میکرومورفولوژی رسوبات آبرفتی متأثر از فرآیندهای خاک زایی، تکنیکی نو در مطالعات ژئومورفولوژی (مطالعه موردی: حوضه آبخیز سقز) (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
شناخت فاکتورهایی که فرآیند تشکیل خاک های قدیمی و رسوبات خاک زایی کربناتی را در بین نهشته های آبرفتی نظم می دهند، مورد مهمی برای بازسازی محیط های دیرینه، تکامل محیط های رودخانه ای در شرایط مختلف اقلیمی و در نهایت سناریوهای مدیریتی است. در پژوهش حاضر، به منظور انجام مطالعات میکرومورفولوژی خاک، یک پروفیل رسوبی متشکل از خاک قدیمی مدفون، لایه کربنات خاک زایی، نهشته های دشت سیلابی (کنگلومرا و لنز ماسه ای)، نهشته های رسوبی منفصل، خاک قدیمی ظاهر شده و نهشته های رسوبی عهد حاضر در یکی از شاخه های فرعی رودخانه سقز مورد بررسی قرار گرفت. جهت بررسی میکرومورفولوژی خاک، دو نمونه از رسوبات آبرفتی متأثر از خاک زایی با حفظ کامل بافت و ساختار خاک برداشت شد. پس از خشک شدن نمونه ها، مقاطع نازک از آن ها تهیه شده و در ادامه مطالعه، تشریح، تفسیر و عکس برداری از آن ها با استفاده از میکروسکوپ پولاریزان در دو حالت نور پولاریزه ساده (PPL) و نور پولاریزه متقاطع (XPL) انجام گرفت. نتایج بررسی میکرومورفولوژی رسوبات آبرفتی پلیستوسن پایانی نشان از آغشتگی زمینه به اکسیدهای آهن و تشکیل رگچه های کوارتز و اکسید آهن دارد که به طور مشخص حاصل فرآیندهای خاک زایی برجا در این نوع رسوب می باشد. بارزترین تأثیر فرآیندهای خاک زایی را می توان در تشکیل لایه های کربنات متأثر از خاک زایی میان رسوبات آبرفتی مشاهده نمود که لایه های سخت کلسیتی را به وجود آورده است. بر اساس شاخص ژئوشیمیایی محاسبه شدت هوازدگی و یافته های میکرومورفولوژی، رسوبات مطالعه شده در این پژوهش، در شرایط آب و هوایی سرد و خشک پلیستوسن پایانی تحول پیدا کرده اند.Micromorphology of pedogenically modified alluvial deposit, a new technique in geomorphological studies (Case study: Saqqez Basin)
IntroductionThe soil formation process is a complex interplay between specific pedogenic processes, creating a set of solid-phase pedogenic features. Pedogenic carbonates, containing evidence for paleoenvironmental conditions are widely distributed in arid and semi-arid regions. The heterogeneity of soils and pedogenically modified alluvial deposits and developed from slope deposits can be seen in macroscopic observations of morphological features such as sudden changes within the grain size distribution the presence of rock fragments with different lithology, a significant change of percentage content and/or shape of coarse fragments, various degrees of soil and deposit weathering, and changes in the colour mantle or the soil conciseness. However, transported, displaced and mixed material on slopes can be also efficiently recognized by a number of micromorphological features. Sometimes, even in the absence of macroscopic indications, the analyses of micromorphological features can give even more detailed insight. Unfortunately, micromorphological features in soils or alluvial deposits affected by slope processes have not received much attention in recent years.MethodologyThe section investigated in this study includes the pedogenically modified alluvial sediments was formed in one of the abandoned anabranches of the Saqez River. Saqqez River basin with an area of 835 km2 is located in Saqqez county, Kurdistan province, western Iran. Saqqez River originates from Pierbodagh and Vazneh Mountains, near Baneh, and flows from southwest to northeast towards Saqqez City as a branch of Zarrinehrud River. Zarrinehrud River flows into Urmia Lake. The area has a Mediterranean continental climate, with hot, extremely dry summers and cold, snowy winters. The minimum winter temperature drops to -30 ̊ C and the summer maximum rises to 40 ̊ C. The average annual precipitation is between 350 and 650 mm and relative humidity ranges from 14% to 95%. The area is located at the margin of the northern part of the Sanandaj-Sirjan Zone (SSZ) which is a metamorphic–magmatic belt associated with the Zagros Orogen and part of the Alpine-Himalayan orogenic system. The area is actually at the intersection of Sanandaj-Sirjan, Khoy-Mahabad and Alborz-Azerbaijan Zones. The geology of the area is composed of meta-sedimentary, meta-volcanic and meta-plutonic rocks of SSZ and the sedimentary rocks of Alborz-Azerbaijan Zone.Two undisturbed sediment blocks from pedogenically modified alluvial deposit unit were collected. Epoxy resin impregnated for micromorphological studies to investigate the origin of sediments, the process and/or agent responsible for deposition of sediments and post-deposition pedogenic features. The intact sediment blocks grabbed from the profile were dried and then vacuum-impregnated in a vacuum desiccator with epoxy resin mixed with hardener and accelerator. Impregnation of micromorphology samples were done in the Soil Micromorphology Laboratory at the Faculty of Natural Resources, University of Tehran. The mammoth-sized thin-sections were made and examined in the Micromorphology Laboratory at Zaminrizkavan Research Co. Results and Discussion Thin-section view of resin-impregnated sediment block from pedogenically modified alluvial unit showing dark-colored basaltic constituents in fine-grained groundmass. Although nearly completely eroded away now, this basaltic lithology should have existed once in the Saqqez River basin. Outcrops of Plio-Quaternary basaltic rocks do occur in the nearby regions in West Azerbaijan and East Azerbaijan provinces. Two studied pedogenically modified alluvial deposit samples marked differences in weathering intensities and degree of replacement by clay minerals in two neighboring rock fragments of similar lithological types indicate that part of the weathering effects might be inherited from the source region of the sediments. Aslo micromorphology study of the samples showed highly vesicular basaltic rock fragments (scoria) in fine-grained matrix stained by iron oxides indicating in-situ pedogenic effects.ConclusionThe mineralogical composition of fluvial sediments and their textures are useful indicators of past environmental conditions. This diversity indicates fluctuations in climate conditions during the Quaternary in most parts of the world. After a period of erosion, the first sedimentation event on the volcanic bedrock in the study section is represented by alluvial sediments apparently as valley fill deposits. This alluvial unit shows alteration features due to subsequent pedogenic processes. Although the unit includes a variety of rock types, the Plio-Quaternary basaltic constituents prevail reflecting denudation of the youngest lithological unit in the source region. Erosion of Plio-Quaternary basaltic rock and transportation from the source region possibly occurred in Pleistocene when the current fluvial system was not developed and sediment transport was largely through flood events; So, the alluvial sediments in this unit could be considered as debris flow or flood deposits. Marked differences in weathering intensities among detrital constituents with the same lithological types are possibly due to inherited weathering features from the source region of the sediments. However, the fine-grained matrix of the sediments partially replaced by iron oxides/hydroxides and replaced by carbonates clearly represent in-situ pedogenic features. The most conspicuous pedogenic feature in this unit is the development of pedogenic carbonates leading to the formation of carbonate nodules and calcic/petrocalcic horizons. A variety of sources for the origin of calcium in pedogenic carbonates could be considered including weathering of andesitic and basaltic volcanic rocks and calcium release from calcium-rich minerals such as plagioclase and calcium-rich minerals in airborne dust.