Origin and Mineral Composition of Clastic Sediments on the Karst Surface Around Divača

Abstract

During the motorway construction around Divača, construction works had unearthed a large number of dolines and caves filled with mechanic sediments. The clastic sediments in some of them have been subjected to in-depth analyses. A sediment analysis has shown that some of these depressions were in fact old caves, filled with fluvial allochthonous sediments which the lowering of the surface has opened up on top, transforming them into surface depressions. All cases described undoubtedly reveal cave sediments formed in different periods or which differ due to a different course of development in the caves. According to their mineral composition, the samples of gravel, sand and loam originate from weathered remains of flysch rocks. When caves open up to the surface or are severed by an emerging doline, the cave fluvial sediments reach the surface and become exposed to the surface conditions, no longer protected against weathering. Denuded cave sediments represent an important source of soil in the Karst. Sediments and cave passages of the Brezstropa jama unroofed cave enable us to partially depict the time in which these caves were formed. It can be assumed that the cave was completely filled with fluvial sediments when the surface above it was still at the altitude of around 450 m. Today’s speleohydrological conditions are different. In caves, the water level of the free-flowing rivers and the level of karst water dropped from the altitude of 400 m to 180 m, while the surface was lowered by at least 50 m. According to the calculations, the surface is said to have lowered by app. 60 m over the last million years. With the presumed thickness of the ceiling ranging from 50 to 200 m, the Brezstropa jama unroofed cave was filled with sediments around 0.8 to 3 million years ago.