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

Abstract

Caves and karst-like features in Proterozoic gneiss and Cambrian granite, southern and central Sri Lanka: An introduction. There has been little study of the geology and geomorphology of the caves and karstlike 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 significant 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 significant size are developed in siliceous rocks in Sri Lanka. while metamor phosed dolomites are interfoliated within the gneissic suite, simple removal of carbonate by solution from within the sur rounding 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 fissures 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 carbon ate karst counterparts. Continuing research will allow them to hold a mirror to our understanding of speleogenesis, mineral ization and sedimentation in carbonate karst caves.

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

“…Unlike the Sri Lankan caves in gneiss described by Osborne et al (2013), no speleothems, neither silica nor carbonate, with the exception of one tiny silica stalagmite, have been found in Sthreepura Cave at Kiriwanagama. The only cave mineral so far identified in the cave is a small deposit of a white coloured pasty material sitting on a tree root adjacent to quartzite close to the ceiling of the East Passage ( Fig.…”

“…Network caves in saprolite are uncommon and probably short-lived phenomena but may play an important role in large-scale cave development in granitic rocks in Sri Lanka and elsewhere. Sri Lankan arch caves in gneiss described by Osborne et al (2013) appear to have formed by the removal of large volumes of phantom rock. An archaeological excavation in the floor of Fa-Hein Lena, a large gneiss arch cave, penetrates through 6 m of phan-tom rock.…”

“…So, like the passages in Sthreepura Cave at Kiriwanagama these tubes are caves in weathered rock, and have weathered rock walls remote from the unweathered bedrock. Osborne et al (2013) also described, Foliation Plane Cave at Karannagoda Rajamaha Viharaya near Ratnapura formed by the removal of a large volume of phantom rock from between two resistant folia. Holes in an isolated thin sheet of phantom rock in the cave suggest that a system of caves was developed in the mass of phantom rock before the phantom rock was almost completely removed.…”

“…1A). Osborne et al (2013) described the range of cave types developed in Proterozoic gneiss in Sri Lanka and noted that phantom rock processes could be recognised at four localities.…”

“…The cave we describe below is largely formed by the removal of an in-situ deeply weathered compositional layer (folium) in the gneiss without significant iron enrichment, so we have decided to call this weathered material a saprolite, a term used by Braga et al (2002) to describe "in situ weathering products of granitoid rocks". Osborne et al (2013) described caves in Sri Lanka developed in granitic gneiss by a weathering and removal process that mimicked the phantom rock process described in limestone caves by Dubois et al, (2014). In these caves granitic gneiss was weathered into a friable material that was washed out to expose a cave wall composed of un-weathered gneiss.…”