Smoganica – a Cave Developed in Upper Cretaceous Breccia

Abstract

Na planoti Banjšice (SZ Slovenija) je najdaljša izvirna jama Smoganica, dolga 492 m. Jama (na nadmorski višini 505 m) leži na jugozahodnem pobočju Čukle (770 m) vzhodno od reke Soče (153 m nad morjem). Smoganica se je razvila v apnenčevi breči, ki je tu debela 10 m in je del zgornjekrednih flišnih plasti. V okolici jame so zeleni laporji kot vključki v breči ali pa je breča vključena v plasti zelenega laporja. Klasti v debelozrnati breči, v kateri je tudi jama, merijo od nekaj cm do več dm v premeru. Klasti večinoma izvirajo iz mlajših rudistnih apnencev, spodnje in zgornje krede ter jurskih mikritnih in oolitnih apnencev. Jama Smoganica leži okrog 2 km južno od Idrijskega preloma in okrog 200 m severno od Kobariškega preloma. V jami zasledimo dve glavni razpoklinski smeri: N30-45°E in N120-135°E. Smoganica se je oblikovala kot sistem manjših rovov, ki so se razvili znotraj breče v vseh smereh. Jama je nastala v freatičnih razmerah. Kasneje je bila v celoti zapolnjena s sedimenti, nad katerimi so nastale nadsedimentne skalne oblike. Sedimenti so bili nato iz jame odnešeni. Današnji vodni tok oblikuje dno jamskih rovov. Višje vode oblikujejo fasete in draslje, nižje pa talne žlebove. Jamo lahko opredelimo kot poligenetsko, saj je njene dele že povsem preoblikovala voda, ki prenika s površja in polzi po jamskih stenah.

On the Banjšice plateau (NW Slovenia) the longest spring cave is 492 m long Smoganica. The cave (505 m above sea level) is situated on the SW slope of Čukla (770 m) E from the Soča river (153 m above the sea level). Smoganica is developed in limestone breccia, which is 10 m thick and belongs to the Upper Cretaceous flyschrocks. On the territory around the cave, the green marls are included in breccias or breccias are included into the green marls. Clasts in coarse-grained breccia, in which the cave is located, vary from some cm to several dm in diameter. Clasts are mostly deriving from younger rudist limestones, Lower and Upper Cretaceous and Jurassic micritic and oolitic limestones. Smoganica is situated about 2 km south from Idrija fault and about 200 m north from Kobarid fault. There are two principal fissure orientations in the cave, N30-45ºE and N120-135ºE. Smoganica was formed from the system of smaller passages that have been developed inside the breccia in 3D. The cave was formed in phreatic conditions, later it was completely filled with cave sediments.In the next stagethe above-sediment rock forms were developed. Cave sediments were later removed from the cave. Today the active water stream is cutting rock forms in the bottom of the cave passages. Higher water quantities are forming scallops and potholes and lower quantities floorchannels. Smoganica can be described as polygenetic cave because the percolating water is reshaping the passages.