Caves and karst-like features in Proterozoic gneiss and Cambrian granite, southern and central Sri Lanka: An introduction

Abstract

There has been little study of the geology and geomorphology of the caves and karst-like features developed in the Proterozo-ic gneiss and Cambrian granite of Sri Lanka. This lack of study is surprising given that caves and rockshelters in these rocks contain signifcant archaeological and cultural sites. Caves and karren, both mimicking those developed in carbonate rocks, have formed both in gneiss, which is the dominant rock type of the Proterozoic crust of the island and in granite. In addition to overhangs, boulder caves, soil pipes and tectonic caves, tunnel caves, arch caves and block breakdown caves of signifcant size are developed in siliceous rocks in Sri Lanka. while metamorphosed dolomites are interfoliated within the gneissic suite, simple removal of carbonate by solution from within the surrounding rock cannot account for all or most of the speleogen-esis observed. while spalling and breakdown are responsible for cave enlargement, cave initiation is probably due to either phreatic solution of silicates and/or phantom rock processes. Speleothems and cave minerals including silicates, phosphates, gypsum, carbonates and niter are found in the caves. Active silicate speleothems are not restricted to joints and fssures and suggest that solution of silicates is currently occurring within the body of the rock in the vadose zone. while guano is the likely source of the phosphate, sulfate and nitrate, the source of the calcium in the carbonates remains unclear. Caves in the intrusive and metamorphic rocks of Sri Lanka are enigmatic. They are unexpectedly similar in appearance to their carbonate karst counterparts. Continuing research will allow them to hold a mirror to our understanding of speleogenesis, mineralization and sedimentation in carbonate karst caves.