Dolomitization

Required Geological Setting

Neogene and Quaternary sediments, Accretionary complexes

Occurrence

Dolomite-dominant dolostone formed by dolomitization. Comparing to calcite-dominant limestone, dolostones are formed at more restricted circumstances. Dolostones are more observed in Permian-Cretaceous and older than Silurian sediments, and less in Devonian-Carboniferous and Cenozoic sediments. Mechanisms of dolomite formation are still under discussion. Ideas are largely divided into the direct precipitation of dolomite from seawater and reactions of seawater with limestone.

Models of the direct precipitation of dolomite from seawater are as follows: 1) dolomite precipitates directly from high salinity seawater formed as residue of seawater evaporite, and 2) protodolomite precipitates directly from seawater formed as residue of seawater evaporite and it alters to dolomite.

Models of reactions of seawater with limestone are 3) evaporative model, 4) tidal pumping model, 5) mixing-zone model, and 6) ocean-floor model. 3) Evaporative model: Seawater evaporites are formed at seashores subjected to periodic flooding of seawater and at lagoons, and residual seawater having high Mg/Ca ratio reacts with underlying limestone to form dolomite. 4) Tidal pumping model: Seawater evaporites are formed at seashores subjected to periodic flooding of seawater and at lagoons, and residual heavy seawater having high Mg/Ca ratio refluxes as groundwater to the sea. Seawater seepages to compensate it. During this circulation activities, the seawater having high Mg/Ca ratio reacts with limestone at subtidal to tidal zones to form dolomite. 5) Mixing-zone model: The flesh water mixed with 10-40 % seawater is undersaturated with respect to calcite and is oversaturated with respect to dolomite. This solution reacts with limestone under the water table to form dolomite. 6) Ocean-floor model: The seawater at a depth of 1,000 meters in low altitude areas is undersaturated with respect to calcite and is oversaturated with respect to dolomite. This solution reacts with limestone on seamounts and oceanic plateaus to form dolomite.