Petrography and Joint Study of Bisotoun Area, Implications for Deciphering Factors Controlling Damages on Bisotoun Iscription

Bisotun inscription located on the mid-way of Hamedan- Kermanshah road (40 km to Kermanshah city), is carved on limestone cliffs exposed in the area. Dissolution of the inscription caused serious damage to the most important heritage of the country. The area comprises massive gray limestones of the Lower Cretaceous as a part of high Zagros thrust belt. Accordingly, the limestones are extensively tectonized and two main joint sets are developed in the area. Joint study was performed in an area of one km around the inscription. Thereby, it is revealed that these joint sets had impact on the dissolution of limestones. Limestone beds containing inscription are composed of mudstone to bioclastic algal wackestones. Original mineralogy of these algal bioclasts was aragonite. A few foraminifers and pellet also occur in some samples. Extensive jointing and unstable original mineralogy of limestone components lead to the dissolution of limestones in a meteoric condition. Evidences such as moldic, vuggy, canal, and cavern porosity suggest a meteoric diagenetic environment. Vugs up to centimeter scale in the field observations are evidences of karstification in the area. Jointing and dip direction of the bedding plane cut across the middle part of the inscription; conduct the corrosive rainfall precipitation toward the inscription surface. Consequently dissolution pit and holes were developed on it. Apparently the only practical way for remediation is to keep the inscription surface away from these corrosive waters. Considering the dip direction of the bedding plane, jointing on limestones and karstification in the area, it is recommended to drain the waters from bedding plane through a canal in a considerable distance from the inscription surface.