عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Karst evolution is governed by different factors. Generally karst system depends on geological and tectonic processes in one hand, and erosion and dynamic processes in the other hand. Some of the karst surveys and models concentrate on geologic and tectonic factors. While other surface processes of regional importance, have so far been largely neglected. For example, glacial erosion has been regarded as an important process in mid- to high latitude active orogenic regions, which have experienced repeated large-scale glaciations during the last two million years (Kaufmann, 2003). Markedly during the past decades, evidences for development of Pleistocene glaciations in the mountainous parts of Iran have increased. Some of the studies have reconstructed the physiographic and paleoclimate conditions of Western Iran (e.g. Wright 1961; Van Zeist and Wright 1963; Hutchinson and Cowgill 1963; Megard 1967; Mahmodi 1988; Snyder 2001, Stevens and et al 2001; Wasylikowa 2005; Wasylikowa and et al 2006). These studies mostly assume lower temperatures as well as probable increase of snowfall in western Iran during Pleistocene. Karst features have developed extensively in Shaho Ridge (Northwest Zagros) in western Iran. In this paper, different factors influencing the karst development in Shaho ridge - contain lithology, tectonic, slope, elevation and climate elements - are evaluated. The main hypothesis is that the climate elements especially glaciations during Pleistocene are of grater importance than the other factors.
Materials and Methods
In this research, in addition to descriptive method, determining the extension and distribution of karst landforms in respect to each influencing variable and Pleistocene glaciations features was analytical. The technique was comparison of karstic (sinkhole area) with non karstic parts. The research tools were topographic maps (1:50000), geology maps (1:100000), aerial photographs (1:50000), Google Earth and fieldwork tools such as GPS, compass and Camera. Slope and elevation map layers were derived from digitized topographic maps as well as lithology and fault and joint layers were derived from digitized geology maps. Estimation of the number of sinkholes per square kilometers was based on fieldwork. These features were counted in 9 stations and the results were generalized for the whole area. The survey of Pleistocene glaciations features like cirques, moraines and solifluction layers was based on fieldwork as well. Moreover, the authors have used the results of previous studies.
Results and Discussion
Karstic features are dominant features in Shaho ridge landscape. Sinkholes mostly have concentrated in two parts, respectively called Paveh karst region and Ravansar karst region. These two regions are separated by three broad valleys. Also karst sinkholes have been formed on the relatively vast interflows. Karst sinkhole regions occupy about 140 Km2 of study area (approximately 800 Km2). Geologically karst system has developed on Bistun limestone Formation that is very capable for karst development. For the assessment of influencing factors, the map of each variable (lithology, fault and joints, slope, elevation) was overplayed by sinkhole map layer. The results showed that the sinkholes mainly have been formed in Bistun formation along faults, joint systems and bedding planes as well as slopes less than 10 degree. While the most parts of Shaho ridge consists of Bistun formation and faults and joint systems and bedding planes extensively present and even slopes are less than 10 degree but there is no sign of sinkholes. The question is: why these features have been developed in specific parts? Overlaying elevation variable as a climatic index with sinkhole layer showed that these features mostly have been formed in parts with elevation more than 1900 meters. Therefore this hypothesis formed that a climatic agent such as glaciations has controlled the sinkhole formation in this region. Mahmodi (1988) believes that based on the height of bottom of glacial cirques, in western Iran during cold stages of Pleistocene, the elevation of permanent snowline had been 1800 meters above the see level, and so that, the western altitudes of Iran have been affected by glacial processes. Evidences for the occurrence of glaciers, contain cirques and glacial moraines, were found in both flanks (northern and southern) of Shaho relief. Along with the study of glacier features, the upper bound of solifluction layers as indicator for periglacial realm was studied. The ancient solifluction layers were found at the height of 1700 m. It is evident that this phenomenon has occurred under the permanent snowline (1800 m).
The results of this study indicate that the absence of karst sinkholes in the most parts of the area that geologic and even slope requirements are capable for karst development is because of the absence of appropriate climate conditions. There is a strong correlation between the occurrence of surface karst features (above 1900 m) and Pleistocene permanent snowline (1800 m). The evidences of glacial and periglacial in the area, support this hypothesis that karst features, belong to glacial cold stages. More likely, during the glacial periods the broad sinkholes (approximately 1 Km2) have had a glacial cirque role as well. Nowadays, mechanical degradation breaks down karstic features dominantly.