چينه‌نگاري سنگي، زيستي و رخساره‌هاي رسوبي سازند شاه‌كوه در برش كوه تنگل بالا، جنوب غرب خور (ايران مركزي)

Lithostratigraphy, biostratigraphy and sedimentary facies of the Shah Kuh Formation at Kuh- e- Tangale Bala section, southwest of Khur (Central Iran)

اين پژوهش در برگيرنده مطالعات چينه‏نگاري سنگي، زيستي و توصيف رخساره‌هاي رسوبي سازند شاه كوه به سن كرتاسه پيشين در جنوب غرب شهرستان خور در شرق ايران مركزي مي باشد. بدين منظور يك برش چينه شناسي در كوه تنگل بالا اندازه‏گيري، نمونه‏برداري و مطالعه شده است. سازند شاه كوه در اين برش 950 متر ضخامت داشته و به طور عمده از سنگ آهك هاي ضخيم لايه تا توده‌اي فسيل‏دار تشكيل شده است. مرز زيرين اين سازند با سازند نقره تدريجي و مرز بالايي آن با يك ناپيوستگي فرسايشي در زيرسازند بيابانك قرار مي‏گيرد. بر اساس گسترش چينه‌شناسي اربيتوليند‌هاي شناسايي شده دو زون زيستي Palorbitolina lenticularis Taxon Range Zone و Praeorbitolina cormyi Subzone در نهشته هاي سازند شاه كوه تشخيص داده شده است. زون‌هاي زيستي شناسايي شده مويد سن بارمين پسين- آپسين پيشين براي سازند شاه‏كوه در برش مورد مطالعه مي باشند. مطالعه رخساره‌هاي ميكروسكوپي نهشته‌هاي سازند شاه‏كوه نشان مي‏دهد كه اين رسوبات در چهار كمربند رخساره‌اي ساحلي، لاگون، سدو درياي باز كم عمق ته‌نشست يافته‏اند.

Introduction The Khur area is located in eastern part of central Iranian Basin. In this area, the Late Jurassic and the Neocomian successions (Chah Palang and Noqreh formations) are characterized by terrigenous rocks produced by erosion of paleohighs after the Late Cimmerian orogeny events. By the end of the Neocomian, the tectonic regime of the area stabilized and is followed by development of a shallow water carbonate platform and deposition of carbonate sediments of the Shah kuh Formation. For the purpose of lithostratigraphy, biostratigraphy and facies description of the Lower Cretaceous Shah Kuh Formation in southwest of Khur city, one section was measured, sampled and studied in detailed at Kuh- e Tangal- e Bala. This section is located 30 km west of Bayazeh village and 80 km southwest of Khur city (base co-ordinates N 33°19? 05?, E 54°50? 01?). In this section, The Shah Kuh Formation with a total thickness of 950 meters mainly consists of thick bedded to massive fossiliferous limestone. This formation rests transitionally on the Noqreh Formation and is overlain by Biabank Formation with a disconformity Materials and Methods Based on changing in lithology and facies, a total of 315 samples were collected and 420 thin sections were prepared. The age of the Shah Kuh Formation has been determined based on identified biozones which had been introduced by previous researchers in Tethyan basin. In order to facies classification (Dunhum 1962; Folk 1974; Tucker and Wright 1990) and facies interpretation (Flugel 2010) have been used. Based on micropaleontological studies 21 genera and 19 species of benthic foraminifera and 10 genera and 8 species of calcareous algae accompanied by skeletal elements of cononial octocorals were identified. According to stratigraphic distribution of identified orbitolinds taxa, two biozones including Palorbitolina lenticularis Taxon Range Zone and Praeorbitolina cormyi Subzone were recognized within the Shah Kuh Formation deposits. Palorbitolina lenticularis Taxon Range Zone This biozone with a total thickness of 947m almost includes the whole interval of the Shah kuh Formation. Thin biozone is recognized by the total occurrence of Palorbitolina lenticularis. Based on the identified taxon and benthic foraminifera and calcareous algae association, the Late Barremian- Early Aptian is ascribed to this biozone. Praeorbitolina cormyi Subzone This subzone is a part of Palorbitolina lenticularis Zone and could be considered as a Taxon Renge Zone.This biozone includes 42m thickness of upper part of the Shah kuh Formation and characterized by the total occurrence of Praeorbitolina cormyi. According to the identified taxon and other microfossils association, the Early Aptian is assigned to this biozone. Based on the identified biozones, the Late Barremian- Early Aptian is ascribed to the Shah Kuh Formation in studied section. Microfacies studies of the Shah kuh Formation deposits reveals that, these sediments were deposited in four facies belts, including beach, lagoon, shoal/barrier and shallow open marine. Beach facies belt includes oolitic fine to medium grained calcite cemented submature calcarenite and sandy oolitic bioclastic packstone. The presence of quartz and other terrigenous grains show the proximity of a siliciclastic source area and short distance of transportation. These facies are observed in the lower part of the Shah kuh Formation. Lagoon facies belt consists of three microfacies including silty bioclastic wackestone, peloid bioclastic packstone and bioclastic packstone. Type of fabric, diversity of foraminifera with porcelaneous test accompanied with calcareous algae represent low energy shallow water lagoonal environment restricted by a shoal/ barrier. These facies mostly occur in the lower part of the Shah kuh Formation in studied section. In the Shah kuh Formation deposits, Shoal/ barrier facies belt includes four microfacies such as ooilitic grainstone, bioclastic ooilitic grainstone, bioclastic grainstone and coral boundstone. These facies represent shallow water sediments of moderately high energy environment related to shoal/barrier. These facies are observed in the middle part of the Shah kuh Formation in studied succession. Shallow open marine facies belt consists of three microfacies such as crinoids packstone, orbitolind packstone and bioclastic wackstone. Crinoid packstone represents shallow open marine sediments with medium to high energy and normal salinity. Orbitolinids with a long diameter and short length generally are observed in sediments related to shallow open marine to slope. These facies occur in the upper part of the Shah kuh Formation. In bioclastic wackestone, skeletal remains predominately consists of bryozoans, brachipods and bivalve shells accompanied by small foraminifera with hyaline tests, which represent shallow open marine environment. Conclusions The Shah Kuh Formation with a total thickness of 950 meters mainly consists of thick bedded to massive fossiliferous limestone. This formation rests transitionally on the Noqreh Formation and is overlain by Biabank Formation with a disconformity. In this research 21 genera and 19 species of benthic foraminifera and 10 genera and 8 species of calcareous algae accompanied by skeletal elements of cononial octocorals were identified. Based on stratigraphic distribution of identified orbitolinds taxa, two biozones including Palorbitolina lenticularis Taxon Range Zone and Praeorbitolina cormyi Subzone were recognized within the Shah Kuh Formation deposits. According to the identified biozones, the Late Barremian- Early Aptian is assigned to the Shah Kuh Formation in studied section. Microfacies studies of the Shah Kuh Formation deposits reveals that, these sediments were deposited in four facies belts, including beach, lagoon, shoal/barrier and shallow open marine. Analyses of sedimentary facies and their lateral and vertical changes as well as the geometry of succession indicate a shallow marine carbonate platform setting for the shah kuh Formation in study area. Keywords: Shah Kuh Formation, Biostratigraphy, Late Barremian- Early Aptian, Khur, Central Iran