رخساره‎‍ها، محیط رسوبی و رخساره‌های الکتریکی سازند هجدک (ژوراسیک میانی) در معدن زغال‌سنگ پرودۀ طبس، جنوب طبس

سازند هجدک با سن ژوراسیک میانی در شمال حوضۀ زغال‌دار پروده-نایبند و در فاصلۀ 90 کیلومتری جنوب طبس در محدودۀ معدن زغال‌سنگ پروده‌ و از‌نظر ساختاری بر بلوک طبس در زون شرقی ایران مرکزی مطالعه شده است. این سازند در برش‌ مطالعه‌شده، ضخامت 40 متر دارد. در این توالی رسوبی پنج واحد چینۀ سنگی شناسایی و تفکیک شده است که عمدتاَ شامل ماسه‌سنگ، شیل، کنگلومرا و سیلتستون است. بر‌اساس مطالعات صحرایی و مطالعات پتروگرافی در سازند هجدک چهار دسته رخسارۀ سنگی دانه‌درشت (gcm و gt)، متوسط (st، sp، sh و sr)، ریز (fl و fm) و کربناته (براکیوپود اکینوئید گرینستون-پکستون) شناسایی شده است. به‌طور کلی عناصر ساختاری کانال و پرکنندۀ کانال مربوط به کانال‌های جزر و مدی و باتلاق دشت ساحلی در رسوبات سازند هجدک شناسایی شده است و شرایط رسوب‌گذاری آن‌ به محیط پهنۀ ساحلی از یک محیط حاشیه دریایی آواری نسبت داده می‌شود. به‌منظور بررسی ویژگی‌های الکتریکی رخساره‌های سازند هجدک در زیر سطح این سازند در یک گمانۀ اکتشافی در محدودۀ معدن پروده نیز‌ بررسی شد. در این گمانه سازند هجدک 48 متر ضخامت دارد و با در نظر گرفتن تغییرات شکل لاگ گاما و مقاومت چهار رخسارۀ الکتریکی تشخیص داده شد که با زیرمحیط‎‍های پهنه‌های ماسه‎‍ای ساحلی، کانال‎‍های جزرومدی کوچک و باتلاق دشت ساحلی انطباق‌دادنی‌اند.

facies, sedimentary environment and log facies of the hojedk formation (middle jurassic) in parvadeh tabas coal mine, south of tabas

the middle jurassic hojedk formation in the parvadeh coal mine (90 km south of tabas) has been studied. the study area is structurally located on the northern tabas block in the north of the parvadeh–nayband coal-bearing basin, east-central iran. the hojedk formation has a thickness of 40 meters in the studied outcrop section. five lithostratigraphic units have been identified, which mainly consist of sandstones, shales, conglomerates, siltstones and thin bioclastic sandy limestone. based on field and petrographic studies, four groups of lithofacies including coarse- (gcm and gt), medium- (st, sp, sh and sr) and fine-grained clastic (fl and fm) as well as carbonate facies (echinoid brachiopod grainstone-packstone) have been identified in the hojedk formation. the architectural elements of channels and point bars of a meandering tidal creek and coastal plain swamp have been identified in the hojedk formation and its palaeodepositional conditions can be attributed to the tidal flat of a marginal marine siliciclastic environment. to investigate the characteristics of the hojedk formation in the subsurface, the electrical facies were defined in an exploration borehole in the eastern part of parvadeh mine. in this borehole, the hojedk formation is 48 meters thick, and four electro/log facies have been identified by considering the gamma-ray motif and resistivity logs. these electro facies are compatible with the sub-environments of coastal sand flats, tidal channels and coastal plain swamps.keywords: tabas block, hojedk formation, middle jurassic, facies, log facies  introductioncentral-east-iranian-microcontinent is a part of iran plate, which can be structurally divided into lut, tabas and yazd blocks (takin 1972; aghanabati 2004). the tectono-sedimentary evolution of east-central iran during the late triassic to the late jurassic is governed by the cimmerian orogenic events (fürsich et al. 2005; seyed-emami et al. 2004, 2020; wilmsen et al. 2009, 2021). the hojedk formation is the last lithostratigraphic unit of the shemshak group in east-central iran, which during the middle jurassic in this area was affected by cimmerian tectonic events resulting in large thickness and facies variations (wilmsen et al. 2009). biostratigraphic studies have led to determining the age of this formation (early bajocian) in eastern central iran (seyed-emami et al. 2020). macrofloras, palynology and sedimentary environment analysis have been conducted on the hojedk formation in tabas block, which has led to the reconstruction of the palaeoenvironmental and palaeoclimatic conditions of this formation (vaez-javadi and mirzaei-ataabadi 2006; vaez-javadi 2016; badihagh et al. 2019; khalilizadeh et al. 2023; salehi et al. 2023). based on preliminary studies conducted in the northern part of tabas block, the hojedk formation was formed in different sub-environments including lakes, coastal plains, deltas, fluvial and flood plains as well as the marine environment (seyed-emami et al. 2006). due to the importance of the hojedk formation in the reconstruction of the tectonic-sedimentary evolution of northern tabas block during the middle jurassic as well as its coal resources, this succession has been studied in the outcrop and subsurface sections in the parvadeh coal mine. the facies and sedimentary environment of this formation are still less known in many areas of the northern tabas block, including the parvadeh coal mine area. considering the extensive coal reserve of 1.1 billion tons in the northern tabas block, this region is important for detailed geological studies for the exploration of workable coal seams. to reconstruct the palaeoenvironmental deposition of the hojedk formation, field, subsurface and laboratory studies were carried out and all the data were integrated with the geological information of the area. furthermore, the identification of log facies of this formation in the exploration boreholes, drilled in eastern parvadeh mine, is useful for coal exploration, discovery and production from the coal-bearing strata of the studied area. material & methods the outcrop section of the middle jurassic hojedk formation was logged and measured by jacob’s staff (sdzuy and monninger 1985) in the western part of the parvadeh coal mine. sampling was done systematically and sometimes according to the identical facies changes. in total 25 hand specimens were collected and thin sections were prepared from the sandstone and limestone samples. clastic facies have been identified and classified based on miall’s classification (miall 1985, 2014). carbonate facies have been classified based on dunham’s scheme for carbonate rocks (dunham 1962), and microfacies character defined based on criteria described by flügel (2010). in addition, a subsurface study has been carried out on an exploration borehole (bh#341), which was drilled in the eastern part of the parvadeh mine. the lithostratigraphy and sedimentological characteristics of this formation in the borehole were compared with the adjacent studied outcrop section. graphical and gamma-ray well logs motifs of the borehole have been used in this study for electrical/log facies identification.  discussion of results & conclusionsin this research, the hojedk formation is studied in the north of parvadeh-nayband coal basin, 90 km south of tabas (parvadeh coal mine). based on field studies, five lithostratigraphic units have been identified.  the succession mainly consists of sandstones, shales, conglomerates, siltstones and thin bioclastic sandy limestone. clastic facies were defined in the field by considering lithology, sedimentary structure, geometry, stratal surface, palaeocurrent and fossil/trace fossil content. four groups of facies including coarse-, medium- and fine-grained clastic facies and one carbonate facies have been identified. two coarse- (gcm and gt), four medium- (st, sp, sh and sr), and two fine-grained facies including fl and fm were identified in the field. some medium-grained facies are bioturbated and sometimes show vertical cylindrical burrows (e.g. skolithos). the carbonate microfacies recognized based on rock texture (echinoid brachiopod grainstone-packstone), is equivalent to the standard microfacies (smf) 18. the latter facies were formed in carbonate bioclastic sand shoals in a marginal shallow marine mixed environment. in order to identify the architectural elements in the hojedk formation, features such as the lithology, stratal surface, geometry, and their associated facies have been considered. in the studied succession, three architectural elements including channel and point bars of a meandering tidal creek and coastal plain swamp have been identified in the hojedk formation. this formation also has been investigated in the subsurface section (exploration borehole no. 341) in the eastern part of the parvadeh coal mine. in this borehole, the hojedk formation is 48 meters thick and consists of sandstones, shales, limestones and siltstones. in this study, four electrical facies (log facies) were identified in the hojedk formation, which is compatible with the sub-environments of coastal sand flats, tidal channels, crevasse splay and coastal plain swamps. according to the identified lithofacies, microfacies, architectural elements and log facies in vertical and lateral distribution, the sedimentary environment of the hojedk formation in the studied outcrop and borehole is attributed to a tidal flat of a marginal marine siliciclastic environment.