Speleogenesis of the world's longest cave in hybrid arenites (Krem Puri, Meghalaya, India).

Meghalaya (NW India) is one of the regions in the world with highest recorded rainfalls. Because of these peculiar climatic conditions and intense solutional weathering, karstic caves are widely reported in the numerous lime- stone areas of this part of India. Likewise, the extremely high rainfall and the tropical monsoon climate have fos- tered the formation of several caves in silicate-rich lithologies. The most important of these caves is Krem Puri, over 25.5 km long, characterized by a fracture-controlled labyrinth forming a bi-dimensional maze with mostly phreatic morphologies and some recent, still active, vadose entrenchments. A detailed petrographic study of the host rocks has shown this cave to be formed along a set of quartzarenite, hybrid arenite and calcarenite layers in a mostly siliciclastic sedimentary succession. XRF elemental analysis of the different rock strata guiding the speleogenesis confirms that the early stage of cave development is confined mainly in the calcium carbonate- rich layers. Geochemical analyses carried out on water flowing in the cave (drips, streams) have shown that car- bonate dissolution is the dominant chemical weathering process contributing to Krem Puri water solutes, whereas the dissolution of quartz, feldspars and clays play only a secondary role in rock weathering. The morphology of the cave points to a lithology-guided dissolution of the carbonate-rich layers and piping of the residual quartz sand in phreatic conditions, when the surface river network was not yet evolved and the water table was at the level of or higher than the cave. This phreatic speleogenetic phase predates the start of river net- work entrenchment (that started not earlier 5 Ma ago, and probably accelerated around 3.5 Ma), which deactivated the karst maze. With a low denudation rate of 0.15 mm/yr, this active phreatic cave level, which cur- rently lies beneath a more or less 120 m overburden, was originally covered by a sedimentary sequence at least 1–2 km thick. After its decoupling from the underground drainage system, denudation slowly brought the surface close to the maze system gradually introducing infiltration of vadose waters into the conduits. This caused the formation of the vadose entrenchments probably b100 ka ago, and carbonate speleothem deposition in the cave.