KARSTIFICATION WITHOUT CARBONIC-ACID - BEDROCK DISSOLUTION BY GYPSUM-DRIVEN DEDOLOMITIZATION

Aggressive karstification can take place where dolomite and gypsum are in contact with the same aquifer. Gypsum dissolution drives the preci pitation of calcite, thus consuming carbonate ions released by dolomit e. Lake Banyoles, in northeastern Spain, is a karst lake supplied by s ublacustrine springs, and karstic collapse is occurring in the immedia te vicinity of the lake. Lake water is dominated by Mg-Ca and SO4-HCO3 , and is supersaturated with calcite that is actively accumulating in lake sediments. Water chemistry, sulfur isotope composition, local str atigrapy, and mass-balance modeling suggest that the primary karst-for ming process at Lake Banyoles is dedolomitization of basement rocks dr iven by gypsum dissolution. Karstification takes place along the subsu rface contact between the gypsiferous Beuda Formation and the dolomiti c Perafita Formation. This process is here recognized for the first ti me to cause karstification on a large scale; this is significant becau se it proceeds without the addition of soil-generated carbonic acid. G ypsum-driven dedolomitization may be responsible for other karstic sys tems heretofore attributed to soil-generated carbonic acid.