نوع مقاله : مقاله پژوهشی

نویسندگان

1 دانشگاه آزاد اسلامی واحد علوم و تحقیقات

2 دانشگاه آزاد اسلامی واحد آشتیان و ریاست مرکز پژوهش های کاربردی سازمان زمین شناسی و اکتشافات معدنی کشور

چکیده

کواترنری بخشی از تاریخ تکوین زمین است که تا دو میلیون سال پیش را در بر می‌گیرد. این زمان زمین‌شناسی در ایران نیازمند پژوهش‌های اساسی است. لذا، بررسی رسوبات آبرفتی یکی از ارکان اساسی در این زمان و بسیار حائز اهمیت است. در این پژوهش سعی شده تا ویژگی‌های این نهشته‌ها در جنوب بلندی‌های البرز، باختر شهر کرج و خاور آبیک بررسی شود. برای بررسی نهشته‌های منطقه، 34 ایستگاه مشخص و نمونه‌برداری شد. نمونه‌ها از رخنمون‌های طبیعی و ترانشه‌های حاصل از راه‌سازی، ساختمان‌سازی و شبکه‌های نصب لوله فاضلاب برداشت شده است. مطالعات روی نمونه‌های برداشت‌شده انجام شد، شامل دانه‌بندی، تعیین لیتولوژی، مورفوسکوپی دانه‌ها، آزمایش پراش اشعه ایکس (XRD)، جورشدگی، گردشدگی و کج‌شدگی. نتایج به دست‌آمده به شکل جداول و نمودارهای گوناگون آمده است. در نهایت، با استفاده از نتایج بررسی‌های صحرایی و آزمایشگاهی، جهت جریان‌های دیرینه در محل نهشته‌های شمال خاوری- جنوب باختری منطقه مورد مطالعه مشخص شده است. مطابق آزمایش‌های انجام‌شده بیش از 50 درصد نهشته‌ها در رده گراول ماسه‌ای گلی قرار دارد که نشان‌دهنده سهم بزرگ مواد دانه‌درشت در اندازه گراول در نهشته‌های مورد بررسی است که بیانگر نزدیکی نهشته‌ها به منشأ خود است. همچنین، جورشدگی بسیار بد تا بی‌نهایت بد رسوبات از ویژگی‌های محیط بادزن آبرفتی است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Sedimentology and paleo current direction of the Fan deposits (Quaternary) in Hashtgerd area

نویسندگان [English]

  • Shiva Navidi 1
  • Fariborz Gharib 2
  • Akram Mirzaei 1

1 Islamic Azad University Science and Research Unit

2 Islamic Azad University, Ashtian Branch, and the Head of the Applied Research Center of the Geological Survey of Iran

چکیده [English]

In the recent decades, with increasing in human need to produce more food and their encountering with high risk geological phenomena like earthquake and bioenvironmental pollutions, the importance of the quaternary system and realizing its different aspects for geologists were increased. This research considers the scientific phenomena occurred in Hashtgerd area. It is located between 35°52′30″N and 36°00′00″N latitudes and 50°37′30″ E and 50°45′00″E longitudes. Topographic elevations in the province vary between 1310 and 1610 m. This area has placed in the southern slopes of the Alborz Mountains and in the northern edge of the central desert of Iran and 40 km west of Karaj. Morphologically this area located on an alluvial fan which is 16 km2. In the northern part, sediments seem older and brighter and have higher cementation and often are related to Paleo-quaternary sediments. The young quaternary sediments have been placed on this alluvial Fan and the sediments are getting more loos and more red toward south. The age of deposits in this area is Pliocene up to present. The Pliocene deposits are the oldest and the agricultural soil and the alluvial deposits are the youngest. The most available dominant lithology is also Tuff.
Materials and Methods
After determination of study area, the geological maps (Karaj 1:100000), topography maps (Hashtgerd 1:25000) and Aerial photographs of the area in the scale of 1:50000 were provided and investigated. After second time field study for getting familiar with the area, access routes and determination of sampling points, 34 point sampled and the coordinate of these point were recorded. The samples were sent to the laboratories of applied geological research center of Iran. For sediment grading test, shaker sieve, for fine sediment grading tests, lasers and XRD test for identification of clay minerals were used in order to understand their sources. In each sampling station physical feature of sediments has been noted and 20 rubbles above 3cm in size sampled for analyzing shape, size using caliper and also their rounded and sphericity. Required Cumulative graphs (in phi and MM) for each station was calculated by the Sediment Sizer software. Sedimentary structures of the region, the granulation and direction of the ancient flow have been studied and the photographs were taken of all of the features. To obtain the ancient flow direction in the sediments, measurements were carried out by compass include pebbles deep and strike, then using obtained data from the region and by help of Rockwork software, the direction of ancient flow has been determined. 
Results and Discussion
In the most of stations, the grain size varies between 20 cm up to sand, silt and clay. So that at the beginning of fan, grain size are coarser and closer to the end grains, the grain size are smaller which indicates high energy in the upper part of the fan and low energy in the final part of the fan. Pebbles are often made of tuff and the fabrics are more Imbrications. The color of sediments due to deposition in the oxidation environment is mostly red. The grading results show that 56% of deposits are muddy sandy gravel, 29% of them gravelly mud, 9% muddy gravel and 6% gravelly muddy sand. Pebbles in this area vary from very angular to very round and their sphericity vary from slightly stretched to almost isodiametric. Roundness and sphericity of grains increases in the region with increasing in distance from the source. More than 90% of samples placed in positive category based on tilting or asymmetry and distribution of the grains. Positive tilting and bad sorting is characteristic of alluvial deposits and alluvial fans. This is due to large amounts of suspended materials such as silt and clay in the environment that after sedimentation, large amount of this material remained in it and caused bad sorting and positive tilting of sediments. In terms of grains form, it is not seen any specific trend in sticks and blades pebbles, but the percent of iso-dimention pebbles increases from east to west and the percent of disc form pebbles increases from north to the south.
Conclusion

According to the Folk nomination 1974, more than 50% of deposits in the measurement stations in the area are placed in the range of muddy sandy gravel indicating the sediments are near to their origin.
Very bad sorting and positive tilting in the sediments are characteristics of the alluvial fans.
The spherical and rounded shape of the grains from north to south and east to west of the region increases and this represents an increase of displacement in the sediments in this direction and shows the direction of ancient flow direction (northeast - southwest) on the region.
Fractures of the cumulative curves in different stations presented three sources for deposits in the region.
More than 70 percent of the constituent particles of sediment in the area are related to the Tuffs. This indicates the main source of Karaj Formation sediments.
Disc form pebbles increase from north to south and pebbles with equal dimension increase from east to west in these stations related to the way that pebbles move and also their lithology.
Based on XRD test results existence of Sio2 in Tuffs, causes the highest frequency of quartz minerals. Calcite and feldspar are the highest abundance after Quartz. Existence of Chlorite in the most of stations also confirms dominant lithology of Tuff in this area.
The ancient flow direction in more than 90% of the measurement stations is North East - South West.
All plotted graphs used for identifying ancient flow direction are modal ionic with the high dispersion and indicate alluvial fan environment.

کلیدواژه‌ها [English]

  • Alluvial fan
  • ancient flow
  • Hashtgerd
  • Quaternary