ویژگی‌های زمین‌شناختی، رسوب‌شناختی و شرایط دیرینه‌محیطی برش رسوبی- باستانی گسکرک شهرستان رودبار

در زمین‌باستان‌شناسی با تشخیص و بررسی محتوای رسوبی و چینه‌نگاری لایه‌ها و مواد باستان‌شناسی، می‌توان درک کامل و صحیحی از پیشینه‌های باستانی به ‌دست آورد. نوشتار حاضر گزارشی از مطالعات زمین‌شناختی و رسوب‌شناختی در محل برش رسوبی باستانی گسکرک واقع در شهرستان رودبار است که به‌منظور بازسازی شرایط محیطی دیرینه انجام گرفته است. برش گسکرک شامل دو لایه رسوب آبرفتی واریزه‌ای ریزدانه، با تمایز رنگ مشخص می‌باشد که مرز زیرین آن‌ها محدود به سنگ بستر رسوبی و مرز بالایی آن‌ها توسط نهشته‌های استقراری جدیدتر پوشیده شده است. رسوبات طبیعی تشکیل‌دهنده این دو لایه را ذرات در حد سیلت شامل می‌شود که تراکم بالایی را نشان می‌دهد. لایه پایینی (لایه i) عمدتاً از رسوبات ریزدانه سیلتی با ناخالصی مواد آهکی تشکیل شده است که درون آن قطعات آهکی به‌صورت پراکنده مشاهده می‌شود. لایه ii که در مرز بالایی خود توسط نهشته‌های طبیعی فرهنگی عصر مفرغ پوشیده شده است با وجود خرده‌های زغالی مشخص می‌شود که آن را از لایه زیرین خود متمایز می‌کند. مطالعه نمونه‌های عهد حاضر شرایط آب و هوایی خشک تا نیمه‌خشک را برای تشکیل افق‌های کربناتی شده در خاک پیشنهاد می‌کند. محتوای کم مواد کربناتی، رنگ قرمز قهوه‌ای و همچنین وجود خرده‌های زغالی در لایه ii که متمایز از لایه i می‌باشد، شرایط دیرینه محیطی متفاوتی را برای لایه ii پیشنهاد می‌کند که به‌نظر می‌رسد با تغییر شرایط آب و هوایی، افزایش بارندگی و رطوبت و گسترش پوشش گیاهی جنگلی در منطقه همزمان بوده است.

Geological and sedimentological characteristics and paleo-environmental conditions of sedimentary - ancient section of Gaskarak in Roudbar county

IntroductionGeoarchaeology is a new and interdisciplinary concept that studies the past of human history using geological methods. In geoarchaeology, a complete and accurate understanding of ancient records can be obtained by identifying and examining the sedimentary content and stratigraphy of archeological layers and materials. Geology has been available to archaeologists over the past two decades as a basic tool for reconstructing ancient environments and understanding the longterm climatic and anthropogenic conditions and interactions of prehistoric human and surrounding environments. These studies are particularly influential in understanding the Pleistocene and Holocene archaeological and geological backgrounds and materials. Meanwhile, geoarchaeological studies of Iran are practically linked to Paleolithic observations and exploration and the use of geological methods such as sedimentology to describe the details of ancient records in caves and rock shelters. Geoarchaeology is an interdisciplinary specialty between geological sciences and archeology that examines the role of geological factors in the formation, continuation, and weakening of ancient settlements. In this field, techniques and methods common in earth science such as aerial photography and satellite imagery, sampling, microscopic studies, chemical analysis, etc. are used to solve archaeological problems. On the other hand, often referred to as archaeogeology, archaeological data are used to solve geological problems, particularly in relation to dating of Quaternary deposits, ancient seismological studies, and ancient mining. Largescale archaeologists study most of the natural landforms and anthropogenic structures and smallscale archaeologists study the soil, natural sediments, and anthropogenic deposits. Archaeogeology also occasionally covers other interdisciplinary studies such as pottery petrography, ancient mineralogy, ancient metallurgy, dating, etc. and socalled archaeometry.Materials and methodsIn order to determine the frequency of sediment grains based on their size and to investigate the pattern of particle size distribution to determine the textural properties, the sorting of grains and the sedimentary abundances of two natural sediment layers I and II were extracted from Gaskarak section and analyzed by laser particle size analyzer. Organic matter and carbonate bind the sediment particles to each other and therefore, to determine the true distribution of particle size, they must be removed prior to analysis of the samples to separate the sediment particles. The mixing and rotating system of the device will cause the particle to move and be exposed to the laser beam. So after the laser light from a high voltage source is exposed to the sample, the laser beam will reflect from the sample surface and then pass through it. The size of the deposited particles is directly proportional to the magnitude of the reflected laser light and to the angle of refraction of the laser beam to the surface of the particle, so that with increasing diameter of the deposited particles, the intensity of the reflected laser light increases, but its angle of failure decreases.Results and discussionGaskarak section consists of two layers of finegrained alluvialdebris sediment, with distinct color which their lower boundary is confined to the sedimentary bedrock and their upper boundary is covered by more recent deposits. The natural sediments that form these two layers are silty particles that show high density. The lower layer (I) is mainly composed of silty finegrained sediments with calcareous fragments. Layer II, covered at its upper bound by the natural and cultural deposits of the Bronze Age is characterized by a charcoal inclination that distinguishes it from its lower layer. Study of the recent samples suggests dry to semiarid climatic conditions for the formation of carbonated horizons in the soil. Low carbonate content, reddishbrown color, as well as the presence of charcoal fragments in layer II, which is distinct from layer I, suggest a different environmental condition for layer II that seems it has been facing simultaneous climate changes, such as increasing rainfall and humidity and expanding forest vegetation in the region.ConclusionField evidence and results of laboratory studies indicate that the natural deposits of the Gaskarak section probably formed as abnormal sediments on ancient hillslope surfaces of sedimentary bedrock. As the morphology and slope of the sedimentary layers show a valleylike position, the eroded material accumulates from sections with higher topography within it. The natural sediments of the Gaskarak section are distinguished by two distinct layers (I and II) that show distinct differences in color, carbonate content, and mineral and organic fragments.