پديد آورندگان :
اقبالي, معصومه سازمان زمين‌شناسي و اكتشافات معدني تهران - پژوهشكده علوم زمين , حمدي, بهاءالدين سازمان زمين‌شناسي و اكتشافات معدني تهران - پژوهشكده علوم زمين , مجيدي فرد, محمودرضا سازمان زمين‌شناسي و اكتشافات معدني تهران - پژوهشكده علوم زمين
كليدواژه :
ﺑﺮش ﺣﻮض دوراه , دوﻧﯿﻦ ﭘﺴﯿﻦ , ﺳﺎزﻧﺪ ﺷﯿﺸﺘﻮ 1و2 , ﮐﻨﻮدوﻧﺖ , ﻓﺮازﻧﯿﻦ- ﻓﺎﻣﻨﯿﻦ
چكيده فارسي :
به منظور مطالعه عناصر كنودونتي، سازند شيشتو 1 برش در حوض دوراه انتخاب و مورد بررسي قرار گرفت. برش مورد مطالعه داراي توالي تخريبي – كربناته است كه شامل 152 متر سنگ آهك، ماسهسنگ و شيل است. همبري زيرين سازند شيشتو 1 بر روي سازند بهرام به صورت ناپيوسته و همبري بالايي آن با سازند شيشتو 2 بهطور پيوسته است. اين سازند داراي ماكروفسيلهايي همچون بازوپا، مرجان، ساقه لالهوش، گونياتيت و ميكروفسيلهايي همچون، كنودونت، فرامينيفر و بقاياي موجودات ديگر (خار اسفنج، گاستروپود، ساقه لالهوش) ميباشد. براساس مطالعات ميكروسكوپي توالي سازند شيشتو 1 در برش حوض دوراه 4 جنس، 13 گونه، 4 زير گونه با 4 زيستزون كنودونتي به ديرينگي دونين پسين (فرازنين پيشين– فامنين پسين) شناسايي و معرفي گرديد كه معادل با زيست زون استاندارد جهاني ميباشند:Polygnathus webbi- Polygnathus dubius Concurrent Range Zone = Falsiovalis - jamieae BiozoneIcriodus symmetricus-Icriodus expansus Interval Range Zone= Rhenana – linguiformis BiozoneIcriodus cornatus- Palmatilepis grasilis grasilis Concurrent Range Zone= Triangularis –trachytera BiozonePolygnathus delicates Total Range Zone= Expansa – praesulcata Biozoneو 6 جنس، 13 گونه و 2 زيرگونه مربوط به بخش زيرين سازند شيشتو 2 به ديرينگي تورنزين پسين – ويزين پسين با 2 زيست زون كنودونتي شناسايي و معرفي گرديد. Gnathodus girtyi cuneiformis- Gnathodus typicus Interval Range Zone = Crenulata – typicusBiozoneGnathodus pseudosemiglaber- Gnathodus collinsoni Concurrent Range Zone = Ancholaris- latus – texanus Biozone
چكيده لاتين :
IntroductionThe study area is located in the zone of Central Iran based on the structural divisions of Iran. Late Devonian deposits in central Iran are widespread. The Shishtu Formation is a two-stroke stratigraphic unit that is subdivided into two sub-formations called Shishtu 1 and Shishtu 2. Below the Shishtu 1 Formation is the Late Devonian (Frasnian-Famenian) and the Shishtu 2 Formation is early Carboniferous (Tournisian-Visean). The boundary between them is marked by a horizon of black charcoal shales called the Mouse horizon. The Paleozoic and Lower Triassic sediments in the Lut Zone are essentially similar to the Shotori Mountain Formations.Materials and MethodsHowz-e-Dorah section is located 57 km from Tabas town and 5 km from Deyhok village. After conducting library studies and selecting the appropriate shear in terms of thickness and separation of layers, the tectonic status of the area and the intended purpose by measuring layer thickness, fossil record collection, photography and sampling, extraction was conducted to extract the conodont elements.Results and DiscussionThe Shishtu Formation 1 is exposed at a thickness of 152 m in the Howz-e-Dorah section. The lower boundary of this sequence with the Bahram Formation is in disconformity with Middle Devonian and its upper boundary is in conformity with the Shishtu 2 Formation. Lithology of the Shishtu Formation 1 alternates from medium to thick bedded brown sandstones, green shales and thin bedded dolomitic white limestones, thin to thick bedded gray limestones, and highly diverse masses. It is high in conodont microfossils and macrofossils such as brachiopods, bryzoas, corals and crinoids. Notable points in this section are the presence of coral limestones, especially brachiopods, which are of late Devonian age in central Iran. The uppermost and last rock unit of the studied section is a 28 m outcrop of cephalopod horizon, outcropped as intermediate red limestone, indicating a short period of no sedimentation. Based on conodonts element studies, the sequence of late Shishtu Formation 1 to Late Devonian (Early Frasnian - Late Feminine) introduces 4 genus, 13 species, 4 subspecies with 4 conodonts biozonesThe anterior part of Shishtu 2 Formation ages back to Late Tournisian - Late Visean. 6 genus, 13 species, 4 subspecies, 13 species and 2 subspecies were identified in Howz-e-Dorah section. According to the studied conodonts elements, the sedimentary environment of the studied section is shallow and close to shore, and the conodonts species in the Shishtu 2 Formation are formed in oxygen and continental slope environments.ConclusionThe study of Shishtu Formation 1 and the basal part of Shishtu Formation 2 resulted in the identification of a high diversity of conodonts in the Howz-e-Dorah section. In addition to the high diversity of conodonts, the diversity of invertebrates, especially brachiopods, has been very high. Depending on the biofacies, depth and depositional environment of the Shishtu Formation 1, due to biofacies, the environment was offshore. Also, the presence of conodonts species is characteristic of tidal environments in Shishtu 1 Formation. The base part of Shishtu 2 is formed in low oxygen environments which are common in lower continental slopes.
