دانشگاه اصفهان پژوهش های چینه نگاری و رسوب شناسی 2008-7888 42 3 2026 09 23 Lithostratigraphy and petrofacies of the Middle Triassic (Upper part of the Elika Formation) and the Upper Triassic–Lower Jurassic strata (Basal part of the Shemshak Group, Shamirzad Formation) in the Jajarm Bauxite Mine, NE Iran سنگ‎‍چینه‎‍نگاری و پتروفاسیس نهشته‎‍های تریاس میانی (بخش بالایی سازند الیکا) و تریاس بالایی-ژوراسیک زیرین (بخش قاعده‎‍ای گروه شمشک، سازند شهمیرزاد) در معدن بوکسیت جاجرم 1 22 30392 10.22108/jssr.2026.148175.1326 FA عزیزاله طاهری استاد، گروه زمین‎‍شناسی، دانشکده علوم‌زمین، دانشگاه صنعتی شاهرود، شاهرود، ایران مهدی جعفرزاده دانشیار، گروه زمین‎‍شناسی، دانشکده علوم زمین، دانشگاه صنعتی شاهرود، شاهرود، ایران Journal Article 2026 01 31 Abstract In the northeastern Jajarm region, within the structural zone of the Eastern Alborz, a complete sequence of the Elika Formation (Early–Middle Triassic) and the Shemshak Group (Late Triassic–Middle Jurassic) is exposed. In this study, the upper part of the Elika Formation and the Upper Triassic Shahmirzad Formation of the Shemshak Group were investigated based on lithostratigraphic and petrographic analyses. The upper part of the Elika Formation, approximately 80 m thick, mainly consists of dolomite and dolomitic limestone with stromatolitic structures and lacks fossils, indicating deposition in a tidal environment. The Shemshak Group, with an approximate thickness of 2,050 m, comprises sandstones, silty sandstones, siltstones, shales, and fossiliferous limestones, and is subdivided into the Shahmirzad, Alasht, Shirin-Dasht, Fil-Zamin, and Densrit formations. The Shahmirzad Formation, about 285 m thick, predominantly consists of shale and sandstone and includes fossiliferous marker beds and diverse petrofacies such as litharenite and lithic arkose. The results indicate that the Shemshak Group exhibits significant lateral variations due to deposition in a fluvial–deltaic environment, and the observed litharenites reflect uplifted sequences associated with orogenic activity and foreland basin subsidence. The results of this study provide a detailed lithostratigraphic framework and fundamental data for identifying bauxite horizons and guiding future exploration. Keywords: Jajarm, Shemshak Group, Elika Formation, Shahmirzad Formation, Petrofacies Introduction The Shemshak Group in the Gol-e-Bini section, similar to other exposures of this group, is predominantly composed of siliciclastic deposits, including sandstone, siltstone, and silty sandstone. This succession was previously investigated by Fürsich et al (2009). According to these studies, the total thickness of the Shemshak Group was measured as 2,050 meters, comprising, from base to top, the Shahmirzad, Alasht, Shirindasht, Fil-e-Zamin, and Dansirit formations. In the Gol-e-Bini section, the lower boundary of the Shemshak Group unconformably overlies the dolomitic strata of the Elika Formation, whereas its upper boundary is disconformably overlain by marly limestones and marls of the Dalichai Formation. Both the lower and upper contacts are erosional disconformities associated with two major tectonic phases, namely the Early Cimmerian and Middle Cimmerian tectonic events, respectively. Variations in sedimentary successions among different depositional basins emphasize the importance of stratigraphic investigations, particularly lithostratigraphic subdivision, as one of the fundamental components of regional geological studies. The relatively homogeneous lithological characteristics and the scarcity of key beds have made the recognition of lateral facies variations within the Shemshak Group extremely difficult. Therefore, the present study focuses on a detailed stratigraphic investigation of the upper part of the Elika Formation and the basal formation of the Shemshak Group, including the Shahmirzad Formation. These formations were subdivided into lithological units, and where possible, key beds and marker horizons were identified and introduced. Materials & Methods The study of the Shemshak Group has been carried out in two stages: field and laboratory. Due to the lithological homogeneity of the succession and the absence of distinct key beds, tracing lateral facies variations within the Shemshak Group is inherently difficult. In this research, after a thorough review of previous studies and field surveys, suitable stratigraphic sections were selected for examination. Because mining activities and tectonic deformation have disrupted parts of the original sedimentary succession, no single complete stratigraphic section could be identified. Consequently, the succession was examined in three separate sections: Gol-e-Bini 4, Gol-e-Bini 6, and Zoo. During field investigations, the sedimentary succession was subdivided into lithological units based on bedding characteristics, sedimentary structures, fossil content, lithology, and both vertical and lateral facies variations. For further description and analysis, samples were collected from certain beds to prepare thin sections. In total, 100 sandstone samples were collected during fieldwork, of which 70 thin sections were prepared and examined using a polarizing microscope. Discussion of Results & Conclusions The upper part of the Elika Formation, with a thickness of about 80 meters, mainly consists of dolomites and dolomitics limestone containing stromatolitic structures and lacking any fossil content, indicating deposition in a tidal flat environment. The considerable thickness and lithological composition of the Shemshak Group suggest sediment accumulation within a shallow basin characterized by a high subsidence rate, most likely within a foreland basin setting. Progressive subsidence of the basin floor resulted in gradual landward transgression and rising relative sea level, eventually leading to the establishment of a carbonate platform upon which the sediments of the Dalichai Formation were deposited. Petrographic and Petrofacies analysis indicates that carbonate and chert rock fragments are abundant in the basal part of the Shemshak Group, whereas metamorphic rock fragments appear in its upper part. These variations in rock fragment composition are related to tectonic conditions and provenance. The carbonate lithic fragments were most likely derived from erosion of the Elika Formation, while the metamorphic fragments were sourced from the Gorgan Schists, which stratigraphically underlie the Elika Formation. The dominant lithology of the Shahmirzad Formation consists of shale interbedded with sandstone layers. A fossiliferous limestone bed in the upper part of the formation constitutes an important marker horizon within this siliciclastic succession. In addition, sandstone beds containing autochthonous calamites in the middle part of the formation represent another distinctive stratigraphic horizon. The predominant petrofacies in the Shahmirzad Formation include siltstone, litharenite with clay cement and matrix, litharenite with carbonate cement, feldspathic litharenite with carbonate cement, feldspathic litharenite with siliceous cement, lithic arkose with carbonate cement, and lithic arkose with clay cement. The Shahmirzad Formation representing the basal unit of the Shemshak Group that was deposited predominantly in a fluvial depositional environment characterized by channel sandstones, floodplain deposits, and swamp sediments. The occurrence of thin coal seams, root horizons, plant fossil remains and wood fragments, collectively indicate deposition under warm and humid climatic conditions. Evidence for a subsequent marine transgression is provided by carbonate beds in the upper part of the formation containing bivalve and echinoid fossils. در شمال ‌شرقی جاجرم، حوضۀ ساختاری البرز خاوری، توالی کاملی از سنگ‌های سازند الیکا (تریاس پیشین–میانی) و گروه شمشک (تریاس پسین–ژوراسیک میانی) رخنمون دارد. در این مطالعه، بخش بالایی سازند الیکا و سازند شهمیرزاد از گروه شمشک، بر‎‍اساس مطالعات سنگ‌چینه‌نگاری و پتروگرافی بررسی شدند. بخش بالایی سازند الیکا با ضخامت حدود ۸۰ متر، عمدتاً شامل دولومیت و آهک دولومیتی با ساخت‌های استروماتولیتی و فاقد فسیل و نشان‌دهندۀ ته‎‍نشست در محیط جزر و مدی است. گروه شمشک با ضخامت تقریبی ۲۰۵۰ متر شامل ماسه‌سنگ، سیلت‌سنگ، شیل و آهک فسیل‌دار است و به سازندهای شهمیرزاد، آلاشت، شیرین‌دشت، فیل‌زمین و دنسریت تفکیک می‌شود. سازند شهمیرزاد با ضخامت حدود ۲۸۵ متر، عمدتاً از شیل و ماسه‌سنگ تشکیل می‎‍شود و شامل لایه‌های شاخص فسیل‌دار و پتروفاسیس‌های متنوع نظیر لیت‌آرنایت و لیتیک آرکوز است. یافته‌ها نشان می‌دهد که سازند شهمیرزاد تحت تأثیر رسوب‌گذاری در محیط رودخانه‌ای–دلتایی، دچار تغییرات جانبی درخور توجه است و لیت‌آرنایت‌های موجود، نمایانگر توالی‌های بالا‎‍آمدۀ ناشی از فعالیت کمربندهای کوهزایی و ته‌نشست‌های مرتبط با حوضه فورلند هستند. نتایج این مطالعه، چارچوب لیتواستراتیگرافی دقیق و اطلاعات پایه‌ای ارزشمندی را برای شناسایی افق‌های بوکسیتی و مطالعات اکتشافی آینده ارائه می‌دهد.

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