Document Type : پژوهشی
Authors
1 Dept. of Earth Sciences, Faculty of Natural Sciences, University of Tabriz, Tabriz, Iran
2 M.S. Student at Hydrogeology, University of Tabriz, Tabriz,Iran
Abstract
1- Introduction
The rocks that can be karstified are divided into two categories of carbonate rocks (limestone and dolomite) and evaporates (salt and gypsum). One of the karstic landforms are caves. Hydrogeologically, caves are dissolved cavities with diameter larger than 5 to 15 mm. This is the threshold of turbulent flow. One of the important characteristics of the caves is the cross-sectional pattern and this parameter is a controlling factor of the cave development mechanism. Caves are developed in general due to tectonic factors are structure control with irregular cross section. These types of caves are developed along faults, joints and bedding. On the other hand, caves which are caused by hydraulic phenomena (hydraulic control), have a circular or ellipsoidal cross-section. However, the first factor in the creation of such caves may also be tectonic structures, but the most important factor for their development was the flow of water (Karimi, 2010).
The karstic flow model was divided into two types of diffuse and conduit types according to velocity and governor flow equations. In diffuse flow systems, the water moves linearly through the connected fractures less than a centimeter. In this type of flow, the output of the
springs is numerous and with low discharge. While in conduit system, water moves through the joints and channels larger than one centimeter and is usually exposed as a large discharge spring. Due to the high distribution of carbonate rocks in Kurdistan province and extensive exploitation of karst springs, recognizing these resources and identification of the role of karstification in supplying the water resources of the province is a necessity. Karstic water sources are strategic water supply sources specially in crisis of drought period. In this research, after determining the karst areas of the province, caves and karstic springs as two important indicators of karst development were studied to evaluate the characteristics of caves and springs of the province in terms of development model and flow system.
2- Methodology
In order to investigate the development of karst and formation of caves and karstic springs, at the first, comprehensive knowledge of the region's petrology is necessary. The formation and structural conditions of the region should also be identified in order to recognize areas with high potential of karstification. Understanding the types of karstic units also helps to identify the level of karstification in different parts of the areas. Using plan of the caves, it is possible to compare the passage strike of the caves with fault's rose diagram. Study of spring density and the discharge rate of karstic springs in different parts of the area can be done to find out whether the karst development system is conduit or diffuse type. Also, to study the relation between the water quality of the springs with the geological formations, hydraulic conductivity (EC) was focused on. The lower electrical conductivity (EC) values are belonged to springs originated from hard rock and karstic units of the area. One of the ways that can be used to evaluate the permeability of hard and carbonate formations is Special Discharge assessment for each formation.
4-Discussion and conclusion
About 29% of the province area is formed of karstic units including impure and pure lime, lime with volcanic layers, and dolomite with the most coverage percent. The results of this study showed that all caves of the area are located on the karstic units of the province and their dominant strikes are concordant with dominant strike of the faults. By comparing Rose diagrams of fault lines around several caves with that’s of cave passage the effect of water or structure on controlling the development pattern of these caves was determined. The total discharge of the springs is 13.7 m3/s which 9.5 m3/s (about 70%) of that belongs karstic springs.
Most springs with discharge above 5 L/S originated from karstic formations are located especially in limestone, impure limestone and limestone with volcanic layers. Average discharge of total karstic springs are 0.4 liters per second, showing poor development of karst or development of diffuse model of flow system. However, karst development system seems to be conduit dominated in the southern parts of the province and diffuse dominated at the other areas. The minimum electrical conductivity (EC) of the study area belongs to springs that discharge fractured and karstic units.
The results of this study showed that all caves of the area are located on the karstic units and their dominant length is consistent with the dominant length of the faults. By comparing the Rose diagrams of fault lines around several caves with the cave passage strike revealed that structure and tectonics have the main control on development, pattern and profile of the caves. About 69.5 % of the total discharge of the springs of study area are originated from the karstic units. Karstic springs provide 79.32% of the water use in agriculture and 4% of urban and rural drinking water in Kurdistan province. The karst development system seems to be conduit dominated in the southern parts of the province and diffuse dominated at the other parts. The minimum electrical conductivity (EC) is belongs to springs which discharge fractured rock and karstic units. The most specific discharge rates are
related to carbonate rocks and lowest discharge rates are related to crystalline rocks of the province.
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