Using GIS to evaluate degree of karstification according to some important factors in carbonate rocks in Iran

Abstract

This study has been carried out with the aim of mapping karst and evaluation of degree of karstification of carbonate rocks in Iran. Mapping of karstic regions of Iran has been performed by some authors previously, but scale and limited information availability have limited the accuracy of their works. In addition, rock type was considered as the main factor to make a karstification map, and other important factors such as tectonic and climate conditions were ignored. One objective of this paper is a spatial evaluation of karst development in Iran considering the role of the chief factors affecting karst development. In this study, a theoretical karst map was developed on the basis of what is considered to be a proper combination of these factors. The main considered factors are those classified by White (Geomorphology and Hydrology of Karst Terrains, Oxford University Press, Oxford 1988) into three driving forces (i.e., chemical driving force, physical driving force and hydrogeological setting). Precipitation, temperature, relief, rock type, tectonic setting and stratigraphic thickness of soluble carbonate rocks are the main factors classified to describe the mentioned driving forces. In this study, precipitation and temperature are represented by the chemical weathering conditions during the modern and glacial periods prepared on the basis of Peltier’s graphs. The required data were gathered from the updated maps with proper scale and detailed references. Then the data were mapped, classified, weighted and managed in separate layers in GIS environment. The karst index (K _i ) is introduced to define the modeled degree of karstification computed by proper combination of six weighted layers, and the final theoretical karst map of Iran is prepared based on the ranked karst index. According to the combination method, values of K _i could be in a range 0–400 in which higher values show more degree of karst development. The classified theoretical karst map shows that out of 205,589 km² of carbonate units (12.62 % of the surface area of Iran) about 78,253 km² (38.1 % of carbonate rocks) have very low to low degree of development of karstic features. About 98,145 km² (47.7 % of carbonate rocks) have moderate degree and about 29,192 km² (14.2 % of carbonate rocks) have high to very high degree of karstification. In other words, 7.81 % of Iran (62 % of the carbonate units) is underlain by carbonate rocks with moderate to high degree of karstification. Densities of caves and karstic springs are hydrogeological features used to check the calculated degree of karstification. For this purpose, complete inventories of caves and karstic springs in Iran were made. Out of 590 caves, 491 caves are located in carbonate rocks. From 1,074 karstic springs (with discharge above 10 l/s), 338 springs have discharges above 50 l/s; 146 caves and 330 springs (with discharges above 10 l/s) occur in rocks in areas of very low to low degree of karstification. About 243 caves and 430 springs fall in the moderate class and 102 caves and 314 springs in areas with high to very high degree of karstification. Cave densities (number of caves per area) are 0.0017, 0.0019, 0.0025, 0.0031 and 0.0046 caves per square kilometer for very low, low, moderate, high and very high classes, respectively. In addition, densities of springs are 0.0041, 0.0043, 0.0044, 0.0103 and 0.0122 springs per square kilometer for very low, low, moderate, high and very high classes, respectively. In other words, densities of caves and springs show a good correlation with the evaluated karstification. Final result is also checked by surface karstic landforms (such as karren and doline) on some examples of well-known karstic regions.