Sinkholes in Neovolcanites of the Manětín Basin

Abstract

The authors have described sinkholes in neovolcanites of the Manětín Basin (about 30 km north of the Plzeň). On the platform of the table mountains Kozelka (659,8 m) and Chlumská hora (650,3 m) representing denudation remnants of lava sheets of Lower Miocene age the shallow sinkholes have been developed in dependence on the jointing of trachybasalt and nepheline basanite. Four evolution stages of suffosion forms have been distinguished. The shallow dish-shaped sinkholes of small and middle dimensions (of a length up to 20 m and a depth up to 2,5 m) belong to the first stage. Fissure type of sinkholes, created on joints parallel mostly with the border of the platforms, represent the second stage; they reach a length mostly of 20-50 m and a depth up to 5 m. The third stage is characterized by few sinkholes of joint type. The destroyed joint sinkholes in the upper parts of the border slopes of the table mountains represent the fourth stage. Disturbing by periglacial frost processes shows to Pleistocene age of the most progressive forms of sinkholes (the joint and fissure sinkholes). Sinkholes in neovolcanites of the Manětín Basin, found in the territory of Bohemia for the first time, are from point of view of form, development and geomorphological position similar to sinkholes created in block sandstones of the Jičín-Hilly Land (B. Balatka - J. Sládek 1968). From the study of these surficial pseudokarst forms in various rocks of the Bohemian Massif (sandstones, neovolcanites) it follows that besides the geological structure the geomorphological position (the high situated, mildly inclined and isolated platforms) is the important condition of the origin of suffosion processes. The suffosion processes are significant morphogenetic agents that contribute to relative fast destruction of the summit platform parts of neovolcanite table mountains of the Manětín Basin.