عنوان مقاله [English]
Playa is one of the important geomorphology types in arid and semi-arid regions and it is sensitive to climatic changes. Composition of sediments is varying in playa and sometimes, it can reflect the past climate. Recognition of the minerals and origin of sediments in these regions has a determinative role in geomorphology of playa, for example, existence of paligorskite and sepiolite clay minerals which are formed and become sustainable in alkalic environments such as playa and sediments of arid regions, and they are recognized as an indicator of the past environment and a sign of evaporation conditions in lake sediments. Sabzevar playa, as an old lake sedimentary environment in the north east of Iran, is important from regional and environmental aspects, and recognition of its constituting sediments can significantly help the recognition of the past climate and the current conditions of the playa. This research is aimed at recognizing the minerals constituting different geomorphic surfaces of Sabzecar playa and studying the late Holocene climate in the region by mineralogy of the sediments.
Materials and methods
In this research, by using the visual data of satellite images and geological maps of the region with 1:100000 scale, the border of playa was determined and its different geomorphic surfaces were recognized. In the field studies performed during summer, 12 samples of surface sediments and deep sediments were by handy auger along the playa geomorphic areas including clay-flat areas, puffy ground clay area, and salt-clay area, and the location of sampling points was recorded by GPS. Then, sedimentary physical and geochemical analyses were done of the samples. Distribution of the particles size was done after screening by using a 2-mm screen and by laser separation method and statistical criteria of the sediments were done. Analysis and measurement of Potential of hydrogen (pH) and Electrical Conductivity (EC) was done in 1:1 ratio of water to sediment. Calcium Carbonate Equivalent (CCE) percentage of the samples was determined by volumetric method. Mineralogy of the sediments was measured by X-ray diffraction device in the range of 3 to 60 degrees (ϴ2), and for more detailed study of clay minerals, 5 samples of dry intact deposits were studied by scanning electron microscopy.
Discussion and conclusion
In Sabzevar playa, 3 geomorphologic facies including clay-flat facies, puffy ground-clay facies, and salt-clay facies were recognized which have been formed affected by the surrounding lithology of rocky outcrops, the existing evaporating layers, and the special topography of the playa. Playa (clay-flat facies) has the lowest electrical conductivity which suggests the low level of underground waters in this area. The dominating mineral quartz of this facies is at the surface which suggests the intense wind deposition as a result of increased aridity of the environment. High amounts of sand particles at the surface of this facies approve this assumption. Smectite clay mineral exists at the deep parts of this facies in an autogenic manner which suggests the weak drainage and increased aridity. The central part of the playa (puffy ground-clay facies) has been formed caused by high water table and salty water and lack of seasonal overflows. Low levels of gypsum aggregation at the surface levels and high levels of gypsum aggregation ar the deep parts approve the existence of puffy ground-clay surface. Decreased calcite and increased gypsum and sepiolite in the deep parts and existence of paligorskite can be probably caused by more arid climate and salt changes in this facies. The west part of the playa (salt-clay facies) is the lowest area of the region and it is covered by halite evaporating mineral and clay layers, and it shows the dominating evaporation process. Surface evaporation through capillary force cause upward movement of underground waters and creation of salt layers. The surface of this facies has a high level of electrical conductivity in the region. At the deep parts of this facies, we can see paligorskite and sepiolite fiber minerals in an autogenic form which suggest the dominance of arid and alkaic environment. Distribution of particles size show the highest percentage of caly in this facies which suggests the lowest average of particle size and the lowest amount of environmental energy at the time of deposition.
The results of this research suggest the existence of clastic and evaporating minerals such as quartz, paligorskite, calcite, and gypsum in Sabzevar playa. First carbonate minerals (calcite) and then, sulfate minerals (gypsum) have deposited and at the end, chlorides (halite) have deposited. The size distribution chart of particles has the average of 46.20 µm at the surface and 23.98 µm at the deep parts and the average kurtosis of 8.56 µm at the surface and 5.29 µm at the deep parts and the skewness is towards the small particles. The average values of EC and pH in all the deep samples are less than surface samples and it suggests more salt in the surface water. Fiber clay minerals such as paligorskite and sepiolite in playa have been formed in the deep part in an autogenic way and in the surface in an allogenic way, and it suggests the dominance of arid climate of the region in past and present time. Also, surface deposits suggest wind deposition in a quite arid environment and deep deposits suggest deposition in a windy alluvial environment.