زیست چینه‏نگاری نهشته‏های دونین‏ میانی- بالایی سازند بهرام در برش کوه رضاآباد (جنوب غربی دامغان)

در این تحقیق نهشته‏های کربناته – تبخیری دونین میانی بالایی برش کوه رضاآباد (55 کیلومتری جنوب غربی دامغان) متعلق به سازند بهرام، مورد بررسی و مطالعه قرار گرفته است. ضخامت این نهشته‏ها حدود 166 متر و از دو واحد کربناته پائینی و بالایی و یک واحد تبخیری در بخش میانی تشکیل گردیده‏‏اند. نمونه‏برداری منظم برای بررسی کنودونت‏ها به شناسایی 12 گونه کنودونتی زیر منجر گردید:icriodus excavates; i. subterminus; i. expansus; i. brevis; i. iowaensis; polygnathus alatus; p. webbi; p. aequalis, p. prepolitus, p. xylus, polygnathus sp. a; polygnathus sp. b.بررسی گستره سنی کنودونت‏ها، حاکی از سن ژیوتین پسین (زون کنودونتی subterminus) تا فرازنین پسین (تا زیست زون rhenana zone) برای نهشته‏های مورد مطالعه است. همچنین پراکندگی گونه‏های بازوپایان: douvillina sp., adolfia sp., aullacella sp., athyris sp., schizophoris sp., athrypa sp., cyrtospirifer sp., uchtospirifer sp.دلالت بر حضور نهشته‏های دونین دارد. در محدوده واحد کربناته پایینی نمونه‏های b4 و b5، دو لایه مملو از صدف‏های استراکدهای چند لایه وجود دارد. این استراکدها متعلق به زیر راسته eridostracina از راسته palaeocopida هستند که در این مطالعه دو جنس cryptophyllus و schaefericoncha شناسایی گردیدند. جنس‏های مذکور از نهشته‏های استوایی نیمه استوایی سیلورین تا کربونیفر مناطق مختلف جهان از جمله: لهستان، آلمان، روسیه و آمریکا نیز گزارش شده است.

Biostratigraphy of MiddleLate Devonian deposits of Bahram Formation in KuheRezaAbad section (SW Damghan)

IntroductionIn the Lower Devonian, western parts of Iran were almost out of the water while central and East Iran and Eastern Alborz have covered by the shallow marine to continental facies. The Central Alborz, Lot and part of central Afghanistan have provided sediment supplements to lower Devonian shallow environments (Weddige, 1984a, 1984b). Padeha, Sibzar and Bahram formations are the representative of The Middle – Late Devonian deposits of Central Iran which Bahram formation more or less corresponds to the Khoshyeilaq and Jeiroud formations in Central and Eastern Alborz. Devonian deposits in KuheRazaabad located at the northern border of Central Iranian structural unit that lithologicaly has similarities with Bahram formation in central Iran except the evaporate unit which is characteristics of the KuheRezaabad section (AlaviNaini, 1972; Wendt et al., 2005). MiddleUpper Devonian carbonateevaporate deposits of Bahram Formation in KuheRezaAbad section (SW Damghan), have been studied by this research. The studied profile is about 166 ms thick includes, lower and upper carbonate and middle evaporate units. Materials and MethodsEighteen conodont samples has systematically collected and processed with conventional acetic/formic acid. Besides a few samples, the others were barren, let us to stablish the conodont zonation for the studied interval. Furthermore 4 polished slabs and 10 thin sections have provided due to the study of multi lamellar ostracods which have been reported for the first time from the studied locality. DiscussionTotally eighteen samples 4 to 5 kgs has processed systematically with conventional acetic/formic acid, 5 samples were prolific with about 225 conodont elements. twelve species belonging to two genera has discriminates as following; Icriodus excavates; I. subterminus; I. expansus; I. brevis; I. iowaensis; Polygnathus alatus; P. webbi; P. aequalis, P. prepolitus, P. xylus, Polygnathus sp. A; Polygnathus sp. B.The studied conodonts assigned Late Givetian (subterminus zone) to Upper Frasnian (rhenana zone) to the studied interval.Douvillina sp., Adolfia sp., Aullacella sp., Athyris sp., Schizophoris sp., Athrypa sp., Cyrtospirifer sp., Uchtospirifer spThe studied levels (B4, B5) has also yielded a spectacular fossiliferous layers reach in multilamellar ostracods of Eridostracina suborder for the first time. Eridostracina are a less known extinct tropicalsubtropical marine ostracods of Palaeocopida Order which ranges from Ordovician to Carboniferous time interval. Precise microscopic studies, extracted isolated forms by the acid leaching method from the sediments, slabs and thin sections, revealed their real identity as ostracods. In contrast to their relatives, the multi lamellar ostracods retain their exuviate during periodic ecdysis. In this case, during every molting stages, a new carapace grows internally inside the carapace, whereas the older layers are abandoned externally forming multi layered shell. In this study, two genus of Eridostracina are identified as: Cryptofillus and Schaefericoncha. These genera also were been reported from: Poland, Germany, Russia, Australia and USA. (Adamczak, 1961; Jones & Olempska, 2013; Schallreuter, 1977; Eichwald, 1860; Clarke, 1882). AcknowledgementThis research undertaken at Department of Geology, Faculty of Sciences, University of Isfahan. Financially and logistically supported by the Vice chancellor for Research and Technology office at University of Isfahan which deeply appreciated.