ريزرخساره‏ ها، محيط رسوبي و تغييرات نسبي سطح آب دريا در سازند روته، برش‏ هاي سنگسر و مكارود، البرز مركزي

Microfacies, Sedimentary Environment and Relative Sea Level Changes of the Ruteh Formation, Sangsar and Makaroud Sections, Central Alborz

سازند روته دومين چرخۀ رسوب‏گذاري پرمين در حوضۀ البرز است. مطالعات صحرايي و پتروگرافيكي در برش‏هاي سنگسر (دامنۀ جنوبي البرز مركزي) و مكارود (دامنۀ شمالي البرز مركزي) نشان‏دهندۀ اين است كه رسوبات سازند روته شامل 19 ريزرخسارۀ كربناته و يك سنگ رخسارۀ سيليسيآواري هستند كه در پرمين مياني در كمربندهاي رخساره‏اي پهنۀ جزرومدي تا درياي باز از يك رمپ كربناتۀ هم‏شيب بايوكلاستي در حاشيۀ جنوبي غيرفعال پالئوتتيس واقع در شمال ابرقارۀ گندوانا در يك منطقۀ گرمسيري نهشته شده‏اند. رسوبات سازند روته در هر دو برش عمدتاً در محيط رمپ دروني نهشته شدند اما ضخامت رسوبات نهشته شده در كمربندهاي رخساره‏اي درياي باز در برش سنگسر از برش مكارود بيشتر است. اجزاي اصلي سازندۀ رسوبات شامل انواع جلبك‏ها، فرامينيفرها و متازوآن‏ها از قبيل اكينودرم‏ها، بريوزوئرها، گاستروپودها، براكيوپودها، استراكودآ، تريلوبيت‏ها، دوكفه‏اي‏ها و اسفنج‏ها توزيع متفاوتي را در زيرمحيط‏هاي مختلف پلاتفرم كربناته نشان مي‏دهند. بررسي تغييرات نسبي سطح آب دريا در اين تحقيق، بر اساس مشاهدات صحرايي و آناليز رخساره‏اي است. اين مطالعات به تشخيص دو سكانس رسوبي ردۀ پايين در برش سنگسر و سه سكانس رسوبي ردۀ پايين در برش مكارود منجر گرديد. مرز سكانس‏ها توسط ناپيوستگي‏هاي فرسايشي، ولكانيك‏ها، افق‏هاي خاك ديرينه، ظهور ريزرخساره‏هاي جزرومدي و ريزرخساره‏هايي كه نشان‏دهندۀ حداكثر افت سطح آب دريا در هر سكانس هستند، مشخص مي‏شود. عميق‏ترين ريزرخساره‏ها نيز به‏عنوان سطوح نشان‏دهندۀ حداكثر پيشروي آب دريا در نظر گرفته شده‏اند. رسوبات سازند روته در هر دو برش بخشي از ابرچرخۀ ترانسپكاس (زيرسكانس آبزاروكاي II) هستند. بيشترين عميق‏شدگي آب دريا در بخش‏هاي مياني سازند روته است و روند كلي تغييرات سطح آب دريا در اين سازند با تغييرات جهاني سطح آب دريا در پرمين مطابقت دارد.

Introduction According to different paleontological and paleomagnetic studies, Iran was part of the Gondwana during the Permian. The Permian lithostratigraphic units in the AlborzAzerbaijan are introduced as Doroud, Ruteh and Nesen Formations. The Ruteh Formation, the second depositional cycle of the Permian in the Alborz Basin, have been studied at two stratigraphic sections in the Central Alborz. The Sangsar section located on the south flank of the Central Alborz, 1 km northwest of Mahdishahr city and the Makaroud section located on the north flank of the Central Alborz, about 37 km south of Chalous city. The thickness of the Ruteh Formation at the Sangsar section is 106 m and at the Makaroud section is 222 m. At the Sangsar section the Ruteh Formation is underlain by the Doroud Formation with gradual contact and is overlain by a lateritic horizon. At the Makaroud section the Ruteh Formation disconformably overlies the Doroud Formation and the upper boundary is faulted and the Chalous Formation overlies the Ruteh Formation at this section. The aim of this paper is to analysis microfacies, interpret depositional environments and delineate relative sea level changes of the Ruteh Formation. Other researchers studied the Ruteh Formation at different sections in the Alborz Basin believe that the carbonate sediments of this formation have been deposited in a homoclinal carbonate ramp and consist of twothree 3rd order depositional sequences. But no sedimentological studies have been done at the selected sections in this study.   Material & Methods Two stratigraphic sections of the Ruteh Formation have been selected, measuted and sampled. One hundred sixty three samples (fifty seven samples from Sangsar and one hundred six samples from Makaroud section)  were collected and thin sections were prepared from all samples. Afew samples were collected from lower and upper formations. Thin sections were stained with potassium ferricyanide and alizarinred S solution according to Dickson (1965) method. The grain and matrix percentages were estimated using visual percentage charts of Bacelle and Bosellini (1965). Dunham (1962) classification were used for carbonate facies nomenclature. Based on lithological and textural characteristics, fossil content, abiotic allochems, facies succession and their comparision with well studied environments by Flugel (2010), microfacies and their subenvironments have been identified. In this study investigation of relative sea level changes is on the basis of field observations and facies analysis. Based on the vertical succession of microfacies and sedimentary paleoenvironmental features, Systems tracts and sedimentary sequences have been recognized. Discussion of Results & Conclusions Field and petrographic studies at the Sangsar and Makaroud sections, indicate that the Ruteh Formation sediments consist of 19 carbonate microfacies and one siliciclastic petrofacies that during the Middle Permian times deposited in tidal flat to open marine facies belts of a bioclastic homoclinal carbonate ramp on the southern passive margin of the Paleotethys located on the northern Gondwana Supercontinent in a tropical region. Tidal flat facies belt includes: Dolomitic mudstone, lime mudstone to sandy lime mudstone, pelloid intraclast packstone and one quartzarenite siliciclastic petrofacies. Lagoon facies belt includes: bioclast mudstone, dasycladacea wackestone, foraminifera pelloid wackestone, bioclast pelloid wackestone,  bioclast tubiphytes wackestone, bioclast algal wackestone/packstone. Shoal facies belt includes: ooid pelloid packstone/grainstone, bioclast pelloid packstone/grainstone, pelloid bioclast grainstone and algal packstone/grainstone. Open marine facies belt includes: tubiphytes gymnocodiacea wackestone, gymnocodiacea packstone, bioclast gymnocodiacea wackestone, sponge spicule bioclast wackestone, crinoid mudstone/wackestone and bioclast mudstone.  The Ruteh Formation at both sections deposited mainly in inner ramp environment but thickness of the sediments deposited in open marine facies belt at the Sangsar section is more than the Makaroud section. The main sediments constituents including algae, foraminifera and metazoans show different distribution in the various subenvironments of the carbonate platform. This studies resulted in identifying two low rank sequences at the Sangsar section and three low rank sequences at the Makaroud section. Sequence boundaries are identified by disconformities, volcanics, paleosol horizons, emergence of tidal flat microfacies and microfacies that are indicative of maximum sea level fall in each sequence. The deepest microfacies are also regarded as maximum flooding surfaces. Deposits of the Ruteh Formation at both sections is part of the Transpecos Supercycle (Absaroka II Subsequence). The most deepening of the sea water occurs in the middle parts of the Ruteh Formation and general trend of the sea level changes in this formation is accordant with the global sea level changes during the Permian.