رخساره‌ها و محیط رسوب‌گذاری نهشته‌های کامبرین بالایی البرز مرکزی، عضو 3 و 4 سازند میلا (سازند ده‌ملا) با تاکید ویژه بر سنگ‌های نواری

کامبرین پسین همراه با تغییرات مهمی در اقیانوس‌ها و خشکی‌ها ازجمله نهشت رخساره‌های خاصی در بخش‌های مختلف دنیاست که به دلیل ماهیت مطبق و نوارمانند به نام «سنگ‌های نواری» شناخته می‌شوند. توالی‌های کامبرین بالایی البرز با نام پیشنهادی «سازند ده‌ملا» شناخته می‌شوند. در پژوهش حاضر، این توالی‌ها در ناحیه‌ البرز مرکزی با تاکید بر سنگ‌های نواری آن بررسی رسوب‌شناختی شده‌اند و سعی شده است با آنالیز رخساره‌ای و تفسیر شرایط محیطی، درک مناسبی از ویژگی‌های رسوب‌شناختی کامبرین بالایی البرز به دست آید. در این زمینه سازند ده‌ملا به 9 واحد لیتواستراتیگرافی و 12 ریزرخساره‌ رسوبی تفکیک و مشخصات رسوب‌شناختی آن به تفصیل بررسی شده است. نتایج نشان می‌دهد این رخساره‌ها در یک پلاتفرم کربناته‌ اپیریک نهشته شده‌اند؛ ولی در طول زمان تغییرات مهمی در فیزیوگرافی و توزیع رخساره‌ای حوضه رخ داده است؛ به نحوی که تکامل پلاتفرم یادشده را می‌توان به سه مرحله تقسیم کرد؛ در مرحله‌ اول رخساره‌های عمدتاً جزر و مدی در کمربندهای رخساره‌ای وسیع نهشته شده‌اند و محیط رسوب‌گذاری در معرض تلاطم‌های بزرگ قرار نداشته است. در مرحله‌ دوم در اثر پیش‌روی سریع دریا، پلاتفرم غرق شده است و رسوبات شیلی سبز روی نهشته‌های پیشین نهشته شده‌اند. در مرحله‌ سوم محیط رسوب‌گذاری به‌شدت متاثر از عوامل آشفته‌ساز محیطی همچون طوفان، جریان‌های دریایی و جزر و مد قرار داشته و این وضعیت به تغییرات شدید رخساره‌ای و نهشت سنگ‌های نواری منجر شده است. سنگ‌های نواری، طیفی از شرایط زیر جزر و مدی تا جزر و مدی را نشان می‌دهند.

Facies and depositional environment of the Upper Cambrian of the central Alborz, members 3 and 4 of the Mila Formation (Deh-Molla Formation), with emphasis on their ribbon rocks

AbstractThe Late Cambrian was associated with tremendous changes in world oceans and lands including the deposition of specific facies in different parts of the world known as “ribbon rocks” due to their thinly bedded undulatory banded nature. In this study, the Upper Cambrian succession of the central Alborz (members 3 and 4 of the Mila Formation) is sedimentologically investigated with an emphasis on its upper part’s ribbon rocks. The study tries to give an understanding of facies analysis and depositional environment of this succession. In this regard, the strata are divided into nine lithostratigraphic units and 12 microfacies representing three subenvironments of supra interand subtidal. The results show that the microfacies were deposited in an epeiric platform with marked changes in the facies distribution and platform physiography in time so that the evolution of this platform can be divided into three stages. During the first stage, dominantly tidal flat facies were deposited in broad belts without significant effects from disturbing agents such as storms. In the second stage, the platform was drowned due to a rapid transgression where deepmarine glauconiterich green shale was deposited on former platform facies. In the third stage, the environment was strongly affected by disturbing factors such as storms and tides where severe facies changes occurred, represented by the deposition of ribbon rocks. The ribbon rocks of the DehMolla Formation show a spectrum of subtidal to intertidalsupratidal depositional settings.Keywords: Upper Cambrian, Mila Formation, DehMolla Formation, Ribbon rocks, Depositional environment  IntroductionThe Upper Cambrian rocks of the Alborz zone are known as the members 3 and 4 of the Mila Formation, newly suggested to be regarded as a new formation, that is, the DehMolla Formation (Geyer et al. 2014). The DehMolla Formation can be subdivided into two informal members of 1 and 2. Member 1 is composed of thickbedded to massive carbonates without significant variations in bedding nature in lateral or vertical directions. Member 2, in contrast, is dominated by thinbedded ribbon rocks, pointing to significant environmental changes. This difference in the lithology, bedding nature and facies distribution of these two members provides a potential research subject. The current study aims to investigate sedimentologic aspects, facies analysis and depositional environment of the DehMolla Formation in the central Alborz with an emphasis on its ribbon rocks.  Materials MethodsThree stratigraphic sections were chosen for this study to encompass all spatiolateral variations of the formation. These include the Shahmirzad, TuyehDarvar and DehMolla sections. During the fieldwork, the sections were measured, logged and sampled carefully with regard to their lithology, bedding geometry and architecture, fossil content, stratal surfaces, sedimentary structures and depositional trends. In total, 257 rock samples were collected from these sections, which were petrographically investigated in the laboratory. The samples were classified according to standard widelyused nomenclatures (Dunham 1962; Embry and Klovan 1971) and were grouped into facies and facies association after comparison with standard facies models as well as with results of coeval studies. Discussion of Results ConclusionsTwelve microfacies are identified in the studied rocks indicating three facies associations/subenvironments of supratidal, intertidal and subtidal. These facies association and related microfacies include: i) supratidal facies association: MF1: Dolomudstone; MF2: Ooid dolograinstone–dolopackstone; ii) intertidal facies association: MF3: Rounded echinoderm ossicle grainstone; MF4: Brachiopod packstone or rudstone; MF5: Oncoid rudstonegrainstone; MF6: Microbial mudstone; MF7: Bioclastic packstonegrainstone; and iii) lower intertidal–subtidal facies association: MF8: Intraclast grainstonepackstone; MF9: Microbiallithistid framestone; MF10: Bioclastic wackestonebindstone; MF11: Bioclastic packstonegrainstone; MF12: Green shalemarl.The domination of peritidal facies in the DehMolla Formation suggests that they were probably deposited in a gently sloping broad carbonate platform (most likely in an epeiric platform) (Pratt and James 1986). The distribution of the recognized facies in space and time indicates that the environment significantly changed during the deposition of the formation so that the evolution of the platform through time can be considered as a threestage scenario:Stage 1 includes Member 1 of the studied formation. The sediment was deposited in a gently sloping carbonate platform under calm and arid environmental conditions. The supratidal region was the site of evaporitic processes leading to the deposition of a thick dolomite unit (MF1). The coastline and adjacent area were occupied by dunes and smaller bedforms of MF2 to MF5, pointing to the constant action of surfs and waves, which led to the accumulation of shells and graindominated shoal and banks. The rest of the intertidal together with the shallower subtidal area was the best region for the development of microbialdominated structures such as microbialsponge small buildups and bluegreen algal biostromes (MF9). Sheltered areas between these bioherms were places with a superior accumulation of mud accompanied by debris derived from these buildups (MF10).Stage 2 is characterized by the deposition of green deepermarine marl and shales. The unit marks the base of the Member 2 and the drowning phase of the carbonate platform during which deeper subtidal facies (MF12) accumulated over previous shallower marine carbonates.Stage 3 includes the uppermost part of the formation that is characterized by ribbon strata. These strata have recorded significant environmental perturbations during their deposition represented by the development of thinly bedded rocks and multiple tempestite horizons. During this stage, the platform was characterized by numerous shoals, banks and bedforms mostly with a medium to smallscale extension, which formed due to  repeated action of tides, waves and storms. The alternation of calm and stormy conditions led to the development of couplets of skeletal (MF7) and microbial (MF6) dominated facies in intertidal settings. Many sedimentary structures and rock aspects are recorded in these strata, pointing to a combined action of tides and storms. These include wavy, lenticular and flaser crossstratification, graded bedding, shell imbrications, flatpebble conglomerates, multiple erosional surfaces, ripple cross stratifications, and etc.This study reveals that the ribbon rocks are mainly composed of five microfacies, including MF6–MF8 and MF10–MF11 associated with interbeds of shale and marls. The facies show a spectrum of environmental conditions from subtidal to intertidalsupratidal. Therefore, these banded thinbedded rocks were deposited in a broad platform with a gentle slope characterized by recurring dynamic depositional processes such as storms and tides shaping the physiography and morphodynamic of the environment. These processes led to the development of numerous bedforms along the platform. Many similar studies also indicate a peritidal origin for these rocks (e.g., Demicco and Mitchell 1982; Demicco 1983; Cañas 1999; Laughrey and Harper 2012).