Quaternary cone forming model using sedimentary and geophysical (geoelectric) data in the southeast of Yazd city, Central Iran

Abstract: Alluvial fan are one of the most dynamic Quaternary landforms in arid areas.An alluvial fan in the southeast of the city Yazd was identified based on satellite images; in this research, the primary objective of this study is to investigate the surface and subsurface sedimentology of alluvial fan flasks. Twenty soil samples were taken for sedimentological studies. The result indicate that, three main sediment types/ lithologies were recognized: gravelly mud, gravelly sand and gravelly-muddy sand leading to three differentiated facies types. The sediment of this alluvial fan haves a high viscosity. The distribution of sediment particles and the causes of changing faces types - were interpreted. Direct current (DC) geoelctric method utilizing Wenner-Schlumberrgerarray was used for subsurface investigations. DC geoelectric surveys were carried out along two crossing profiles with the lengths of 2050m and 270 m, respectively. However more resistivity was observed in the longitudinal profile and the initial sections of the alluvial fan, compared to the last parts, and a sedimentary model was drawn finally. Introduction: Quaternary alluvial fans are important depositional environments evidencing erosional topography and indirectly palaeoclimatic events. Surface runoffs formed along mountains outskirts mainly cause these deposits. Heavy rainfall and rapid (global) warming during the warm time of the Quaternary period have played a significant role in the formation of alluvial fans. Surface runoffs gradually lose energy, speed and a significant part of their solid burden after passing through the mountainous slopes of the valleys and canyons, entering the plain of the mountains. In general, the thickness of the alluvial fan decreases and its extent increases coming from the mountain to the plain. The opening section is characterized by a canal that draws the sediment from the highest point of the mountain to the plain. The alluvial fan is forming a canal from mountain to plain. The longitudinal profile line can be described concave and the transverse sections as convex. River cuts in the surface are observed. Alluvial fans , can be classified to three groups regarding size and expansion: small scale (several square kilometers), medium scale (several dozen square kilometers) and large scale (several hundred to several thousand square kilometers). Initially, the surface of the alluvial sediments has been analyzed. Therefore, common outcrops were studied and traced along the alluvial fan, through depth information is based on the interpretations of indirect information by (geoelectric measurements) and modeling with software. The purpose of this research is to investigate the sedimentology of the alluvial fans, identify subsurface units and provide a suitable sediment model for them. Initial studies of the Quaternary alluvial fan in the southeast of Yazd city, based on analysis of aerial photos and satellite imagery from the last 20 years. Sediment units were generally identified and classified. The overall geological structure of the study area and the impact of geological units on the alluvial fans have been investigated. Firstly, the alluvial fan was completely identified regarding the geologic position its dimensions. The studied alluvial fan in can be described as blade and disk in shape in its proximal and medial part. The grain kurtosis is moderate, while rounding is angled to half-angled. The poor sorting of this alluvial fan is remarkable, because the gravel size particles are present in proximal and distant facies. The results of granulation analyses and field studies revealed high viscosity of the sediments and observed debris flows moving downstream. The debris deposits consist of eroded Eocene rock units These deposits are tenon-shaped. Several factors influence the formation of the tenon-shaped alluvial fan. Rocky clay is one of the most essential components and the conditions for clay formation in this area have been provided due to the type of rock unit and weathering and erosion. Due to the high slope and the lack of vegetation and severe rain in the Pleistocene period, conditions for the formation of the alluvial fans with high viscosity deposits have been provided. Positive skewness is caused by the presence of large amounts of suspended matter in the river water, such as silt and clay, which can be deposited in parts. The statistical analysis revealed to a positive skewness value of the grain sizes within the proximal sediments. This indicates clay existence in the upstream of the alluvial fan. Discussion Surface studies include the sampling,identification and classification of Quaternary sediment. The main reason for the study of rock units is the association of these units with weathering and erosion and the direct effect of these factors on the formation of quaternary units. The third unit, has a direct impact on the alluvial fan of the area, The geometric shape of the studied alluvial fan can be described as blade type due to its highest thickness (proximal part) at the mountain ridge indicating the elevation of the Eocene rock units before the formation of the alluvial fan. According to the detailed studies of the rock units of the area, it was determined that each different lithological material and lithological changes due to weathering could affect the nature and type of the alluvial fans in the area and produce alluvial fans with different length, width and sediment type. For this reason, depending on the type of origin, it is possible to differentiate the alluvial fans of the area. The grain analysis revealed different statistical parameters and three different sediment types. All sections of the alluvial fan were identified and separated in ArcGIS software. Statistical parameters such as mean sorting, skewness, and kurtosis are sensitive to flow energy variations and are controlled by factors of erosion, transport, and rate of deposition. Therefore, the study of the particle size dispersion is one of the basic principles for the study of the conditions for the sedimentation and sediment transport The matter can be discussed with the governing energy and erosion of the alluvial fan of the area by considering the statical parameters and the type of these sediments. Conclusion: According to sub-surface studies (geophysics), a fault was identified between the stone unit of the Cretaceous and alluvial deposits, as indicated in.