud R. Majidifard , Ahmad Raoufian & Jafar Taheri 25.08.2020 1 revision: School of Mining Engineering, College of Engineering, University of Tehran, Iran accepted, 02.09.2020 2 Senckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, available online, 17.11.2020 Sektion Paläozoologie, Germany 3 GeoZentrum Nordbayern, Fachgruppe PaläoUmwelt, Friedrich-Alexander-Universität n ISSN 0373-9627 Erlangen-Nürnberg, Germany n ISBN 978-3-946705-08-6 4 Research Institute for Earth Sciences, Geological Survey of Iran, Box 131851-1494 Tehran, Iran 5 Daneshvar Center, Farhangian University, Neyshapour, Iran 6 Geological Survey of Iran, NE Branch, P.O. Box 91735-1166, Mashad, Iran *Author for correspondence and reprint requests; E-mail: [email protected] Zitteliana 94, 99–156. Abstract The rocks of the Jurassic System in North and Central Iran are represented by very thick sedimentary successions reflecting the deposition during two tectono-stratigraphic megacycles, bounded by three important unconformities. The older (Early to early Middle Jurassic) cycle starts, after the Late Triassic closure of the Palaeotethys and the collision of the Iran Plate with the southern margin of Eurasia (Turan Plate), with an abrupt uplift of the Cimmerian Mountains around the Triassic–Jurassic boundary (Main-Cimmerian Event) and is terminated by the Mid-Cimmerian Tectonic Event in the mid-Bajocian. The predominantly siliciclastic sediments of this tectono- stratigraphic cycle are accommodated in the Ab-e-Haji Subgroup of the (upper) Shemshak Group. The Ab-e-Haji Subgroup consists of thick and widespread, partly coal-bearing siliciclastic strata and also records some important marine ingressions, especially in the Toarcian and Aalenian. The younger tectono-stratigraphic cycle starts with an extensive marine transgression in the Late Bajocian and ends around the Jurassic-Cretaceous boundary with the Late Cimmerian Tectonic Event. In the aftermath of the Mid-Cimmerian Tectonic Event, the Iran Plate was dissected into several structural units, the geological histories of which differed and so did their facies patterns. Thus, in North Iran two main sedimentary areas developed, namely the Alborz and Koppeh Dagh basins, which show moderate diffe- rences in lithology and thicknesses. However, the differentiation is much more pronounced on the Central-East Iranian Microcontinent, which comprises three N–S-oriented and independent structural units, i.e., from east to west the Lut, Tabas and Yazd blocks. During the Jurassic Period, the sea mostly covered the Tabas and Lut blocks, whereas the Yazd Block remained largely emergent. The lithologically diverse and numerous formations of the area are combined in the Magu and Bidou groups. Early to Early Middle Jurassic ammonite fau- nas of North and Central Iran are palaeobiogeographically closely related to Northwest European (Subboreal) ammonite faunas, allowing a similar biozonation. Concurrent with the Late Bajocian transgression, there is an abrupt change in faunal composition and the Middle and Late Jurassic ammonite faunas of North and Central Iran are much more similar to those of epicontinental seas bordering the nort- hern margin of the western Tethys and occupy an intermediate position between the Mediterranean and the Submediterranean Province. The results of the studies on ammonoid palaeobiogeography are in line with Early Jurassic palaeogeographic reconstructions that place the Iran Plate at fairly high palaeo-latitudes of about 45°N, followed by a rapid southward shift throughout the Middle and Late Jurassic to a position of about 30°N. Keywords: Jurassic System, Iran Plate, Lithostratigraphy, Biostratigraphy, Ammonite faunas, Palaeo(bio)geography. 1 Introduction Geological Survey of Iran) and Germany (universities of München, Würzburg and Erlangen-Nürnberg as Jurassic rocks show a wide distribution in North well as the Senckenberg Natural History Collections and Central Iran, attain great thicknesses and are in Dresden). The manifold results of these studies commonly superbly exposed. The present paper is a have been published in numerous papers (see text short synopsis of the extensive investigations of the and references), which are summarized herein. The sedimentary strata of the Jurassic System in North biostratigraphy is based on systematically collected, and East-Central Iran, carried out over the last 20 described, and published ammonites that fortunate- years by teams from Iran (University of Tehran and ly occur in most of the studied formations, allowing Zitteliana 94 100 Figure 1: Structural and palaeogeographic framework of Iran. A, Middle Callovian palaeogeography of the western Tethyan area (modified after Thierry 2000). B, main structural units and sutures of Iran (modified after Wilmsen et al. 2009a). an accurate chronostratigraphic calibration of the li- the Palaeotethys Ocean. The resulting Palaeotethys thostratigraphic units as well as precise intra- and in- suture runs approximately E–W in northern Iran, terbasinal correlations. Therefore, special emphasis bending southeastwards into Afghanistan (Fig. 1b). is put herein on the extensive and diverse Jurassic Towards the southwest, the Iran Plate is bounded by ammonite fauna collected over the years. the Neotethyan suture, separating it from the Arabi- an Plate. Palaeogeographic reconstructions for the Middle Jurassic (e.g., Barrier & Vrielynck 2008) place 2 Geological overview the Iran Plate at the northern margin of the Neotethy- an Ocean at subtropical palaeo-latitudes of ca. 30°N Iran is characterized by a very diverse Phanerozo- (Fig. 1a). ic geology. The Central-East Iranian Microcontinent The Jurassic System in North and Central Iran (CEIM; Takin 1972), consisting of the Yazd, Tabas, (Iran Plate) consists of two large tectono-sedimen- and Lut blocks, together with North/Northwest Iran tary cycles. The older cycle begins, after the closu- forms the Iran Plate, which occupies a structural key re of the Palaeotethys and the collision of the Iran position in the Middle East (Fig. 1a, b; Wilmsen et al. Plate with the active southern margin of Eurasia du- 2009a). As an element of the Cimmerian microplate ring the Late Triassic (Eo-Cimmerian Event; Stöcklin assemblage, it became detached from Gondwana 1974; Sengör et al. 1988; Wilmsen et al. 2009a), with during the Late Permian and collided with Eurasia the so-called Main-Cimmerian Event at the Triassic- (Turan Plate) during the early Late Triassic, closing Jurassic boundary (Fürsich et al. 2009a; Wilmsen 101 Zitteliana 94 Figure 2: Late Triassic to Jurassic tectono-sedimentary and lithostratigraphic framework of Iran (compiled after Seyed-Emami et al. 2004a and Wilmsen et al. 2009a). The Jurassic System with its two tectono-stratigraphic megacycles is highlighted in blue. CEIM: Central-East Iranian Microcontinent. et al. 2009a; Fig. 2). This Early to early Middle Ju- the sedimentary rocks are widely similar. The Shems- rassic tectono-sedimentary cycle is represented by hak Group (former Shemshak Formation of Assereto the predominantly siliciclastic rocks of the Ab-e-Haji 1966a) in North as well as in Central Iran (Iran Plate) Subgroup (new) of the Shemshak Group (Fürsich et is an up to 4000-m-thick siliciclastic succession, wi- al. 2009a; Wilmsen et al. 2009b, c). The cycle was dely distributed across the Iran Plate (Figs. 2, 3). It is terminated by the Mid-Cimmerian Event, around the bounded at the base and at the top by two important early/late Bajocian boundary (Seyed-Emami & Alavi- unconformities, caused by the Early and Mid Cim- Naini 1990; Brunet et al. 2003; Seyed-Emami et al. merian tectonic events. For better comparison with 2004a; Fürsich et al. 2009b; Wilmsen et al. 2009a; Central Iran, Assereto (1966a) proposed to raise the Fig. 2). The younger cycle started with an extensive Shemshak Formation to the rank of a group. During transgression in the late Bajocian and is terminated a symposium on this matter at the Geological Sur- by the Late Cimmerian Event, around the Jurassic- vey of Iran in 1984, it was largely accepted to com- Cretaceous boundary (Wilmsen et al. 2003, 2009b, bine the Nayband, Ab-e-Haji, Badamu, and Hojedk 2010; Fig. 2). In East-Central Iran, this Middle to Late formations within the Shemshak Group (Aghanabati Jurassic tectono-sedimentary cycle includes the 1998). Fürsich et al. (2009a) raised the Shemshak deposits of the Magu and Bidou groups (Wilmsen Formation of the Alborz Mountains formally into et al. 2009b), while in northern Iran it is represented the rank of a group and subdivided it from base to by a lithologically diverse complex of lithostratigra- the top into nine formations: Ekrasar, Shahmirzad, phic units comprising the Shal, Dalichai, Kashafrud, Laleband, Kalariz, Alasht, Javaherdeh, Shirindasht, Chaman Bid, Lar, Mozduran and Shurijeh formations Fillzamin and Dansirit (Figs. 3, 4). However, Krystyn (e.g., Lassemi 1995; Majidifard 2008; Taheri et al. et al. (2019) excluded the Norian-Rhaetian Nayband 2009; Hosseinyar et al. 2019; Fig. 2). Formation from the Shemshak Group, based on li- During Late Triassic to early Middle Jurassic, the thostratigraphic and facies differences, as well as on Iran Plate acted more or less as a coherent mass and the occurrence of the Main-Cimmerian event at the Zitteliana
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