Sediments in the S-647 Cave in the Kastelec Tunnel

Abstract

The S-647 Cave was uncovered during the process of constructing the left tube of the Kastelec tunnel and was unknown until then. It is over 550 m long and 50 m deep. The eastern part descends evenly and reaches its lowest accessible point at the altitude of 315 m, i.e. 47 m under the bottom of the tunnel. The western part ascends gently so that the final point lies at the altitude of 356 m, 6 m under the tunnel’s bottom. In addition to flowstone, the cave also contains larger quantities of sandy and loamy alluvium of flysch origin. In several places, the sandy-loamy sediment is overgrown with a black crust formed by iron and manganese hydroxides. On the floor along the wall of the upper part of the cave, flowstone was deposited over the sandy sediment in the form of minor concretions where the crystals grow outwards from the centre and give the appearance of karstified pebbles. The S-647 cave is interesting because it is located deep under the surface and has been inaccessible until the tunnel began being constructed. What is more, this is not a sink cave in terms of origin, but it contains a lot of alluvium as well as vast quantities of collapsed stalagmites and stalactites. The breakages and fissures can partially be connected with the sliding and subsiding of alluvium, while the broken points of stalactites have yet to be explained. The cave’s special feature is its large quantity of helictites and the presence of stalagmites with a triangular cross-section. Because of its special character, position and speleomorphological features the cave was proposed to remain open for research purposes, which the tunnel constructors complied with, enabling an entrance through a concrete tube.