دانشگاه اصفهان پژوهش های چینه نگاری و رسوب شناسی 2008-7888 42 3 2026 09 23 Reconstruction of the depositional conditions of the Eocene siliciclastic deposits in the Zagros (Kashkan Formation, south of Lorestan) بازسازی شرایط ته‎‍نهشت نهشته‎‍های آواری ائوسن زاگرس (سازند کشکان، جنوب لرستان 79 113 30406 10.22108/jssr.2026.148437.1328 FA یعقوب نصیری استادیار، گروه زمین‎‍شناسی، دانشکدۀ علوم، مجتمع آموزش عالی گناباد، گناباد، ایران سمیرا تقدیسی نیک بخت استادیار، گروه زمین‎‍شناسی، دانشکدۀ علوم، مجتمع آموزش عالی گناباد، گناباد، ایران مصطفی صداقت نیا دانش‎‍آموختۀ مقطع دکتری، کارشناس آزمایشگاه مرکزی دانشگاه لرستان، خرم‎‍آباد، ایران Journal Article 2026 02 22 <strong>Abstract</strong> The aim of this research is to investigation the lithofacies and sedimentary environment of the Kashkan Formation (middle Eocene) in the Lorestan sedimentary basin. In this regard, four stratigraphic sections were selected. The Kashkan Formation in the studied area has a variable thickness of 150 to 200 meters. It is mainly composed of conglomerate and sandstone strata, with lesser amounts of fine-grained lithofacies including mudstone, siltstone, and shale. Field studies on the four sections led to the identification of 12 lithofacies, which include five conglomeratic lithofacies (Gm, Gms, Gp, Gh, Gt), five sandstone lithofacies (Sp, St, Sm, Sh, Sr), and two mudstone lithofacies (Fm, Fl). Based on the vertical and lateral changes of facies, the depositional environment of these deposits was a braided river system with a gravelly and sandy bed in proximal areas. The clastic sequences of the Kashkan Formation overlie the marine carbonates of the Taleh Zang Formation, which indicates a sea regression and the progradation of braided rivers toward the basin. A decrease in accommodation space relative to sediment supply caused the progradation of braided river facies within the LST facies suite for the Kashkan Formation. <strong>Keywords:</strong> Sedimentary environment, Kashkan Formation, Zagros, Lorestan. <strong> </strong> <strong> </strong> <strong>Introduction</strong> Terrigenous rocks in the Zagros region of Iran have received relatively little attention. These rocks (e.g., the Amiran, Kashkan, Aghajari, and Bakhtiari formations) have mostly been studied only from the perspectives of lithostratigraphy or stratigraphic position (Motiei 1993). The Kashkan Formation is most extensively distributed in the Lorestan region, and its main facies include conglomerate, sandstone, and siltstone. This formation is situated between the carbonate Telezang Formation below and the Shahbazan Formation above. Where the Telezang Formation is absent, the Kashkan Formation rests directly on the Amiran Formation. The type section of the Kashkan Formation was introduced by James and Wynd (1965) in the Amiran Anticline near the city of Mamulan. Terrigenous formations are of great importance in the Zagros in terms of tectonics, sedimentary environment, climatic changes, and hydrocarbon generation and storage. The Kashkan Formation is essentially a terrigenous unit and is widely exposed in Lorestan. This formation is named after the Kashkan River that flows through this province. Among the few studies conducted on the Kashkan Formation, Yousefi Yeganeh et al. (2012) investigated the sedimentary environment and trace fossils of this formation within the Lorestan sedimentary basin, focusing on the northern, southeastern, and northwestern parts of Lorestan Province. The main goal of this research is to identify and describe the outcrops of the Kashkan Formation along the Khorramabad–Pol-e Zal Freeway (southern and southwestern Lorestan)—an area that has remained largely understudied. Through detailed analysis of lithological units and sedimentary environments in four selected sections, the depositional processes of this formation will be better understood. Finally, by integrating these findings with previous research, a comprehensive understanding of the depositional history of the Kashkan Formation in the Zagros sedimentary basin will be completed and enhanced.   <strong>Materials & Methods</strong> The sections were selected to maximize thickness, facies diversity, and accessibility while minimizing cover. In this regard, four stratigraphic sections were chosen along the Khorramabad–Pol-e Zal Freeway. In these sections, field characteristics—including grain size, bedding, geometric form of beds, sedimentary structures, contact types between beds, and sediment stacking patterns—were carefully examined and recorded. The thickness of the formation was measured using a Jacob staff and compass. Gravels within conglomeratic units were studied using the ribbon method (Tucker 2003). This method involves dividing conglomeratic outcrops into sequential lateral or longitudinal ribbons; within each ribbon, the size, roundness, sorting, lithology of clasts, and their percentages are systematically recorded to document lateral and vertical depositional changes in a regular, comparable manner. The orientation of the foresets of current ripples was used to determine paleocurrent direction. Sandstone samples were collected based on facies variations using a point-sampling method from the middle part of the beds. Clastic facies were classified according to Miall's (2006) codes, and sandstone classification followed Folk (1980). By integrating data from microscopic and field studies, lithofacies and facies associations representative of sub-environments within a sedimentary system were identified and differentiated. Finally, the sedimentary model of the Kashkan Formation was interpreted. <strong> </strong> <strong>Discussion of Results & Conclusions</strong> Field studies on the examined sections led to the identification of 12 lithofacies. These include coarse-grained conglomeratic lithofacies (Gm, Gms, Gp, Gh, Gt), medium-grained sandstone lithofacies (Sp, St, Sm, Sh, Sr), and fine-grained mudstone lithofacies (Fm, Fl). The architectural elements identified in the four studied sections include channel elements (CH), overbank fines (OF), gravelly bar elements (GB), sandy bar elements (SB), and sediment gravity flow elements (SG). The SG (sediment gravity flow) element is introduced as the main and dominant element in proximal braided rivers with debris-laden flows, which is entirely consistent with the high percentage of conglomeratic facies in the studied deposits. This element primarily consists of massive conglomerates with very poor bedding, a lack of large-scale sedimentary structures, and poor sorting. The formation of the SG element is attributed to high-energy fluvial flows, particularly debris flows and hyperconcentrated flows. These flows typically develop under conditions of sudden discharge increases, bank instability, or intense bedload activity within braided river channels, leading to the rapid deposition of coarse-grained sediment. In terms of depositional setting, the SG element is interpreted mainly as a fill of main and subsidiary channels within a braided river system, representing peak flow energy phases. The abundance of this architectural element indicates the predominance of high-energy conditions, a relatively steep channel gradient, and a continuous supply of coarse-grained sediment to the sedimentary basin. Therefore, the SG element in the studied deposits is presented directly within the framework of a braided river model with an effective role played by debris flows. Sequence stratigraphy of the Kashkan Formation—as a prominent representative of high-energy fluvial depositional environments in the Zagros (Lorestan) basin—provides a powerful tool for reconstructing the basin's evolutionary history and understanding the influence of sea-level fluctuations on river dynamics. The dominant lithological composition of the Kashkan Formation includes thick conglomerates, coarse-grained sandstones, and mudstone, which directly indicates high-energy depositional conditions, such as those found in braided river systems. The significance of sequence stratigraphy for the Kashkan Formation extends beyond understanding sea-level changes; it holds the key to the tectonic history of the Zagros Basin by identifying sedimentary units related to erosion and deposition during periods of base-level fall. The formation of sedimentary sequences, particularly lowstand systems tracts (LSTs) that occur during periods of pronounced sea-level fall (i.e., reduction in river base level), is often associated with deep incision of riverbeds and the development of valleys and channels that are subsequently filled by conglomeratic and sandy sediments. These processes are closely linked to regional tectonic uplifts and changes in basin slope. Therefore, identifying and delineating LST units within the Kashkan Formation can serve as an indicator of tectonic activity and uplift events associated with the Zagros orogeny. Based on the vertical and lateral changes in facies, the depositional environment of these deposits was a proximal braided river system with a gravelly and sandy bed. Paleocurrent direction analysis for this formation shows a northeast-to-southwest trend, which is evidence of unidirectional (fluvial) flows within the Kashkan Formation. The shallow depth and low sinuosity of the channels, along with the presence of coarse-grained gravelly and sandy sediments, support the interpretation that the clastic facies of the Kashkan Formation accumulated in a proximal braided river environment. هدف از این پژوهش، بررسی رخساره‎‍های سنگی و محیط رسوبی سازند کشکان (ائوسن میانی) در حوضۀ رسوبی لرستان است. در این راستا چهار برش چینه‎‍شناسی انتخاب شد. سازند کشکان در محدودۀ‎‍ مطالعه‎‍شده با ضخامت 150 تا 200 متر، از توالی غالب کنگلومرا و ماسه‌سنگ و به میزان کمتر رخساره‌های ریزدانۀ گلسنگی، سیلت‎‍سنگی و شیلی تشکیل شده است. لایه‌های ریزدانه بیشتر به‎‍صورت میان‌لایه‌های پراکنده در بین واحدهای کنگلومرایی و ماسه‌سنگی مشاهده می‌شوند. مطالعات صحرایی بر‎‍ چهار برش، به شناسایی 12 رخسارۀ سنگی منجر شد که شامل پنج رخسارۀ سنگی کنگلومرایی (Gm, Gms, Gp, Gh, Gt)، پنج رخسارۀ سنگی ماسه‎‍سنگی (Sp, St, Sm, Sh, Sr) و دو رخسارۀ سنگی گلی (Fm, Fl) شد. بر‎‍اساس تغییرات عمودی و جانبی رخساره‎‍ها، محیط رسوب‎‍گذاری این نهشته‎‍ها سیستم رودخانه‎‍ای بریده‎‍بریده با بستر گراولی و ماسه‎‍ای در نواحی نزدیک به منشأ بوده است. توالی‎‍های آواری سازند کشکان بر‎‍ کربنات‎‍های دریایی سازند تله‎‍زنگ قرار گرفته است که نشان از پس‌روی دریا و گسترش رودخانه‎‍های بریده‎‍بریده به‎‍سمت حوضه است. کاهش فضای رسوب‎‍گذاری نسبت‎‍به تأمین رسوب سبب پیشروی رخساره‎‍های رودخانه‎‍ای نوع بریده‎‍بریده در دسته رخسارۀ LST برای سازند کشکان بوده است. آنالیز جهت جریان دیرینه در این سازند، روند شمال شرق به جنوب غرب را نشان می‎‍دهد که شاهدی بر جریان‎‍های یک‎‍طرفه (رودخانه‎‍ای) در سازند کشکان است. https://jssr.ui.ac.ir/article_30406_cf0ef620ac5015f9ce884bcc58648903.pdf