The Evolution of an Inland Sea of Marine Origin to a Non-Marine Saline Lake: The Pennsylvanian Paradox Salt

The Paradox Formation has long been identified as having a marine origin. However, the magnesium sulfate-poor potash assemblage in the salt cannot be explained by simple equilibrium evaporation of seawater. Dolomitization of associated carbonate rocks may be the key to the initial depletion of seawater sulfate in the form of additional gypsum and anhydrite deposition, which leads to non-deposition of magnesium sulfate minerals and the precipitation of sylvite and carnallite. The economic-grade potash ore in several of the evaporite cycles occurs as sylvite with only minor carnallite. The deficiency of magnesium chloride minerals indicates that the water chemistry was not merely the result of cation exchange of Ca²¹ for Mg²¹ but that a considerable amount of brine mixing with waters of other compositions beside calcium-enriched seawater took place. Replacement fabrics and depositional textures as well as sedimentary facies relationships indicate probable shallow water deposition of saline minerals in the basin, and subaerial exposure on the basin margins. The chemistry of the evaporite minerals can be explained by an altered seawater brine mixing with meteoric water in the form of stream runoff and flash floods in an arid basin.