Origin of Dolomite in the Eocene Castle Hayne Limestone, North Carolina

Abstract

ABSTRACT A locally dolomitized 11.0-m section of the Castle Hayne Limestone is exposed in the Martin Marietta Aggregates Quarry in New Hanover County. A 1.0-m zone, 6.4 m below the overlying unconformity, consists entirely of sucrose-textured dolomite with intercrystalline and moldic porosity with vague outlines of the original allothems. The dolomite is nonstoichiometric (48 mole % MgCO3), nonferroan, and occurs as very fine to fine, anhedral to subhedral crystals, commonly with cloudy centers. The percentage of dolomite decreases fairly uniformly above and below this zone, and 3.6 m below the upper unconformity, the unit is undolomitized. Above and below the zone of maximum dolomitization, the dolomite selectively replaces the micrite matrix, and calcite allochems are unaffected. s dolomitization increases towards a maximum, calcite allochems are replaced. The lack of sabkha-type sedimentary structures, evaporite minerals, and the presence of a stenohaline marine-shelf fauna suggest hyposaline dolomitization consistent with the mixing-zone model. Analyses of the Na, Sr, and Cl content in acid-soluble material from a core drilled at this locality reveal that the Sr content systematically increases from 302 ppm at the top of the core to 524 ppm at the base. The Sr data suggest that the calcite and dolomite phases have been depleted in Sr, perhaps by hyposaline fluids. The strong positive correlation between Sr content and depth suggests a memory of a stratified (mixing-zone) fluid. Sodium ranges from a low of 252 ppm in calcite to a high of 1,500 ppm in dolomite, and a strong positive correlation exists between percentage of dolomite and Na concentration. The molar ratio of Na:Cl ranges from greater than 2:1 to 11:1; thus, the high Na values cannot be attributed to high salinity fluid inclusions in the dolomite. Although the high Na values associated with the dolomite suggest saline/hypersaline dolomitization, the acid-soluble Na was derived from a heulandite group zeolite which is present in the dolomites. The interlocking nature of the dolomite and zeolite crystals, the euhedral morphology of the zeolite and the strong correlation between percentage of dolomite to Na concentration suggest that both mineral phases are authigenic and formed penecontemporaneously. Unless the Na distribution can be documented, these data suggest that whole-rock Na concentrations in ancient dolomites may not be an accurate indicator for hyposaline versus hypersaline dolomitization. The laterally equivalent, undolomitized biomicrudite facies of the Castle Hayne Limestone has essentially no matrix porosity and little secondary moldic porosity. Thus, dolomitization has greatly enhanced the porosity and permeability of the biomicrudite facies in this area. Mixing-zone dolomitization caused by a lowering of eustatic sea level in conjunction with favorable hydrologic and geologic conditions can have a profound effect on reservoir properties and distribution.