Preliminary results on the sedimentation and speleogenesis of the cave Šimčev spodmol (Slavinski ravnik corrosional plain, SW Slovenia)
Abstract
Šimčev spodmol is a small cave in the corrosional plain of Slavinski ravnik and a bypass channel to the
Unroofed cave Loza. By itself it is a cave of minor importance, but in the context of the speleogenesis of studied contact karst area it plays a very important role. Our aim was to decipher its sedimentological record in order to understand the morphogenesis of the study area. We sampled a profile of allogenic sediments with flowstone and stalagmite in a side passage, and a flowstone dome at the entrance of the cave and the unroofed cave. Allogenic sediments were studied for mineralogical and paleomagnetic properties and paleontology, while the autochthonous deposits were sampled for paleomagnetic properties and U-series dating. We found that the majority of the allogenic sediment consists of quartz and illite (together 65-88%) and to a lesser extent to chlorites and plagioclase (together 10-35%), with the amount of the latter slowly decreasing in depth. Mineral composition of allogenic sediments is consistent with transport from the Postojna Basin flysch and shows strong weathering. Preliminary paleomagnetic results reveal both normal and reverse polarities. Subsequent rock magnetic analysis will help to clarify the primary carrier of magnetization.
The paleomagnetic investigation shows eastward counter clockwise rotation more intensive than in the cave Markov spodmol (Zupan Hajna et al., 2020), located at the northern border of the Slavinski ravnik. In summary, the preliminary results from the cave Šimčev spodmol give us two distinct simultaneous phases of sedimentation: allogenic sedimentation within the epiphreatic zone which was confirmed by fauna remains, and sedimentation of the flowstone crust and speleothems in the vadose zone of cave development. This shows as, that the entrance to Šimčev spodmol was closed, while the 25 m lower inner part was still frequently flooded. This study with comparison with previous research (Zupan Hajna et al. 2008, 2020) has indicated, that the sedimentation of Šimčev spodmol is older than the allogenic sedimentation in Markov spodmol, whereas the formation of the
Unroofed cave Loza is the oldest, which provides new insights into the speleogenesis of the Slavinski ravnik corrosional plain.