The 1 st International Applied Geological Congress, Department of Geology, Islamic Azad University - Mashad Branch, Iran, 26-28 April 2010
An introduction to some Polygnathid conodonts from Shirband, Damghan
B. Valizadeh Kakhki1, S. M. Hosseininezhad2
1- Msc. Student of Stratigraphy, Faculty of Geology, Damghan university of Basic Sciences
2- Assistant Prof. of Faculty of Geology, Damghan university of Basic Sciences
Abstract Late Devonian deposits (Geirud Formation) in Shirband area is located with conformal discontinuity on the Mila Formation and under the carbonate and dark gray shale deposits of Mobarak Formation. These sediments start with a relatively thick layer of limestone and white-brown siltstone and convert to yellow Dolomite and shale and marn limestone with some fossils. Between about 100 conodont samples, some Polygnathuses like Polygnathus alatus, Polygnathus communis communis, Polygnathus brevilaminus, Polygnathus webbi, Polygnathus communis mugodzharicus, Polygnathus symmetricus and Polygnathus delicatus is identified here that assign the upper Devonian age to the studied section.
Keywords: Shirband, Conodont, Devonian, Alborz
Introduction Shirband area is located in 25 Km away from Damghan that belongs to eastern Alborz zone. This area is accessible by Damghan to Dibaj and Damghan to Shahroud roads (Figure 1). To date, no study is carried out on the Formations of this region and studies is limited to preparing of Geological map (1/100000) by Geological survey of Iran and some sparse studies on Shemshak Formation by Eastern Alborz Coal Company. Conodonts are significant in connecting the Iranian studied region and neighbor countries. Also this fauna can be influential in Devonain Paleogeography determination. In order to perform this study, 35 samples of the defined section were collected. After doing preparation procedures like Acid wash, 100 conodont samples were prepared. Apart from that other fossils like scolecodonts, placoderms, and crinoids stems were found. Based on found Conodonts, Upper Devonian age is suggested for this section.
Discussion Probably, Paleothetis oceanic crust genesis and separation of Touran plate from North Iran plate (Gondwana) has occurred in early Devonian, so facies of Irans Devonian stones progressively cover the pre-Devonian discontinuity. At the start of early Devonian, sea level dropped down. By advancement of the sea at the time of formation of upper Devonian deposits, Devonian rocks located on the lower silourian or older rocks in the most parts of Iran. These stone layers have thickness about 167 meters in Shirband area and consist of recurrence of limestone, white to brown siltstone, Dolomite, thin layer limestone frequency to gray brachiopoda middle layer with shale and lime shale inter layers and dark gray limestone layer containing Spiriphids, Briosoer, Coral. This sequence is considered as Geirud Formation and is located with conformal discontinuity on the Mila Formation and under the carbonate and dark gray shale deposits of Mobarak Formation.
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The 1 st International Applied Geological Congress, Department of Geology, Islamic Azad University - Mashad Branch, Iran, 26-28 April 2010
Shirban
Figure 1 – Shirband area with its accessing ways.
Results Fossil and biostratigraphic evidences showed that upper Devonian deposits in Shirband section are marine and surface. Six polygnathid was identified in this study (Polygnathus alatus, Polygnathus communis communis, Polygnathus brevilaminus, Polygnathus webbi, Polygnathus communis mugodzharicus, Polygnathus symmetricus and Polygnathus delicatus) and based on them Upper Devonian (Famenian) age is suggested for this section. Based on these conodonts, surface marine, coastal and warm sedimental environment is characterized. Considering paleontological and biostratigraphic evidences it seems that transit of Famenian facies to Tournaisian facies is gradual in Shirband area.
References 1. Alavi, M. (1991). Sedimentary and stractural characteristics of the Paleo Tethys remanants in northeastern Iran. Bulletin of Geological society of America, No. 103, pp. 983-992. 2. Ashouri, A. (2006). Middle Devonian-Early Carboniferous conodont faunas from the Khoshyeilagh Formation, Alborz Mountains, north Iran. Journal of Sciences, Islamic Republic of Iran, No. 17, pp. 53-65. 3. Assereto, R. (1969). Geologia e Genesi dei sedimenti fosfatici Devoniani dellelburz central. Memorie degli instituti di Geologia e Mineralogia delluniversita di Podova, No. 27, pp. 1-59. 4. Bozorgnia, F. (1973). Paleozoic foraminiferal biostratigraphy of central and East Alborz mountains, Iran. National Iranian Oil Company, Publication No. 4, p.185. 5. Hosseininezhad, S. M. et al. (2005). Bio Events of Meyghan area Famennian rocks in the east Alborz. The 23 rd symposium on Geoscience, pp. 436-436. 6. Sandberg, C. A. and R. Dreesen. (1984). Late Devonian icriodontid biofacies models and alternate shallow water conodont zonation. In: Clark, D. L. (ed.), Conodont biofacies and provincialism: Geological society of America Bultain Special Papers, Vol. 196, pp. 143-178.
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The 1 st International Applied Geological Congress, Department of Geology, Islamic Azad University - Mashad Branch, Iran, 26-28 April 2010
7. Ziegler, W. and C. A. Sandberg. (1990). Late Devonian standard conodot zonation, In: W. Ziegler (ed.), First International Senckenberg Conference and 5l European conodont Symposium Contribution (ECOS V), Courier Forschungsinstitut Senckenberg, Vol. 121, pp.1-115. 8. Ziegler, W. (1971). Conodont stratigraphy of the European Devonian, in Sweet, W.C., and Bergstram, S.M., eds., Symposium on Conodont Biostratigraphy: Geological Society of America Memoir Vol. 1, No. 27, pp. 227-284.
Plate: 1- Polygnathus alatus (Huddle, 1934) 128x 2- Polygnathus communis communis (Branson and Mehl, 1934) 107x 3- Polygnathus communis communis (Branson and Mehl, 1934) 96x 4- Polygnathus brevilaminus (Branson and Mehl, 1934) 65x 5- Polygnathus brevilaminus (Branson and Mehl, 1934) 117x 6- Polygnathus webbi (Stauffer, 1938) 132x 7- Polygnathus communis mugodzharicus (Gagiev, Kononova and Pazukhin, 1987) 169x 8- Polygnathus brevilaminus (Branson and Mehl, 1934) 144x 9- Polygnathus brevilaminus (Branson and Mehl, 1934) 101x 10- Polygnathus brevilaminus (Branson and Mehl, 1934) 108x 11- Polygnathus brevilaminus (Branson and Mehl, 1934) 115x 12- Polygnathus symmetricus (Branson and Mehl, 1934) 114x 13- Polygnathus delicatus (Ulrich and Bassler, 1975) 97x 14- Polygnathus sp. 106x 15- Polygnathus communis communis (Branson and Mehl, 1934) 92x 16- Polygnathus brevilaminus (Branson and Mehl, 1934) 164x 17- Polygnathus communis communis (Branson and Mehl, 1934) 106x 18- Polygnathus communis communis (Branson and Mehl, 1934) 150x
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The 1 st International Applied Geological Congress, Department of Geology, Islamic Azad University - Mashad Branch, Iran, 26-28 April 2010
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