تأثير گنبد نمكي درمدان بر رخساره‌ها و دياژنز نهشته‌هاي سنومانين ـ سانتونين (ناحيه فارس، زاگرس)

Effect of Darmadan salt plug on facies and diagenesis in the Cenomanian-Santonian deposits (Fars area, Zagros

براي تحليل حركت نمك و تاثيرات زماني و مكاني آن بر سيستم رسوبي دو برش در فواصل دور و نزديك گنبدنمكي تاقديس درمدان در شرق ناحيه فارس مطالعه شد. نهشته هاي سنومانين-سانتونين در اين ناحيه، بخش بالايي سازند كژدمي، سازند سروك و بخش پاييني سازند گورپي است. ضخامت برش هاي مورد مطالعه در برش D1 (دور از گنبد) 170 متر و در برش D2 (نزديك گنبد) 255 متر است. براساس آناليز رخساره اي و خصوصيات سنگ شناختي 8 ريز رخساره كربناته شناسايي شدند. آناليز رخساره ها، شيب زياد رسوبات و كم عمق شدن رخساره ها به سمت گنبدنمكي نشان مي دهد كه رخساره ها در يك شلف كربناته نهشته شده اند. فرايندهاي دياژنزي دررسوبات ميكريتي شدن، سيماني شدن، گلاكونيتي شدن، نئومورفيسم، فشردگي، آشفتگي زيستي و انحلال است كه با تغييراتي مانند شكل گيري سيمان هاي دولوميتي و انيدريتي و سيليسي شدن، آهن دار شدن و دولوميتي شدن به سمت گنبدنمكي همراه است. اين فرايندها در محيط هاي دريايي، متئوريك، تدفيني و بالاآمدگي تشكيل شده اند. با انطباق بين برش هاي مورد مطالعه، شواهد متفاوتي در برش D2 نظير ظهور اربيتولين، جلبك قرمز، آنكوئيد، ذرات تبخيري، آذرين و آواري مشاهده شد. با توجه به شواهد مي توان نتيجه گرفت كه گنبدنمكي در زمان رسوبگذاري و پس از آن بر روي نهشته هاي سنومانين-سانتونين تاثيرات زيادي داشته است.

IntroductionUpper Cretaceous period can be considered as the peak of diapirism movements in Zagros sedimentary basin (Bosak et al, 1998). After sedimentation of Hormoz salt series, these salts were rising up towards the surface during the Lower Paleozoic (Jahani et al, 2009). In this study, Cenomanian-Santonian strata in Fars area that consist of Bangestan group (NE-SW trend), have been studied based on effect of local and regional diapirism on these strata.Material and methodOne hundred and sixty samples from two stratigraphic sections (upper part of Kajdumi Formation, Sarvak Formation and lower part of Gurpi Formation) in Darmadan anticline (Near salt dome and far from it) have been studied. All samples shave been stained with alizarin red-s to enhance differentiation of calcite from dolomite (Dickson, 1966). In petrographic studies, skeletal and non-skeletal allochem percentages have been accounted based on Tucker (1991) and Flugel (2010). Identification of microfacies based on Embry and Clovan (1971) and clastisity index based on Carrozi (1989) have been carried out.Results and DiscussionEight microfacies related to the open marine, shoal, lagoon and tidal flat sedimentary environment areas have been identified:Open marine facies:MF1- Pelagic Foraminifera Glauconitic Wackestone/PackstoneMF2- Bioclast Wackestone.MF3- Fine grained Bioclast PackstoneMF4- Intraclast WackestoneMF5- Echinoderm and Rudist debris Packstone to RudstoneShoal microfacies:MF6- Rudist GrainstoneLagoon microfacies:MF7- Orbitolinids Wackestone/PackstoneTidal flat microfacies:MF8- Sandy MudstoneSedimentary environment‌Based on lateral and vertical facies changes, marginal carbonate shelf can be proposed as the sedimentary environment for these strata. In this case, sedimentation has happened in deep, outer shelf, middle shelf and inner shelf areas. Moving towards salt dome, facies change to shoal, lagoon and intertidal shallow facies. This can be due to diapirism movements in the area. Due to upward salt dome movement, some evaporitic clasts have been transported to the sedimentary environment (Giles and Lawton, 2002); this can be observed more in near salt dome sections.Diagenetic process in two sectionsIn areas far from salt dome section, there are some diagenetic processes such as: micritization, cementation (equant), bioturbation and, glaconitization processes, but close to salt dome sections, we can see more diagenetic processes such as: cementation (syntaxial), physical and chemical compactions (stylolite), silicification, dolomitization and hematitization. This fact shows that there are more diagenetic processes in near salt dome sections which can be related to the diapirism movement in this area.ConclusionBased on petrographic studies, eight microfacies related to the carbonate shelf (from open marine to tidal flat) have been identified. Dolomitization, silicification and cementation (anhydrite and dolomite) processes had happened because of supersaturated fluids (high mg) in this area. Some diagenetic processes such as hematitization and physical and chemical compaction have increased once moving towards the salt dome. Comparing two sections to each other based on petrography, microfacies, diagenesis, energy index allochems, siliciclastic particles, gypsum and anhydrite cements, dolomite, etc. show us that diapirism had happened as a syn-sedimentation and post-sedimentation process.