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Stratigraphic sequences of the Hanson Creek Formation are grouped into two categories based on differing paleogeographic settings and diagenetic histories. Sequences in northeastern Eureka County record deposition in shallow subtidal to peritidal environments. Replacement dolomitization was the main diagenetic process affecting the original calcium carbonate skeletal grains, ooids, and mud. Lack of associated evaporite minerals or their pseudomorphs precludes sabkha diagenesis. Stratigraphic sequences in southwestern Eureka County consist of laminated carbonate and shale of moderately deep-water deposition, succeeded by a shoaling-upward sequence capped by oolitic grainstone. Stabilization of original carbonate sediment to low-magnesian calcite was the main diagenetic pro ess affecting these rocks.
Thin sections stained with potassium ferricyanide indicate that Fe++ did not have a role in the formation of Hanson Creek dolomites. Sr++ concentrations of the dolomites (20 to 91 ppm) are much lower than values reported for Holocene dolomites. The low values indicate reaction with solutions of strontium/calcium ratio lower than that of seawater. For Hanson Creek dolomites, ^dgr018 values range from -0.75 to -6.73 parts per thousand PDB. Such distinctly light values reflect the influence of solutions depleted in O18 relative to normal seawater. The data suggest that dolomitization occurred in the shallow subsurface as a result of the mixing of meteoric-derived groundwater and marine pore water. Intrusion of fresh water into subtidally deposi ed sediment took place as a result of lateral extension of fresh-water lenses developed beneath subaerially exposed tracts of the inner carbonate platform. Dolomite-to-limestone transitions mark the lateral extent of freshwater lenses in the subsurface. Undolomitized deeper water carbonates of southwestern Eureka County, remote from areas of freshwater recharge, were positioned beyond the greatest lateral extent of freshwater lenses and were not subjected to the early influence of meteoric derived groundwater.
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