DEDOLOMITIZATION AS A DRIVING MECHANISM FOR KARST GENERATION IN PERMIAN BLAINE FORMATION, SOUTHWESTERN OKLAHOMA, USA

Cyclic deposits of Permian shales, dolomites, and halite and gypsum-be aring strata in the Blaine Formation of Southwestern Oklahoma contain abundant karst features. The present study shows that an important mec hanism of karst development in these sequences is dedolomitization, wh erein gypsum and dolomite in close spatial proximity dissolve and supe rsaturate groundwaters with respect to calcite. The net loss of mass a ccompanying this process (dolomite and gypsum dissolution minus calcit e precipitation) can be manifest in secondary porosity development whi le the coupled nature of this set of reactions results in the retentio n of undersaturated conditions of groundwater with respect to gypsum. The continued disequilibrium generates karst voids in gypsum-bearing a quifers, a mineral-water system that would otherwise rapidly equilibra te. Geochemical modeling (using the code PHRQPITZ, Plummer et al 1988) of groundwater chemical data from Southwestern Oklahoma from the 1950 's up to the present suggests that dedolomitization has occurred throu ghout this time period in evaporite sequences in Southwestern Oklahoma . Reports from groundwater well logs in the region of vein calcite sug gest secondary precipitation, an observation in accord with dedolomite formation In terms of the amounts of void space produced by dissoluti on, dedolomitization can dominate gypsum dissolution alone, especially in periods of quiescent aquifer recharge when gypsum-water systems wo uld have otherwise equilibrated and karst development ceased. Mass bal ance modeling plus molar volume considerations show that for every cub ic cm of original rock (dolomite plus gypsum), there is 0.54 cm(3) of calcite and 0.47 cm(3) of void space produced Only slightly more pore space results if the dedolomitization reaction proceeds by psuedomorph ic replacement of dolomite by calcite than in a reaction mechanism bas ed on conservation of bicarbonate.