Sthreepura cave at Kiriwanagama, south central Sri Lanka: A network cave in saprolite and proterozoic quartzite

Abstract

Sthreepura Cave at Kiriwanagama consists of an upper level of cavities in quartzite formed by arenization and a lower level developed in a saprolite formed from the weathering of gneiss sandwiched between two layers of quartzite. The lower level cave is not like most arenization and phantom rock caves as the walls are composed of saprolite, not exhumed unweathered rock. The cave is not a soil pipe as it is not in soil, lacks the characteristic features of soil pipes and has formed directly below, not above an aquiclude. Sthreepura Cave at Kiriwanagama is a network cave in saprolite. Speleogenesis in the lower level began with the development of joint-guided tubes in the saprolite directly below the upper layer of quartzite, probably under phreatic conditions. Later erosion under vadose conditions removed the saprolite to produce a network of passages with a rectangular profile. Widening of the passages ultimately caused ceiling failure producing breakdown chambers, but fallen slabs of quartzite armoured the walls of the Main Passage of the cave protecting them from undercutting. Key words: Sri Lanka, gneiss, network cave, saprolite, arenization. Jama Sthreepura pri Kiriwanagami, južni del osrednje Sri Lanke: Mreža rovov v saprolitu in proterozojskem kvarcitu Jamo St hreepura pri Kiriwanagami sestavljata gornji nivo arenizacijskih votlin v kvarcitu ter spodnji nivo v saprolitu, ki je nastal s preperevanjem gnajsa med plastmi kvarcita. Tudi stene rovov spodnjega nivoja so iz saprolita, kar kaže na drugacen razvoj od znanih arenizacijskih in fantomskih jam. Jama nima lastnosti preperinskih cevi in je v nasprotju s temi nastala neposredno pod (in ne nad) akvikludom. Jama Sthreepura pri Kiriwanagami predstavlja mrežo kanalov nastalih v saprofitu.. Speleogeneza se je zacela z razvojem cevi vzdolž razpok v saprolitu, neposredno pod gornjo plastjo kvarcita, verjetno v freaticnih pogojih. Kasnejsa erozija v vadozni coni je saprolit odstranila in tako ustvarila mrežo rovov pravokotnega profila. Sirjenje rovov je privedlo do rusenja stropa in nastanka podornih dvoran. V glavnem rovu so padli bloki kvarcita zascitili stene in preprecili nadaljnje spodkopavanje v glavnem rovu jame. Kljucne beside: Sri Lanka, gnajs, mreža rovov, saprolit, arenizacija.