ABSTRACT

Upper Paleozoic (Pennsylvanian) algal bioherms of the western Orogrande basin consist of multiple high-frequency sequences with stratiform dolomitization that preferentially affects peritidal facies and strata proximal to sequence boundaries. Field, petrographic, and geochemical analyses reveal two replacive dolomite phases, and several dolomite cements. The facies-selective (FS) replacive phase affects peritidal facies, and consists of micritic to finely crystalline, fabric-preserving dolomite with high Sr and Na concentrations and low to moderate Fe and Mn. The non-facies-selective (NFS) phase affects facies proximal to sequence boundaries, and consists of finely to medium crystalline dolomite that is both fabric-selective and commonly fabric-destructive. This phase has moderate to locally elevated Sr and Na concentrations, and moderate to high Fe and Mn. Dolomite cements line and occlude pores and molds, and consist of medium to coarsely crystalline rhombs with moderate Sr and Mn, low Na, and high Fe. Stable oxygen isotope values for bulk dolostones exhibit a wide range and include relatively low values, suggesting precipitation and recrystallization over a wide range of conditions.

FS dolomitization occurred penecontemporaneously with peritidal deposition at glacioeustatic fall to lowstand. NFS dolomitization occurred early postdepositionally, by reflux of mesosaline to hypersaline brines during early-middle stages of each glacioeustatic transgression. Virgilian lowstand gypsum deposits of the southwestern Orogrande provided the Mg source for the transgressive brines. Dolomite cements precipitated largely under reducing conditions of the burial environment.

Early dolomite in the western Orogrande basin and its absence in the eastern Orogrande basin reflects local and regional physiographic asymmetry. The western shelf was of low relief, with an archipelago of high-relief mounds, which favored fall-stage peritidal conditions with local restriction in the mounded region. In contrast, the eastern shelf was of high relief and relatively elevated, which precluded tidal-flat development and allowed marine inundation only at late transgressive to highstand stages, by nearly normal-marine waters. Development of both peritidal (penecontemporaneous) and transgressive reflux (early postdepositional) dolomitization was therefore restricted largely to the western Orogrande basin.