زیست‌چینه‌نگاری و چینه‌نگاری سکانسی نهشته‌های کربونیفر زیرین زون البرز بر مبنای فرامینیفرهای بنتیک و ژئوشیمی ایزوتوپی

توزیع و هندسه سازند مبارک به سن تورنزین ویزئن حاکی از رسوبگذاری بر روی رمپ کربناتی هموکلاین نسبتاً کم انرژی با زاویه کم است. این تفسیر براساس ویژگی‌های رخساره‌های تشکیل دهنده و روابط جانبی و عمودی تدریجی بین رخساره‌های رسوبی است. براساس تجزیه و تحلیل رخساره‌ها و مجموعه‌های رخساره‌ای، می‌توان چهار مجموعه رخساره‌ای برای رمپ کربناتی مبارک در نظر گرفت که عبارتند از محیط‌های رسوبی حوضه، رمپ خارجی (مجموعه‌های ساب‌تایدال عمیق)، رمپ میانی (مجموعه‌های ساب‌تایدال کم‌عمق) و رمپ داخلی. سازند مبارک از 4 سکانس رسوبی (s1 s4) تشکیل شده است که حاصل فعل و انفعال بین تغییرات سطح آب دریا و شرایط اقلیمی می‌باشند. این سکانس‌ها شامل پکیج‌های پس‌نشسته1 (دسته رخساره پیش‌رونده (tst)) و پیش‌نشسته2 (دسته رخساره ترازبالا (hst)) هستند. زیست‌چینه‌نگاری سازند مبارک براساس زیرتقسیم‌بندی زون‌های فرامینیفری می‌سی‌سی‌پین3 (mfz) انجام گرفته، 9 بایوزون (mfz1 mfz9) متعلق به زیرآشکوب‌های هاستارین، ایوورین و مولینیسین را نشان می‌دهد. انطباق خوب دسته‌رخساره‌های tst، hst و ژئوشیمی ایزوتوپ کربن 13 (δ13c) حاکی از آن است که نوسانات سطح آب دریا به عنوان مکانیسم اصلی، منجر به تغییر مقادیر ایزوتوپ کربن 13 (δ13c) روی رمپ کربناتی مبارک شده است.  به طور کلی، تلفیق این مجموعه داده‌ها می‌تواند مبنایی برای انطباق سکانس‌های رسوبی در مقیاس ناحیه‌ای ناشی از تغییرات سطح دریای جهانی قابل استفاده باشد.

Biostratigraphy and sequence stratigraphy of the Lower Carboniferous deposits in the Alborz zone based on benthic foraminifera and isotopic geochemistry

IntroductionUnderstanding the cyclical nature of changes in sealevel regime leads to awareness of historical land events. In the functional basin, it leads to predictions about the presence of source rocks and hydrocarbon reservoirs in sedimentary sequences. Cycles are formed during a process of falling to rising sea level. The study of sedimentary cycles and understanding the causes of their formation is done in the form of cyclic stratigraphy. Relative changes in sea level were the first active mechanism for the formation of marine sedimentary cycles and sequences. In general, cyclic sedimentary patterns are the product of tectonic and climatic processes, followed by global and local changes in sea level.Materials and methods The study of cycles and their formation controlling factors in this article has been done using qualitative studies such as field and laboratory evidence of carbonate deposits of Mobarak Formation. In order to achieve the types of cycles and processes affecting their formation, facies and facies association, depositional environment, isotopic geochemistry and relative sensors were determined using benthic foraminifera as well as sequence stratigraphy.Discussions and findingsStudies on field evidence such as sediment erosion pattern, lateral continuity of layers, sedimentary and biological structures, as well as petrographic studies of facies and facies association obtained in this formation, four thirdorder sedimentary sequences of S1 to S4 were identified. The sequences did not have a low stand systems tract (LST) and were identified by a highstand systems tract (HST) and transgressive systems tract (TST) that could be controlled by the interaction between eustatic sealevel changes and climate conditions. Depositional sequences show retrogradational (transgressive systems tract) and prograditional (highstand systems tract) stacking patterns. Together, these sequences cover a large regressive cycle from the basins / open marine facies to the tidal zone. The distribution and geometry of the TournaisianVisean Mobarak Formation imply deposition on a lowangle, relatively lowenergy homoclinal carbonate ramp. This interpretation is based on the characteristics of the constituting facies and their gradual lateral and vertical changes. Based on the facies analysis and their associations, four facies’ belts (associations) can be recognized, i.e., basinal environments, outer ramp (deep subtidal associations), mid ramp (shallow subtidal associations), and inner ramp. The study of fossils in sedimentary deposits is one of the oldest and most common methods for determining the relative age of sediments.  By examining the distribution of fossils in stratigraphic units and organizing stratigraphy in units based on their fossil content, the age of the units can be determined. According to this, the biostratigraphic investigation shows 9 biozones (MFZ1 MFZ9) belonging to the Hastarian, Ivorian, and Moliniacian substages based on the classification of Mississippian Foraminiferal Zones (MFZ). A good agreement between TSTs and HSTs, and δ13C chemostratigraphy implies that sea level fluctuations are the main mechanism that control the δ13C fluctuations in the Mobarak Formation.  Integration of these data indicates that the sequences has been driven by the eustatic sealevel changes and suggests a basic for the regional sequence stratigraphic correlations. The relative ages of the studied calcareous strata were obtained in the Kalariz sections. They are related to the three substage of Hastarian, Ivorian, and Moliniacian. In addition, the study of oxygen and carbon isotopes, which was performed in the Kalariz section, was used to determine the sequence levels and the longterm temperature of the Lower Carboniferous deposits. Therefore, by studying the isotopic values of oxygen and carbon, it is found that the analyzed samples of this formation experienced the least amount of alteration and diagenesis and were close to the waters of the lower Carboniferous Sea.ConclusionIn this study, 12 main facies were identified and classified into 5 facies association. These facies association are spread on a homoclinal ramp carbonate platform. Examining the field and laboratory evidence, four thirdorder sedimentary sequences S1, S2, S3 and S4 were identified by the highstand systems tract (HST) and transgressive systems tract (TST) categories. According to the subdivision of Mississippian foraminiferal zones (MFZ), 9 biozones (MFZ1 MFZ9) were identified in this study, which belong to the subgroups of Hastarian, Ivorian and Moliniacian. In addition, by studying the isotopic values ​​of oxygen and carbon, it was found that the analyzed samples of this formation experienced the least amount of alteration and diagenesis and were close to the waters of the lower Carboniferous Sea. Finally, the studies showed that the main mechanism controlling the fluctuations of isotopic values, cycles and sequences of the third order are the Eustatic fluctuations of the water surface.