عنوان مقاله [English]
Loess deposits in northeastern Iran are deposited sparsely and with thin thickness on the slope. The aim of this study was to identify and determine the distribution of loess sediments, to study the properties of sediment texture and morphoscopic characteristics (SEM), and finally to identify the origin of these sediments. The fine sand was the dominant particle in the samples, and the sum of coarse silt, very coarse silt and fine sand was more than 60%. In morphoscopic study, quartz grains were found to be not very rounded, and the prevalence of fine sand, low rounding of quartz grains and auricularity of most quartz particles were evidence indicating locality and proximity of the harvesting site to the deposition site. There was a negative correlation between height and average size of the collected particles and its correlation coefficient was -0.7.
In Kopet Dag region, loess sediments are scattered and of low thickness, and located around the Kalat Naderi, Sarakhs, Torbat-e Jam, Fariman and Mashhad. The goal of this study was to: 1) determine the characteristics of the particle size distribution, 2) determine the properties of the surface texture and the shape of the quartz particles, 3) determine the possible origin of the loess sediments, and 4) determine the distance from the harvest site.
Materials and methods
The study area was located at the end of the eastern and southeast of the Kopet Dag mountain range at latitudes of 20'35˚ to 59'36˚ north and longitudes of 50'59˚ to 20'61˚ east and included loesses of Sarakhs, Torbat-e Jam, Fariman and south of Mashhad. In order to do this research, map of loesses distribution was first prepared using geospatial maps, then the range of loess sediment was identified by wide field observations based on the geomorphic and topographic characteristics, and totally 119 surface samples were taken from which 20 samples were selected for particle size distribution and 4 samples for SEM study. For particle size distribution, at first the amount of gypsum and lime of the particles was determined, and after the preparation of the samples, the size of the grains was measured using laser particle size analyzer Malvern mastersizer 2000, and the average grain size, standard deviation, elongation, and skewness were calculated. For morphoscopic study, quartz grains from fine sand part are suitable for quartz morphological analysis and electron microscopy analyzes. To prepare and separate quartz grains before scanning the grains by electron microscopy, the geochemical composition of the grains was determined using Energy Dispersive Spectroscopy X-ray (EDAX) to ensure that only quartz particles were analyzed.
Results and discussion
The results obtained from the analysis of the particles size showed that silt particles with the average of 62.68% had the highest frequency in comparison to sand and clay particles with the averages of 28.96 and 8.3%, respectively. The results obtained from the scanning of quartz grains indicated that most of the grains had low roundness, and their fractures were angular and the abrasion coefficient of the samples varied from 100 to 210 (low abrasion). With respect to surface texture, quartz grains were mainly aura (semi-luminous) and sometimes opaque.
Loess sediments were deposited on slopes with east, north-east and north directions, and their dispersion originating from Turkmenistan had a north-easterly southwest transmission path, while the prevailing wind of the region at the present time (According to the above-mentioned green light) has a north west-south east direction. This shows the different conditions of wind blow direction during the deposition of loess deposits, and based on the slopes that loess sediments deposited on them, wind direction at sedimentation time had a northeast-southwest transmission path. The results of the experiment related to the particle size distribution of the loess of the region revealed that the greatest fashion or facade belonged to 59-micron particles, which include particles with the diameter of coarse silt and fine sands, indicating a small distance from the harvesting source. Furthermore, the results showed that there was strong negative correlation between height and average size of the particles (- 0.7), so that the higher the height of sample collection, the smaller the average particle size.
The results obtained from scanning quartz grains showed that most of the grains had low roundness and the fractures were angular, and low abrasion coefficient of quartz samples does not indicate a very long distance. Also, the aura-like nature of the particles represents harvesting from old river beds or alluvial plains, the medium travel distance, and locating in wind sediments areas, and is therefore evidence of the locality of sediments origin. Finally, the frequency particle size curves had almost a uniform and monotonous pattern, and the monoseness and prevalence of sediments with a diameter of 59 microns confirm the belonging of sediments to a single origin. In this regard, it can be assumed that Hari River bed or the dry and desert areas beyond this river in Turkmenistan as well as the beds of dry rivers in the dry and low rainfall Quaternary periods were the origins of the loess sediments of the study area.
Keywords: Loess sediments, Northeastern Iran, Particle size distribution, Morphoscopy, Identification.