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The paper reports the results of SiO2 analyses in the Aonda Cave system, located on the Auyan-tepui, one of the widest table-mountains of the Gran Sabana (South Venezuela), characterised by karst landforms developed in siliceous rock. Chemical analyses underline the very low concentration of SiO2 of the surface water. Percolation and cave drip waters have a SiO2 concentration of about 1 mg/l. The mean silica load of the cave stream is 184 mg/s, mainly derived from surface solution removal in the allogenic recharge area. In the Aonda Cave system, the mean SiO2 dissolved load is 40 mg/s, in part from surface solution (15%) and mainly from underground processes (85%). The low solubility of SiO2 in slightly acidic water implies the importance of the time factor in the formation of cave systems. With the present dissolution rate, about 10 Ma would be necessary to form the known karst system. This estimation can be significant only if we assume that climate has been stable in the last few tens of millions of years. Furthermore, this age can be taken as a minimum estimate, while, according to the geomorphic evolution of the area, the origin of the Aonda Cave system could be reasonably dated back to at least 20–30 Ma, that is, to the Oligocene. |
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