Great Cavernosity Between Dane and Fernetiči Points to Diverse Karst Formation

Abstract

The motorway route between Dane and Fernetiči penetrates the karst ridge near Sežana, running over the karst plain to the border with Italy. The karst developed in Cretaceous limestone, which is intersected by smaller faults; this was best visible during the tunnel excavation under the Tabor Hill. It is quite remarkable the extent to which the surface of this part of the karst aquifer is dotted by old caves which are filled up by fine-grained alluvium and rubble and that had, at one point or another, lost their roofs. The higher parts of the aquifer have been subject to reshaping by long-term percolating water. Old caves reveal glimpses into a time when subsurface water was close to the actual surface of today. The continuous lowering of the karst regions led to the situation where today’s surface is lower than the original karst surface was. It appears that great cavernosity came about as the result of the high permeability of this part of the aquifer. The size of the passages suggests that they were passed by larger water flows which had originally completely filled the passages. It is assumed that a remnant of a flysch nappe was somewhere in the vicinity, feeding the aquifer with surface runoff. This is also suggested by the ponor properties of some caves. The cave bottoms display meandering patterns carved out by small-scale water flows. Even after the subsurface water table had dropped, flysch layers still lingered over the limestone. Despite the water drop, high waters occasionally still reached the caves, eventually filling them up with fine-grained sediments. In the cooler periods of the Pleistocene, some of the old caves were filled up by rubble.