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An 11-m section of the bryozoan biomicridite facies of the Castle Hayne Limestone in the Martin Marietta quarry, New Hanover County, North Carolina is locally dolomitized. About 6.5 m below the overlying unconformity, a 1.0-m zone consists entirely of sucrosic dolomite. The percentage of dolomite decreases fairly uniformly above and below this zone, and 3.6 m below the upper unconformity, the unit is undolomitized. The dolomite is nonferroan and occurs as fine anhedral to subhedral crystals. Above and below the zone of maximum dolomitization, the dolomite selectively replaces the micrite matrix. Where dolomitization increases toward a maximum, calcite allochems are replaced.
Acid-soluble sodium ranges from a low of 252 ppm in calcite to a high of 1,500 ppm in dolomite. Microprobe analysis revealed that sodium is concentrated in heulandite-group zeolite. The interlocking nature of the dolomite and zeolite crystals, the euhedral morphology of the zeolite, and the strong positive correlation between percentage of dolomite to sodium concentration suggest that both mineral phases are authigenic and formed penecontemporaneously from an open-system, stratified fluid (Dorag).
Unless the sodium distribution can be documented, these data suggest that whole-rock sodium concentrations in ancient dolomites may not be an accurate indicator for hyposaline versus hypersaline dolomitization. Dolomitization in proximity to the overlying subaerial unconformity has greatly enhanced postdepositional permeability in the micrite facies of the Castle Hayne Limestone. Dorag dolomitization caused by a lowering of eustatic sea level in conjunction with favorable hydrologic and lithologic conditions can have a profound effect on reservoir properties and permeability distribution in ancient carbonates.
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