بررسی ویژگی‌های رسوبی و کانی‌شناسی رسوبات قدیمی و جدید دشت سیلابی رودخانه سقز

نویسندگان

چکیده

رودخانه­ها، یکی از عناصر موجود در سرتاسر چشم­انداز قاره­ای و حساس به تغییرات بوده که فرم خود را در طی زمان در برابر دامنه وسیعی از نیروهای درونی و بیرونی تنظیم می­کنند. هدف اصلی این مطالعه، بررسی ویژگی­های کانی­شناسی رسوبات قدیمی و جدید دشت سیلابی رودخانه سقز، انطباق پراکنش واحدهای لیتولوژی با یافته­های کانی­شناسی و بررسی تأثیر شرایط زهکشی حوضه موردبررسی بر روی این ویژگی­ها است. بر این اساس، 14 نمونه از رسوبات دشت سیلابی در امتداد شاخه اصلی رودخانه سقز (شامل هشت نمونه از رسوبات جدید و شش نمونه از رسوبات قدیمی) در محل اتصال شاخه­های فرعی به رودخانه اصلی جمع­آوری شد. در ابتدا، کربنات­های سیمانی نمونه­های قدیمی برای بررسی ویژگی­های دیاژنزی و شناسایی نوع سیمان رسوبات قدیمی تحت آنالیز XRD قرار گرفتند. سپس از نمونه­ها مقاطع نازک تهیه شدند تا مطالعات کانی­شناسی، دانه­شماری، انطباق پراکنش واحدهای لیتولوژی در سطح حوضه با یافته­های کانی­شناسی و در ادامه تحلیل­های آماری بر روی داده­های به‌دست‌آمده صورت گیرد. به‌منظور مشخص نمودن تفاوت­های کانی­شناسی و اجزای رسوبی تشکیل‌دهنده رسوبات قدیمی و جدید دشت سیلابی از آزمون من - ویتنی و برای تعیین مؤلفه­های اصلی تأثیرگذار بر تغییرات کانی­شناسی و اجزای رسوبی از تحلیل عاملی استفاده شد. نتایج XRD نشان داد که سیمان بین قطعات گراولی کنگلومراها در تمام رخنمون­های پادگانه رودخانه­ای از نوع کلسیت می­باشد. با توجه به نتایج تحلیل­های آماری، تفاوت میانگین دو گروه رسوبات قدیمی و جدید دشت سیلابی رودخانه سقز به لحاظ کوارتز پلی­کریستالین، شیست و ولکانیک معنی­دار می­باشد. بر اساس نتایج تحلیل عاملی، مؤلفه اول با توجیه 2/32 درصد از واریانس به‌صورت قوی مثبت با کوارتز پلی­کریستالین و شیت و به‌صورت قوی منفی با قطعات ولکانیکی مرتبط است. واحد سنگی شیست در بیش­تر موارد یا در نزدیکی محل آبراهه­های اصلی حوضه و رودخانه سقز قرارگرفته و یا در پایین­دست حوضه و محل تشکیل پادگانه­های رودخانه­ای سخت شده (کنگلومرا) واقع‌شده است.

کلیدواژه‌ها


عنوان مقاله [English]

The study of sedimentary and mineralogical characteristics of old and recent flood plain sediments in Saqqez River

چکیده [English]

INTODUCTION
Rivers, as one of features in terrestrial landscape, are sensitive to changes that regulate their forms against a wide range of internal and external forces over time. River sediments are significant and long, although disconnected, archives of earth landscapes evolution as well as many Quaternary sedimentary sequences that reflect the reactions of drainage systems to changes in sea level and past environments. River systems are very complex due to the dynamics involved in their formation, thus controlling the different phases of aggradation, degradation, and pedogenic processes. Analysis of mineral provenance is an importance task to better understand of river dynamics and climatic changes in this region, which is reflected in different weathering intensities. Aggradation phases can be recognized by selective sorting of particles due to differential entrainment and transport. This is especially seen in characteristics of heavy minerals, such as density, grain size and shape. In general, the sorting may be a result of ancient depositional environments and provenance of sediments. In rivers with high sedimentation rates, heavy minerals are affected by considerable sorting before they reach the alluvial plain. Active deposition across the floodplains of rivers arises through a variety of processes. Floodplains are rarely passive recipients of diffuse overbank sediments across tabular relief. Instead, river floodplains have an intricate ‘depositional web’ with an array of linked depressions and channels that may both span and connect significant expanses of ponded water. Deposition in and beyond this undulating topography takes a variety of forms. The range of out-of-channel sedimentation processes alongside larger rivers may collectively be characterized as spillage phenomena, active at flood-stage, and particularly, but not exclusively, bordering main channels or channel belts raised above valley floor level to create strong lateral elevation contrasts.
STUDY AREA
Saqqez River basin with an area of 865 square KMs is located in northwest margin of Sanandaj-Sirjan zone. From a morphological view, middle parts of the study basin are largely constituted of Cretaceous clastic and volcanic rocks have a mild topography accompanied by low-lying hills and flat erosion levels.
 
METHODOLOGY
The main purpose of this study is the investigate of compliance of lithological units distribution of Saqqez river basin with mineralogical characteristics of old and recent flood plain sediments then determine the effects of study basin qualification on these characteristics. Physical and chemical analyses were used to investigate the characteristics of flood plain sediments.  14 samples were collected from flood plains along main tributary of the river including eight and six samples from recent and old sediments respectively; the junctions of tributaries branches to main river are the sampling locations. Old sediment samples were analysis by XRD to recognizing the cement type of old sediment. Thin sections were prepared from the samples to study of mineralogical, point counting, compliance of lithological unit’s distribution of the basin with mineralogical characteristics of sediments and statistical analysis of mineralogical and sedimentary data. In laboratory, thin sections were investigated using polarizing microscope in two modes of Plain Polarized Light (PPL) and Cross Polarized light (XPL) for the purpose of determining samples shape and sediments type. Point counting of the samples was done through visual method and microscopic interpretation. Mann-Whitney test was used to delineate of mineralogical differences between old and recent flood plain sediments; then factor analysis was used to specify of the effective principal components on differences in mineralogical and sedimentary components.
 
RESULTS and DISCUSSION
The results of XRD analysis showed that the cements type of conglomerate gravel fragments is calcite in all river terraces outcrops. Physical investigations about the components of sediments and identifying the parent rock of sediments revealed that sediments nature are include fluvial sediments, sandy gravely sediments, coarse-grained fluvial deposits (unconsolidated sediments). Volcanic, metamorphic (Schist) and sedimentary (Shale) rock fragments are formed the main components of sandy carbonate mudstone. Regarding to the results of statistical analysis, a difference between two group’s sediment samples in terms of mineralogical and sedimentary components is meaningful due to polycrystalline quartz, schist and volcanic fragments. Based the results of factor analysis, the first principal component with rationalization of 32.2% of variance regards with polycrystalline and schist as strongly positive and regards with volcanic as strongly positive.
 
CONCLUSION
A brief look at dispersion of lithological units across Saqqez river basin indicates that the rock unit of Schist is located either near main channels of rivers or downstream the basin where river terraces are formed. Accordingly, the hydrological and topographical conditions of the basin are organized in a way that transport of schist rock fragments to downstream might happened due to the main tributaries thus increase the frequency of Schist; as it is proved in the present study by the point counting of conglomerate rock fragments.

کلیدواژه‌ها [English]

  • Flood plain
  • Conglomerate
  • Mineralogy
  • Factor analysis
  • Saqqez River