A Comparison of Morphometry and the Effects of Formation Mechanisms on the Morphometry of Gypsum and Limestone Sinkholes

The presence of soluble formations promotes the formation of karstic forms. Among the most important types of these forms are sinkholes. Since the Gachsaran Formation in Jaber and Maroon plains comprise the gypsum layers, the gypsum sinkholes have occurred inside them and the karst zones of Parau and Shahou heights in Kermanshah Province have also formed on the carbonate layers. The field studies confirm that there are major differences in the morphometry and formation mechanism of gypsum and calcareous sinkholes. The present study aimed to investigate the effect of the formation mechanism of these phenomena on their morphometry and the differences and similarities of the two phenomena (gypsum and Calcareous sinkholes). In this study, the parameters of the sinkholes, including the general shape of the sinkholes, diameter, depth, slope, perimeter, and the area of ​​each sinkhole were collected. According to the Basu method, the gypsum sinkholes are categorized as elongated and bowl-shaped types, and on the basis of the Switch method, they are classified as bowl-shaped, pit-like, and funnel-shaped sinkholes. While another investigation conducted on the calcareous sinkholes indicates that these phenomena are mainly categorized in the form of oval and elongated sinkholes, and some of them are formed in the bowl-shaped types (based on the Basu Method). Besides, according to the Switch method, they are classified as bowl-shaped forms as well. Then, the relationship between these components was evaluated by using the multivariate linear regression statistical method and stepwise multiple linear regression analysis. One of the important results in this study was the direct relationship between the morphometry of the shapes and their mechanism of formation. The plastic property of the Gachsaran Formation is the cause of the placement of this formation in synclines. It causes variation in the position of gypsum layers. So, gypsum layers had different places and different positions. The relationship between gypsum layers and effective water flows and the very high rate of dissolution of gypsum (compared to lime) causes gypsum sinkholes to have different mechanisms of formation and variation in form, even in a specific study area. As a result, modeling for gypsum sinkholes is more complex than limestone sinkholes.  

Karst is a geomorphic and hydrological system that is formed by the dissolution of soluble rocks such as limestone, dolomite, and gypsum (Eziot et al., 2014). Gypsum and anhydrite are among the rock types which can be karstified easily and quickly (White, 1988). There is a close relationship between karst geomorphology and its evolution so that karst morphology phenomena are abundant and developed in the area (Ghobadi 2009, Karimi 2010). Generally, four different mechanisms are characterized for the formation of sinkholes. The mechanisms are solutions from above, collapse from the bottom, soil transport, and removal of buoyant support. Sometimes the formation of the sinkholes is due to a combination of different mechanisms. It seems that the above-mentioned mechanisms can also be applied to gypsum sinkholes; the only difference is the formation speed of the landform (Gunay, 2002). This research attempted to investigate the structural and morphometric differences and similarities between the gypsum and calcareous sinkholes and to compare their mechanisms of formation.  

Due to the considerable difference between the dimensions of the gypsum and calcareous sinkholes in this study, two methods were applied to study and extract the sinkhole forms. In a study carried out by Zamanzadehh et al. (2019), the sinkholes of the study areas were extracted based on DEM10 and CCLs methods. Given the much smaller size of gypsum karstic sinkholes in comparison to the calcareous sinkholes, the CCLs method cannot be used to investigate the morphometric features of the gypsum sinkholes in the study areas of Maroon and Jaber plains. Thus, field studies are the only way to identify these sinkholes. After the field investigation and measuring the characteristics of sinkholes in both study areas, the required parameters were prepared by SPSS software.  

In this research, Basu and Switch methods were used to determine the morphology of the sinkholes. According to the calculations, most of the sinkholes of the Maroon plain are categorized in elongated forms, and the most frequent sinkholes of the Jaber plain are of the circular category, while in Zamanzadehh et al. (2019), based on the Basu method, the least and the most frequent sinkholes are of circular and oval types, respectively. Also, some of the sinkholes are classified as elongated ones. Using the Switch method, in the Maroon and Jaber plains, 14% of the sinkholes are of pit-like type, 13% of them are funnel-shaped, and 73% of them are categorized as bowl-shaped, while in the case of calcareous sinkholes (Zamanzadehh et al., 2019), all of them are bowl-shaped. However, based on the studies and observations, it should be noted that the vicinity of marl and gypsum, and marl collapse within the gypsum sinkholes can cause major changes in the morphometry of the sinkholes and disrupt these assumptions.  

According to the results of the conducted research on the gypsum sinkholes, the components of the area with large diameter, the area with small diameter, a deep area, and a large diameter demonstrate the most significant correlation. The results of Zamanzadehh et al. (2019) confirm that the components of the perimeter with large diameter, small diameter with perimeter, large diameter with the area, an area with perimeter, and small diameter with an area have the most significant correlation. The results of simple regression analysis of second and third-degree equations between morphometric parameters of gypsum sinkholes show that there is the highest correlation rate among some parameters such as perimeter and area, small diameter and large diameter of gypsum sinkholes, while the obtained results of Zamanzadehh et al. (2019) confirms that the maximum significant relationship is between the area and perimeter components. The results of stepwise multiple linear regression analysis among the morphometric components of the sinkholes show that the highest quantity of the coefficient of determination is related to the area, small diameter and large diameter, and depth, and the lowest quantity of the coefficient of determination is related to the slope with depth, and large and small diameters. However, according to Zamanzadehh et al. (2019), the maximum quantity of significant relationship is related to perimeter parameter with the parameters of large and small diameters, and the least quantity of significant relationship is related to the components of the slope with depth, small diameter, and large diameter. The reason for the difference between the dimensions of calcareous and gypsum sinkholes can be owing to the thickness of gypsum and calcareous layers. The presence of the limestone on the crest of the elevations and the existence of gypsum sinkholes in the synclines, as well as the thickness of calcareous layers, compared to the gypsum layers, and the difference in dissolution rate of these two layers are the main factors behind the differences in the formation mechanism of these features. The presence of the creeks on the formations and gypsum layers of the Maroon plain form the elongated sinkholes. In the Jaber plain, where most of the sinkholes are formed on the knick line, the intersection point of a hillside concentrated flow and the gypsum layer has created circular sinkholes. However, it should be noted that the presence of gypsum layers with marl layers and their vicinity changes the appearance of these sinkholes rapidly (due to the collapse of marl) and basically influences their morphometry. The dissolution is the most important factor for the formation of calcareous sinkholes, and the fault factor is the only parameter that has caused a high proportion of oval and elongated sinkholes in comparison with circular sinkholes. However, due to the plasticity and thickness of the gypsum layers in the Gachsaran Formation, the effects of tectonic activities on these layers are damping, and the faults do not affect the formation of the sinkholes. One of the most significant reasons that have led to the higher correlations of calcareous sinkholes to gypsum sinkholes stems from the formation type of them.