بررسي بيواستراتيگرافي نانوفسيل‌هاي آهكي سازندهاي سرچشمه و سنگانه در غرب حوضه كپه‌داغ، برش چينه‌شناسي شيخ

Calcareous nannofossil biostratigraphy of Sarcheshmeh and Sanganeh formations in the west of Kopet-Dagh Basin, Sheikh Section

حوضه‌رسوبي كپه‌داغ در شمال و شمال‌شرق ايران و در حدفاصل مرز ايران، تركمنستان و افغانستان قرار دارد. در اين حوضه، توالي‌هاي دريايي كرتاسه از گسترش و پراكندگي خوبي برخوردار هستند. سازندهاي سرچشمه و سنگانه جز واحدهاي رسوبي كرتاسه اين حوضه هستند كه بيشترين ضخامت اين سازندها مربوط به قسمت‌هاي غربي حوضه كپه داغ مي‌باشد. بررسي نانوفسيل‌هاي آهكي در برش شيخ (قسمت‌هاي غربي منطقه) به تشخيص 84 گونه متعلق به 35 جنس از 16 خانواده منجر شد. بر اساس نانوفسيل‌هاي آهكي شاخص در برش مورد مطالعه، قسمت‌هاي فوقاني بيوزون Watznaueria oblonga Zone (NC5)، بيوزون Chiastozygus litterarius Zone (NC6) و قسمت‌هاي ابتدايي بيوزون Rhagodiscus angustus Zone (NC7A) در سازند سرچشمه و بيوزون Rhagodiscus angustus Zone (NC7A) در قسمت‌هاي ابتدايي سازند سنگانه تشخيص داده شده است. بر اين اساس سن سازند سرچشمه در برش مورد مطالعه بارمين پسين- ابتداي آپتين پسين و سن سازند سنگانه ابتداي آپتين پسين- ؟آپتين پسين پيشنهاد مي‌شود.

Introduction The Kopet-Dagh Basin is located in the north and north east of Iran, within the border triangle of Iran, Turkmenistan and Afghanistan. Sediments of the Cretaceous age are well exposed in this basin (Afshar Harb, 1994). Sarcheshmeh and Sanganeh formations belong to the Cretaceous age and have the highest thickness in the west of the basin. In the current study, these two formations have been investigated in the west of the Kopet-Dagh basin with regard to the calcareous nannofossils to determine the exact age of the sediments of the studied interval. Material and Methods In this study Sarcheshmeh and Sanganeh formations have been investigated in Sheikh section which is located at the 25 Km of north east Bojnord. At the studied interval, Sarcheshmeh Formation is mainly consists of grey marls at the lower part and an alternation of grey marls and dark grey shales at the upper part. Sanganeh Formation is mainly consists of dark grey shale and grey silty shale layers along with some interbeds of silty limestone, siltstone and sandstone. For investigation of calcareous nannofossils at the studied interval, a total of 255 samples were processed from Sarcheshmeh (141 samples) and Sanganeh (114 samples) formations. Samples were processed using the gravity settling technique of Bown and Young (1998) and smear slide method of Perch-Nielsen (1985). Slides were studied by an Olympus polarizing microscope (BX51), with 1600x and 2000x magnifications. Discussion of Results and Conclusions For determining the calcareous nannofossil biostratigraphy at the studied interval, the biozonation of Roth (1978) emended by Bralower et al. (1993, 1995) has been applied. In order to identify the biozonation of the studied interval, the first occurrence (FO) and the last occurrence (LO) of the calcareous nannofossils is recorded. According to the index calcareous nannofossils of the studied interval, three biozones are defined that are as follow: 1) Watznaueria oblonga Zone (NC5), 2) Chiastozygus litterarius Zone (NC6) and 3) Rhagodiscus angustus Zone (NC7). At the studied interval Calcicalathina oblongata, Cruciellipsis cuvillieri and Lithraphidites bollii were not recorded, but the first occurrence of Rucinolithus irregularis is recorded at 291 m of the section. So from the first part of the studied interval up to the FO of R. irregularis is considered as Watznaueria oblonga Zone (NC5). The FO of R. irregularis is considered as an index to determine the Barremian/Aptian boundary. The FO of Eprolithus floralis is recorded at 647 m, which marks the top of Chiastozygus litterarius Zone (NC6) and determine the Early Aptian. Prediscosphaera columnata is not recorded at the studied interval and the LO of Micrantholithus spp. is recorded at 816 m of the studied interval, which marks the top of NC7A (early Late Aptian). At the upper part of the studied interval from 816 to 1230 m, no calcareous nannofossil event is recorded, and just according to its stratigraphical position is regarded to be part of ?NC7 Zone with the age of Late Aptian. According to the identified calcareous nannofossil biozones, the age of Sarcheshmeh Formation is considered to be Late Barremian-early Late Aptian and the age of the Sanganeh Formation is regarded as early Late Aptian-?Late Aptian. Keywords: Calcareous nannofossils, Kopet-Dagh Basin, Sarcheshmeh Formation, Sanganeh Formation, Cretaceous.