Scientific Quarterly Journal of Geosciences

Scientific Quarterly Journal of Geosciences

Study of Physical - Chemical Characteristics and the Source of Loess Deposits in Gorgan -Plain Region

Document Type : Original Research Paper

Author
Abbas Pashaei
Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran
Abstract
As an introduction it must be reiterated that the loess deposits of Iran is of aeolian origin. Physically, 70-90 percent of constituent particles of the world loess consists of silt with an average diameter of 15.6- 132 micron. It means that, it is composed of medium-sized silt to very fine-grained sand. The percentage clayey particles in younger loess deposits are less than 10%. But, this may increase to 20-25 percent with age and development of weathering process. Other characteristics are porosity and their apparent low density, being between 1.25-1.65.
Apart from above- mentioned constituent parts of loess, we must mention an appreciable percentage of calcareous material, which in normal condition, may range between 10-15 percent and may also reach up to 40 percent.
Loess investigation was carried out in Agi Soo valley, adjacent to Iran- Turkmenastan border and between Maraveh Tappeh village and Takal Kuh uplands, as well as northern slopes of the Alborz, some 300 km. away from the border, i.e., between Gorgan and Minoodasht. The result of physico-chemical investigation on these deposits showed that, as we approach more and more to the main source of this material, the percentage sandy material of them increases up to 34 percent. Whereas, the percentage clayey material decreases only to 5 percent.
On the other hands, Dadly Qaznin upland, towards the end of Agi- Soo valley, the percentage sandy material decreases to approximately 14 percent. On the contrary, the percentage clayey material increases up to 16 percent, while the percentage silty material straddle between 61-74 percent.
Thus, the rate of interchangeable cations of this material with an increase in percentage clayey material also increases from 8 meq/100 gr to 14 meq/100 gr. Moreover, the percentage calcareous materials are about 14 percent in low elevations, which in turn can reach up to 28 percent towards the high elevations. This trend is more or less repeated in other traverses.
Consequently, with regards to the direction of the dominant winds of the area, which mainly are north to north-westerly. Thus, it is concluded that the source of this material is derived from flood- deposits and sandy hills, which is situated in the Caspian Sea depressional areas and mostly of flood- origin. Otherwise, the center of loess accumulation should have been on the north-eastern slopes of Koppeh Dagh and Alborz and not on its north-western and northern slopes.
Finally, the results of previous studies on loess formation in Khuzestan Plain, even in a very limited extent, in regions such as Kavir to Kolamakan, with tornados of 170 days duration and Sistan-Beluchestan, with winds up to 120 days, as well as other similar regions, studies still being continued.
Keywords
Loess Deposits, Gorgan

Subjects

 
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Scientific Quarterly Journal of Geosciences
Volume 6, 23-24
Spring & Summer 1998, Vol. 6, No. 23-24
Autumn 1998
Pages 67-78