ماکروفسیل‌های گیاهی، اثرفسیل‌ها، آنالیز رخساره‌ها و تفسیر محیط رسوبی سازند هجدک در برش چینه ‌شناسی ده‌شیخ، شمال کرمان (ایران مرکزی)

پژوهش حاضر به شناسایی 30 گونه متعلق به 16 جنس از ماکروفسیل‌های گیاهی راسته‌های اکوئیزتال‌، سیکادال‌، بنه‌تیتال، کنیفرال، پلتاسپرمال‌، ژینکگوآل، کوریستواسپرمال و فیلیکال منجر شد. بر اساس انتشار چینه‌شناسی گونه‌های شاخص ماکروفسیل‌های گیاهی شناسایی‌شده نظیر anomozamites major، coniopteris hymenophylloides، elatides thomasii، nilssonia bozorga، nilssonia tenuicaulis، nilssoniopteris mosafolia و otozamites furoni، سن ابتدای ژوراسیک میانی (باژوسین باتونین) برای لایه‌‌های دربرگیرنده پیشنهاد می‌شود. در پژوهش حاضر، اثرفسیل‌های نهشته‌های سازند هجدک در برش چینه‌شناسی ده‌شیخ، شمال کرمان مطالعه شدند که درحقیقت، نخستین گزارش از اثرفسیل‌های این سازند است. این اثرفسیل‌ها که 8 اثرگونه متعلق به 4 اثرجنس planolithes، rhizoliths، taenidium و thalasinoides را شامل می‌شوند، در کنار ساقه‌های برجای اکوئیزتال‌ها یافت شده‌اند. مطالعۀ حاضر کمک شایانی به شناسایی همراهی اثرفسیل‌ها با فسیل‌های گیاهی در این ناحیه و شناخت محیط رسوبی دیرینه است. به‌طور‌کلی، گیاهان یادشده شاخص محیط‌های مرطوبند، اما اثرفسیل‌های شناسایی‌شده در کنار مطالعه‌های رسوب‌شناسی نشان‌دهندۀ محیط‌های دلتایی تحت‌تاثیر سیستم‌های رودخانه‌ای و کانال‌های رودخانه‌ای‌اند. پژوهش حاضر حاصل تلفیق سه مطالعۀ فسیل‌شناسی گیاهی، اثرفسیل‌شناسی و رسوب‌شناسی در توالی‌های ژوراسیک ایران مرکزی است و نتایج آن نقش مهمی در شناخت بهتر محیط دیرینۀ افق‌های فسیل‌دار سازند هجدک دارند.

Plant macrofossils, ichnofossils, facies analysis and palaeoenvironment interpretation of the Hojedk Formation in the north of Kerman (Central Iran)

AbstractThis study led to identify 30 species and 16 genera of eight different plant macrofossils orders including Equisetales, Cycadales, Bennettitales, Coniferales, Peltaspermales, Ginkgoales, Corystospermales, and Filicales at the DehShiykh section, north of Kerman. According to stratigraphic ranges of the index taxa such as Otozamites furoni, Nilssoniopteris mosafolia, Nilssonia tenuicaulis, Anomozamites major, Coniopteris hymenophylloides, Nilssonia bozorga and Elatides thomasii the age of early Middle Jurassic (Bajocian–Bathonian) is assigned to the DehShiykh section. Middle Jurassic ichno fossils of the Hojedk Formation in the DehSheikh section have been studied for the first time. The study also led to the determination of terrestrial ichno fossils including eight ichno species, belonging to four ichnogenera such as Planolithes, Rhizoliths, Taenidium and Thalassinoides. They have been found alongside the stems of Equisetals. Combination of the results of ichno fossil studies with plant fossils help us to identify the palaeoenvironment. These plant fossils are generally indicator of humid conditions. Moreover, the identified ichnofossils in accordance with the sedimentological studies, represent deltaic environments influenced by the river systems and channels. This is the first study of the combination of three studies of paleontology (Plant macrofossils), sedimentology and ichnology in east central Iran. The results have contributed to the accurate understanding of the paleoenvironment of fossiliferous horizons of the Hojedk Formation.Keywords: Plant macrofossils, Tracefossils, Middle Jurassic, Delta, Kerman.  IntroductionThe northern region of Kerman is one of the richest areas of Iran for the study of plant fossils and so far more than 80 taxa have been reported (Khalilizadeh 2010). The first studies on coal deposits in the Kerman region have been carried out by HoutumSchindler (1881), Stur (1886) and Stahl (1897). In recent years, MirzaeiAtaabadi (2002) has studied the plant macrofossils of the Hojedk Formation in the Pabdana, BabNizu and Hashuni areas. VaezJavadi and MirzaeiAtaabadi (2006) studied the plant macrofossils of the Hojedk Formation. Ameri et al. (2014a, b) have also studied the biostratigraphy of the Hojedk Formation deposits in the BabHutk section. Studies on these plant macrofossils have been more focused on coal areas and mines, noncoal areas in this respect has been received less attention. In addition, there are no reports of ichnofossil studies of the Jurassic layers of the Kerman region, especially in association with plant fossils. Trace fossils are very useful for understanding the paleoclimates and generally interpreting paleosedimentary environments (Buatois and Mangano 2004). In the present study, which focuses on the study of plantmacrofossils of the Hojdak Formation in DehSheikh section north of Kerman, trace fossils have also been studied along with this macrofossils and the general sedimentology of these layers is also considered. Material & MethodsImportant factors in the identification of plant macrofossils include the characteristics of veins and pinnules, rachis, the grooves on the stem as well as the nodes and inter nodes in sphenophyte. In this study, about 100 plant macrofossil specimens were studied and they were photographed using a Nikon 90D digital camera. The images were processed using Adobe Photoshop CS5. All these specimens are kept in the Museum of Ferdowsi University of Mashhad. Trace fossil studies are usually carried out in the field on a centimeter scale and samples are taken to the lab for further and more detailed examination. In this study, about 20 trace fossil samples were collected. The lithofacies described in the field are divided mainly on the sedimentary texture (especially grain size) and structures; according to the Miall (1985) classification. The identified petrofacies are based on Folk (1980) classification scheme with polarizing microscope. Also, identification of sedimentary environment and subenvironments were interpreted based on the facies characterizes. Discussion of Results & ConclusionsBased on the plant macrofossils, especially Coniopteris hymenophylloides, Elatides thomasii and Nilssonia friziensis, Middle Jurassic age (Bajocian–Batonian) is confirmed for the studied section. The studied plant macrofossils, especially filicales and horsetails, grow in humid environments. The in situ Equisetal stems have paleoecological significance. Horsetails, are hygrophytes, and adapted to humid conditions (Brea et al. 2009). Osmundaceae have been to warm and humid conditions along the rivers or marshes (Gordenko 2008). Due to the abundance of cycadophyta and ferns in the study area, tropical to subtropical paleoclimate have been reported (Salyukova et al. 2013). The ichno fossil diversity of the deltaic regions is very low (Hobday and TavenerSmith 1975). Salinity changes, water turbulence, mud influx and etc. are the most important stress in the delta area. The study of 8 ichnospecies in the DehSheikh section shows these traces, are simple. They are similar to those of deltaic systems (Buatois and Mangano 2011). These trace fossils resemble specimens of deltaic systems and are related to the heteropod invertebrates of Scoyenia ichnofacie. According to sedimentological studies, the sedimentary environment of the Hojedk Formation in the DehSheikh section is often interpreted as a deltaic environment (coarsening upward cycles) under the influence of the river (fining upward cycles) which indicate of channel filling and energy depletion periods. From this study, the combination of paleontological, sedimentological and lithological studies with the study of ichnofossils will be very useful and effective in better understanding the palaeoenvironment and with proper interpretation, it will helpful to recognition details and create a clear picture of the environment.