Climate variability analysis for evolution effect over the Garmsar Alluvial fan, Based on field measurements

Alluvial fans are conical – shaped landforms by gentle slops which are produced and reshaped by erosionand deposition of material on the base of mountains after river arrival in gentle slopes after long times,from thousands to millions of years. Climate is a control factor which has a sever effect on morphology anddepositional size in quaternary alluvial fans. There are many differences between fans in different climaticregions, and it seems that there is a relationship between fans morphology and its available deposits anddischarge regime. Climatic fluctuations changes floods power, available sediments amount and sodepositional environment affected by it. In this research we try to analyses climatic fluctuation effect onGarmsar alluvial fan by using well logs and comparing them by climatic conditions that made changes insize and form of deposited material. Research literature derived from documentaries such as books,scientific articles appeared in related quarterlies and so on. Then we used Garmsar, Aradan, andKohanabad ۱:۵۰۰۰۰ Topographic maps and Landsat ETM+ images of ۲۰۰۱ by pass and row ۳۶, and ۱۶۴ toextract Garmsar alluvial fans’ boundary. For obtaining depositional layers of the fan we got and used welllogs of the region from ministry of Power of Iran. We try to get logs that can cover all alluvial fans’ extentfor use in Arc GIS software. To draw well logs we used EXCEL software. Field survey completed ourresearch for studying and observing superficial layers of Garmsar alluvial fan, especially in river beds.Alborz mountain system evolution occurred in many stages. Reliefs which formed during quaternary,disturbed by permanent hydrologic system of Hable –roud and sediments of this region of AlborzMountains carried to Garmsar plain, consequently. Garmsar fan is a typical alluvial fan which formed byerosion works and deposition of this river in the region during quaternary period and fluctuations causedby climatic conditions that affected on the fan. The river drains high central Alborz lands and has an obliquedirection in respect to general mountain trend (East – West direction). In cutter section of the mountain, itcrossed the mountainous area and made a vast alluvial fan which we know it as Garmsar Fan now. Becausepresence of different geologic formations by different texture and hardens in the river basin and longdistances in transported sediments, grains are well rounded and sorted. Upper deposits are gravels insidea matrix of silts and clays and have a ۷ kilometer extent from the apex of the fan to its base. This section offan has a high infiltration capacity. In middle section of the fan there are finer grains and more variationscan be seen. We can classify this matterial as water deposits which are cutted by river flows. Infiltration islittle here in comparison to upper sections of the fan. In the lower parts of the fan (base of fan), there arefine particles which are silts and clays, dominantly. Gravel beds inside this section of fan are results of flashfloods and deposits of channel. In some parts of these channels there are many springs in the end sectionsof the fan. Garmsar alluvial fan which is among great extended Iranian alluvial fans, formed by manycontroller factors. Climate and its’ fluctuations affected its extent, genesis and evolution. Climaticfluctuations investigations shows that there were frequent periods of dry and moist periods during itsformation and extension , but the number of wet and calm periods was dominant in the research area. Maindeposits of grain at the top of the alluvial fan are clay, fine, moderate and coarse sands, which its claycomponent is noticeable. Well logs drilled in different parts of Garmsar alluvial fan shows that there weremore moist periods than dry ones. Duration of dry periods was shorter, also. These conditions causeddifferences in materials deposited in Garmsar alluvial fan, which are fine grained clays. By increasing intemperature in studied basin in recent years, another factors such as moisture and precipitation changedand the basin experienced more precipitations than before. So, the snow reserves of the basin decreasedand snowmelts decreased, too. These factors resulted to extension of the fan by increasing of deposition ofmaterial on the Garmsar alluvial fan.

کلیدواژه ها:

Morpho- climate ، Climatic Fluctuations ، Depositional Sequences ، Garmsar Alluvial fan