Dedolomitization and the Development of Rhombohedral Pores in Limestones

Dedolomitization, the reverse process of dolomitization, is brought about by solutions with a high Ca+÷/Mg +÷ ratio reacting with dolomite to form calcium carbonate. Experimental work by others uggests that dedolomitiza-tion can only proceed at or near the earth's urface. Textural evidence of the process, therefore, provides a use-ful indicator of near-surface diagenesis. Two previously undescribed textures of calcite-after-dolomite ar treated. I n one of these, the pre-dolomitiza-tion limestone fabric appears to have been partially regenerated by dedolomitization, and in the other, a clotted or "grumeleuse " texture has resulted from the replacement. These and several of the previously published e-scriptions of dedolomltization textures can be explained in terms of the growth of the new generation of calcite nucleated on inclusions derived from the original imestone and carried over in the intermediate dolomite crystal. Calcite-after-dolomite has been found grading into rhombohedral pores. It is proposed that the pores con-cerned result from the selective leaching of mineralogically unstable high-magnesian calcite or aragonite, be-lieved in many cases to be the initial product of dedolomitisatlon. I certain samples rhombohedral porosity has been destroyed by subsequent calcite cementation. The resulting cementation mosaics are distinctly different from the textures of direct dedolomitlzation. The detection of dedolornitizatlon textures is valuable both for the interpretation f diagenetic environment and possibly for the prediction of sucrosic dolomite reservoirs for such fluids as hydrocarbons and water.