The relationship of brine chemistry of the Pennsylvanian Paradox Evaporite Basin (southwestern USA) to secular variation in seawater chemistry

Abstract

To establish the brine chemistry associated with the evaporites in the Pennsylvanian Paradox Basin of southeastern Utah and southwestern Colorado (USA), the composition of primary fluid inclusions was determined for sedimentary halite from two drill cores, one near the central part of the basin (Shafer Dome No. 1) and one from a more marginal location of the basin (Gibson Dome No. 1). Chemical analysis of halite fluid inclusions was done on six samples from three different evaporite cycles of the Paradox Formation; cycle 10 in the Shafer Dome core and cycles 6 and 18 from the Gibson Dome core. The inclusions that range in size from 2 to 80 microns across, were analyzed using the Petrychenko method. Large inclusions (40 to 80 microns across) that were used for the chemical analyses contain one fluid phase with a carnallite or sylvite daughter crystal. Also reported in this study are fluid inclusion homogenization temperatures for sylvite or carnallite from primary halite crystals in the Gibson Dome core and in Shafer Dome. The relationship between K⁺ and Mg²⁺ in chloride rich inclusions corresponds to their proportion in MgSO₄-depleted marine waters concentrated to the stage of carnallite deposition. A correlative relationship was observed between K⁺ and Mg²⁺ sulfate rich inclusions and their predicted proportions in seawater not depleted in sulfate. In this suite of measurements, the sulfate-poor mineralogy and sulfate-poor inclusion brine compositions occur in the lower cycles of the Paradox Formation, while the sulfate-rich mineralogy appears to be better developed in the shallower cycles. The mineralogy of the Paradox Basin Evaporite Formation has previously been explained by one of the authors (SWS) as dues to the dolomitization reaction of seawater brine with associated carbonates where mixing of seawater and meteoric water occurred in an evaporite basin that was intermittently closed from direct seawater inflow. However, the apparent temporal relationship of the mineralogy is also consistent with global seawater chemistry changes between MgSO₄-rich to MgSO₄-poor compositions that have been proposed by other workers. A transition from MgSO₄-rich to MgSO₄-poor seawater composition may have occurred between Pennsylvanian and Permian times. This paper presents a possible alternate explanation to those already proposed in the literature, that the Paradox Formation mineralogy resulted from an intermediate seawater composition that records the global transition from MgSO₄-rich to MgSO₄-poor seawater.