Morphology, mobility and grain size characteristics in new sand dunes (Case study: young Erg of Abshirin)

ABSTRACT
The aim of the paper is to assess the morphological characteristics, mobility, and distribution of sediment particle size as maturity indicators of young dunes. The results indicated that according to the Equivalent Sand Thickness (EST) parameter and the wind direction variability parameter, the morphology of the sand dunes was determined as linear. The climatic index of sand dune mobility using meteorological data of Qom, Kashan, and Ardestan synoptic stations in a 27-year period showed that the sand mobility index (M) for sand dunes is 210, which is in the range of fully active dunes. The Grain Size Distribution and the scatterplots diagrams of sorting, skewness, and kurtosis versus the mean size of particles in differentiating the types of sand dunes showed that the relationship of sorting and skewness versus the mean size is effective in showing the dune's mobility. Sediment grain size parameters can be used as an indicator for transport environment and spatial changes. The studies of this research showed that based on grain size characteristics and climatic index, the sand dunes are the simple linear type with full activity. The transport environment in the dune sediments of the upwind sector is more energetic than the dune sediments of the downwind sector
Extended Abstract
Introduction
Dune fields in arid and semi-arid regions typically form part of local to regional scale sand transport systems, which comprise source areas, transport pathways, and depositional sinks. The range of states of new sand dunes morphology and mobility generally follows the ratio between wind energy for sand transport, aeolian depositions characteristics, and many other environmental factors such as vegetation cover, humidity, and topography. Several parameters have been proposed to account for the morphology and mobility of the sand dunes. Wasson and Hyde considered dune forms as a function of the equivalent sand thickness (EST) parameter and wind direction change (RDP/DP) parameter. The range of states of dune mobility generally follows a climatic gradient. The climatic index of dune mobility developed by Lancaster has been applied to various environments. This index provides a measure of sand mobility (M) as a function of the ratio between the annual percentage of the time the wind is above the threshold for sand transport (W) and the effective annual precipitation (P/PE), where PE is potential evapotranspiration calculated using the Thornthwaite method. The grain size characteristics of the dune sands are closely related to factors such as the dynamic processes of the dunes, sand availability, vegetation, mode and distance of transportation from the source zones, and the energy conditions of the transporting medium. Textural and compositional variables widely used in grain-scale studies are the grain-size parameters (mean size, sorting, skewness, and kurtosis), and the specific gravity mean grain size is widely employed in dynamic interpretations, transport equations and sedimentary environment differentiation. Sorting is very useful in studies of sedimentary environments and aeolian dynamics. Dune sands tend to be better sorted than river sands. Skewness is likewise used to describe grain size distributions in aeolian environments and models to pattern the sediment transport trends. The distinction between the sand types can be numerically stated by computing the distribution curve's skewness (the third moment). On the phi scale, the skewness of dune sands is generally positive, whereas that of beach sands is generally negative. Finally, kurtosis is the less employed grain size parameter, and even Friedman (1961) affirms that it is not an environment-sensitive parameter. An aeolian sedimentary environment dominates the studied area and includes Active depositional dunes that have been formed in the last few decades. This research aims to analyze the characteristics of new sand dunes, including morphology, sediment physics, dunes mobility, and the relationship evaluation between factors to understand the nature of new sand dunes as one of the indicators of dry environments.
 
Methodology
The characteristics of new sand dunes were evaluated based on meteorological, remote sensing data, observations, and field sampling. MODIS remote sensing data is used to study the sand dunes' morphology. The meteorological data were derived from two stations of Qom and Kashan. The analysis of the elongation and form of the dunes as the wind direction indicator showed that the area is affected by the winds region of the Qom. The samples were collected from linear sand dunes within varying morphologies. A total of 16 dune sites were studied. Grain size analysis of all samples was carried out using standard dry sieving and sedimentation techniques. Graphic grain-size parameters were estimated following Folk and Ward and using GRADISTAT software. The four size parameters were calculated, namely, mean size, sorting, skewness, and kurtosis. Scatter plot diagrams of mean size versus sorting, skewness, and kurtosis were plotted as scatter diagrams to evaluate their interrelationship and effectiveness in differentiating between the various sand dunes.
 
Results and Discussion
Anemometry analysis shows that the wind in the region blows from three directions as westerly, north-westerly, and easterly, respectively, based on frequency and speed. Sand-moving winds in the area are strongly controlled from two primary directional sectors, westerly and north-westerly. Total potential sand transport (drift potential, DP) ranges from 202 (Qom station) to 87 (Kashan station) vector units (VU). Different types of sand dunes were identified in Erg Absherin; (a) prebarchanic dunes, (b) wedge-shaped dunes, and (c) simple immaturity linear dunes to silk dunes. The grain size distribution of the samples showed that the sand dunes have an average size range of medium to fine sand. The histograms of the size distribution indicated that they are all unimodal, with a modal class varying between medium to fine sand size. The sands of the studied dunes are poorly sorted; they range in size from 0.46 to 1.12 phi. The young and immature dunes of the northern area are relatively less sorted than the mature dunes. The interrelationship between mean size and skewness shows a general trend of skewness from medium to fine particles. Positivity of skewness increases with the increase in the mean grain size. In the same way, a general decreasing trend is recognized in the interrelationship between mean size and kurtosis, so sediments with a smaller mean size (larger phi) have leptokurtic. The state of dune mobility was determined based on the Lancaster dune mobility index test. The data showed that the sand mobility index (M) for sand dunes is 210, which is in the range of fully active dunes.
 
Conclusion
The landscape of Kashan deserts is dominated by desert sand dunes, which occupy a considerable area of this region. Many of these new dunes have been formed and developed in the last few decades. Therefore, they provide the form and aeolian deposits with special features. The immaturity of crescent-shaped sand dunes (prebarchanic) to Seif dunes, lack of vegetation cover, and topographical characteristics indicate that the Erg is active. The spatial distribution of sand dunes showed that the linear morphology is consistent with the behavior of Qom station's wind and sand flux patterns. The harmony of the wind and storm rose patterns indicated that both effectively shape the dunes. The value of the dune mobility index exhibits that the dunes are fully active. The Aeolian Sediment Availability was compared with Glaser's diagram. Most of the samples were located in the aeolian mobility sector according to Gläser criteria. In terms of grain size distribution, there are differences in the grain size distributions for different dune types. The dunes are mostly composed of medium to magnificent sands. The sorting parameter indicates that the sands on the southern dunes (downwind) are better sorted than on the northern dunes (upwind). Under the conditions of low wind activity in the south of Erg, the frequency of finer particles and better sorting will increase. In general, the study of the analysis of this new and young Erg indicates that the dunes are characterized by linear or elongated, active, and mobile in an Aoelaian high-energy environment with sands of medium to fine, poorly to moderately well-sorted and finely skewed.
 
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved the content of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.