The relationship of brine chemistry of the Pennsylvanian Paradox evaporite basin (Southwestern USA) to secular variation in seawater chemistry

To establish the brine chemistry associated with the evaporites in the Pennsylvanian Paradox Basin of southeastern Utah and southwestern Colorado (USA), the composition of presumably primary fluid inclusions was determined in sedimentary halite from two drill cores, one near the central part of the basin (Shafer Dome 1) and one from a more marginal location of the basin (Gibson Dome 1). Chemical analysis of halite fluid inclusions was made 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. Inclusions that range in size from 2 to 80 microns across were analysed using the Petrychenko (1973) method. Large inclusions (40 to 80 microns across) that were analysed contain one fluid phase with a carnallite or sylvite daughter crystal. Also reported in this study are fluid inclusion homogenisation temperatures for sylvite or carnallite from primary or recrystallised halite crystals in the Gibson Dome 1, Shafer Dome 1, Cane Creek 1 and Elk Ridge 1 cores. The relationship between K ⁺ and Mg ²⁺ in chloride-rich inclusions corresponds to their proportion in MgSO ₄ - depleted marine waters concentrated to the stage of carnallite deposition. A correlative relationship was observed between K ⁺ and Mg ²⁺ sulphate-rich inclusions and their predicted proportions in seawater not depleted in sulphate. In this suite of measurements, the sulphate-poor mineralogy and sulphate-poor inclusion brine compositions occur in the lower cycles of the Paradox Formation, while the sulhpate-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 (S. Williams-Stroud) as dues to the dolomitisation reaction of seawater brine with associated carbonates where mixing of seawater and meteoric water occurred in an evaporite basin that was intermittently closed to direct seawater inflow. However, the apparent temporal relationship of the mineralogy is also consistent with global seawater chemistry changes between MgSO ₄-rich to MgSO ₄-poor compositions that have been proposed by other workers. A transition from MgSO ₄-rich to MgSO ₄-poor seawater composition may have occurred between Pennsylvanian and Permian times. This paper presents a possible alternative 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 MgSO ₄-rich to MgSO ₄-poor seawater.