Abstract

Dedolomitization, the reverse process of dolomitization, is brought about by solutions with a high Ca (super ++) /Mg (super ++) ratio reacting with dolomite to form calcium carbonate. Experimental work by others suggests that dedolomitization can only proceed at or near the earth's surface. Textural evidence of the process, therefore, provides a useful indicator of near-surface diagenesis. Two previously undescribed textures of calcite-after-dolomite are treated. In one of these, the pre-dolomitization 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 descriptions of dedolomitization textures can be explained in terms of the growth of the new generation of calcite nucleated on inclusions derived from the original limestone and carried over in the intermediate dolomite crystal. Calcite-after-dolomite has been found grading into rhombohedral pores. It is proposed that the pores concerned result from the selective leaching of mineralogically unstable high-magnesian calcite or aragonite, believed in many cases to be the initial product of dedolomitisation. In certain samples rhombohedral porosity has been destroyed by subsequent calcite cementation. The resulting cementation mosaics are distinctly different from the textures of direct dedolomitization. The detection of dedolomitization textures is valuable both for the interpretation of diagenetic environment and possibly for the prediction of sucrosic dolomite reservoirs for such fluids as hydrocarbons and water.

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