Barchan dunes and nebkhas are great geomorphologic phenomena of desert areas in south of Damghan. The purpose of this study is to analyze the sedimentology of sand dunes and nebkha deposits in the Haj Aligholi Desert.
More than two-thirds of the Iran's area is arid and semi arid land. The diversity of flora is limited in these areas because of low humidity. This causes the wind to easily affect on the surface of these lands and carries the sand size and finer grains and deposited in another place. Barchan sand dunes are the most abundant type of sand dunes in the Haj Aligholi Desert. Dispersed vegetation plays an important role in sediment transport and nebkhas formation in this area. Nebkhas or sand dunes are formed usually in hot and humid and semi-arid regions and widespread where the sand can be trapped by vegetation. The study area is located about 35km south of Damghan in 54°, 10' to 54°, 30'E longitude and 35°, 45' to 36°, 00'N latitude.
2-Materials and methods:
27 and 15 samples were collected respectively from the sand dunes and nebkhas to determine the sedimentary parameters. Grain size analysis were carried out using dry sieve with a set of 0.5 phi screens for measurement of statistical parameters (mean, median, sorting, skewness and kurtosis). For morphometric investigation the parameters of sand dunes (lee side, stoss side and crest) and nebkhas (long axis, short axis and height length) were measured.
3-Results and discussion:
The mean diameter of the grain size, the average of sorting, skewness and kurtosis in the sand dunes are about 2.435 phi (fine sand), 0.530 phi (moderately well sorted), -0.268 (coarse skewed) and 1.054 (mesokurtic), respectively. The mean diameter of the grain size, sorting, skewness and kurtosis in the nebkhas are about 2.800 phi (fine sand), 0.540 phi (moderately well sorted), -0.070 (near symmetrical) and 1.640 (very leptokurtic), respectively. The comparison between the index of kurtosis and sorting show that with increasing of the grain's kurtosis, the grain's sorting increased.
After doing regression for each of the independent variables versus the dependent variable to height was calculated. The results show that the attribution of the stoss side and the crest show the best correlation. Therefore, with stoss side and crest components, we can calculate the height of sand dunes from the high-resolution images satellite. To compare the relationship between the long axis's length and short axis's length, as well as the long axis's length and height of the nebkhas obtained the regression equation in the study area. The results show that this both variable are suitable for measured of long axis in the nebkhas.
In this study, both types of nebkhas (active and passive) were identified that cause of the deactivation of them is dropping in the level of groundwater, which its witness is embayment and cracking on the surface of the ground. Due to the prevailing of northwest wind seems to be the accumulation of sediments at this area is closely related to the groundwater table and soil moisture of this region.
The average of sorting calculated for sediments of sand dunes and nebkhas, were moderately well sorted can be show two source for sediments (bimodal) and it cause to decrease the amount of sorting. The average of skewness of sand grains that skewed to the coarse grains which we can found type movement of sediments that it is saltation. Northwest wind that blowing in study area controlled sand dunes type and position and cause of this claim is morphology and situation of sand dunes. The results of regression revealed that the attribution of the stoss side and the Crest in sand dunes shown the best correlation and also in nebkha dunes the attribution of the short axis and height show the best correlation. In the study area both nebkha (active and passive) found which phenomena annihilator of nebkha is falling groundwater level that it's increasing the desertification.