Abstract:
The surfaces of table mountains (Tepuis) in southeastern Venezuela display well-developed karst topography including caves, sinkholes and karren-features. Although the rock (orthoquartzite of the Precambrian Roraima Formation) has a very low solubility, active cave systems are present with passages more than one kilometre in length, descending to more than 300 metres depth. These dimensions are greater than any so far reported in quartzitic rocks. There is strong evidence that corrosive rather than erosive processes are responsible for the karstification. Thin-sections of rock samples show dissolution not only of the amorphous silica cement, but also of the crystalline quartz grains themselves. Together with field observations in and near an active cave system on the Kukenan Tepui, this indicates a close similarity between the processes active on the Venezuelan table mountains and karstification processes in rocks of greater solubility. A combination of factors including high precipitation (4000-7000 mm/year), rock of very high purity (98 % silica) and the absence of other significant geomorphological processes prevailing for at least several million years are thought to have enabled a spectacular karst landscape to evolve in a rock that in the past has been regarded as almost immune to chemical weathering