Study of Morphometric Characteristics of Gol Square Playa during Quaternary Period

Playa refers to internal regions, which are typically flat and covered by salt or salty fine-grained clastic sediments. Playa is the deepest part of a closed basin of a desert area, which has smooth surface with slight slope and is covered by fine-grained sediments. These surfaces have either no or little vegetation. Basically, due to the high water table of underground waters and the lack of appropriate drainage, the soils around the playas in Iran are always subject to saltiness because intense evaporation and presence of capillarity force result in accumulation of salt in different soil horizons and surface zones of the playas, which leads to inappropriate ecological conditions for establishment of plants. Creation of playas in Iran is a geological phenomenon in which human plays no role; since, as a result of dynamic movements during Mio-Pliocene period in recent 7million years, many of the Iranian deserts such as Dasht-e-Lut and Dasht-e-Kavir (also known as Great Salt Desert) have been created, which currently have growing desertification trend. During the rainy (glacial) stage, surface of the playas was expanded, saltiness of water was reduced, and sediments were mainly clay-silty at center and coarse-grained at boundaries; however, during the interglacial stage, similar to the current period, surface of the playas were smaller, waters were saltier, and sediments were mainly evaporite and gypsum. The internal depressions in Iran in quaternary (fourth) period have been formed as a result of the folds during Oligocene period and then have been filled with erosive matters such as molasses (soft greensand along with maroon and conglomerate). These sediments have been folded by the last Plio-Pleistocene phase of Alpine orogeny. The playas in Iran have generally surrounded all the internal plates as well as many of the inter-mountain depression. The major environmental conditions that cause formation of playas in Iran include: petrologic characteristics, structure, depth and elevation, drainage status, and climatic conditions of environment.
Methods and Material: Respecting the subject and objective of the present study, it was conducted via statistical analysis and field survey. For this purpose, various data, statistics, and information were collected from the study area, and various methods were selected and used for data analysis. Therefore, in general, the present study is aimed to investigate the geomorphological changes of the playa in Meydan Gel in Qatruyeh during quaternary period. The topographic maps used in this study, which covered the entire Iran, included maps with 1:250000 and 1:50000 scale compiled by Geographical Organization of Armed Forces. Also, these maps covered the playas in Meydan Gel including: Neyriz NH40-9, Meydan Gel 6948II, Beshneh 6948I, Vazireh 6947I, Darab NH40-13. As for the geological maps of the study area, the 1:250000 map pf Neyriz (H-II) and 1:100000 map of Qatruyeh provided by GSI Organization (Geological Survey & Mineral Explorations of Iran) were used. Moreover, the geological 1:100000 map pf Darab provided by GSI was used as well.

Since, under hydromophological conditions rivers, waterways network, and lakes exhibit different behaviors as regular flow, mixed flow, mud flow, and flood flow, measurement of the sediment particle's diameter and determination of their weight percentage is called granulometry. During the quaternary period, the climate has had some fluctuations and has passed warm and cold epochs. During the cold epochs in regions such as Zagros, internal regions of Iran, and Alborz, some pluvial lakes have been formed, most of which are currently dried due to evaporation resulting in formation of playas in Iran.
In quaternary period, three main processes were important: erosion, sedimentation, and soil formation. These processes have been active from the highest to the deepest points of the world, including Iran. Therefore, from high elevations toward internal basins (playas), various processes have been active, which include: glacial processes, aeolian processes, limnic processes, glaci-aeolian processes, glaci-aquatic processes, marine processes, mass wasting (also known as mass movement), volcanic activities, fluvial processes, soil formation processes.
Sediments are of an inevitable significance in aeolian and aquatic erosion studies, so that qualitative and quantitative investigation of sediments plays an outstanding role in identifying and controlling the dominant erosive processes. In both quaternary glacial and interglacial epochs, various erosion processes have occurred. In quaternary period, at glacial regions of physical weathering (degradation), the weathering process is dominant, and consequently the small and large pieces become angled. As a result of freezing process, the soil is disrupted and some disturbances are created in its structure, resulting in sedimentation. Once the ices are melted ad water is flown, the rocks collide together, the smashing of which results in sedimentation in depressions and lakes.

Results of the present study showed that at the borderline of playa and erosive plain, the water table would be at higher level, which is dependent on the genus of sediments in this part; so that, the medium-grained sediments are placed at this section and then tend toward fined-grained and wet salty sediments by moving toward the playa. The water level variations occur as a result of fluctuations of the rate of input and output water (evaporation) from the basin. The high water level causes carbonated (calcite and dolomite) sedimentation, while in case of low water level, the sulfated sediments, including gypse and then magnesium sediments, are deposited. Through a complete deposition cycle in playas and limnic environments, the clastic, carbonated, sulfated, and chlorinated sedimentation process occur in a downward (top-down) direction. In such case, the mineral's content variations from clastic to chlorinated along the playa's core must exhibit a regular reduction; whereas, due to the climatic changes and water level variations, such trend is not seen in Qatruyeh playa. The water level variations can be investigated using the minerals found in playa. The water level variations are commonly associated with sulfated-to-carbonated phase conversion (increased water level) and carbonated-to-sulfated phase conversion (reduced water level). Alternation of evaporative-clastic minerals in playas is an excellent paleoclimatological evidence, so that during periods with wetter climate, due to the increased sedimentary load of rivers, mainly the clastic minerals entered the river; while, during the warmer periods, the evaporative sediment's content was increased.