Dolomitization of Ordovician Red River carbonates has produced 3 types of dolomite. Mottled dolomite and bedded dolomite developed during early diagenesis, and saccharoidal dolomite is a product of late diagenesis of limestone. Effective porosity is limited to the late diagenetic dolomite.

The mottled dolomite in the section is present as patches in carbonate mudstone and resulted from selective dolomitization of burrows in a shallow marine environment. Anaerobic bacteria, acting on buried organic material produced a chemical environment favorable for dolomite genesis. The concentration of this finely to coarsely crystalline mottled dolomite is proportional to the degree of burrowing and hence to the rate of sedimentation.

The bedded dolomite consists of thin beds of argillaceous, microcrystalline dolomite closely associated with laminated and nodular anhydrite. The dolomite was formed by replacement of carbonate mud under hypersaline conditions in a supratidal environment.

The saccharoidal dolomite ranges greatly in thickness and was produced by subsurface replacement of carbonate mudstone. Fractures permitted entry and circulation of dolomitizing solutions in nonporous limestone resulting in fine-grained porous dolomite. Distribution of saccharoidal dolomite can be predicted by mapping fracture zones which are commonly related to local structure.

End_of_Article - Last_Page 536------------