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 Šri 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 drugačen 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 začela z razvojem cevi vzdolž razpok v saprolitu, neposredno pod gornjo plastjo kvarcita, verjetno v freatičnih pogojih. Kasnejša erozija v vadozni coni je saprolit odstranila in tako ustvarila mrežo rovov pravokotnega profila. Širjenje rovov je privedlo do rušenja stropa in nastanka podornih dvoran. V glavnem rovu so padli bloki kvarcita zaščitili stene in preprečili nadaljnje spodkopavanje v glavnem rovu jame.
Ključne beside: Šri Lanka, gnajs, mreža rovov, saprolit, arenizacija.