Geochemical and Sedimentary Changes of the Mighan Playa in Arak, Iran

The Mighan Playa is the largest playa in Markazi Province. Sedimentary and soil structures show that during all phases the playa was ephemeral. The duration and frequency of flooding varied causing variations in the relative amounts of accumulation, and soil formation. The clastic fraction of the playa sediment consists of quartz, mica and chlorite while the non-clastic evaporite fraction is dominated by gypsum, calcite, glauberite, thenardite and halite. Using the principle of chemical divides, such variations in mineralogy have been explained in terms of a change in brine chemistry from Na-Ca-Mg-SO4-Cl to Na-SO4-Cl type. It is also suggested that at an earlier stage, Mighan playa brine underwent evaporation under the conditions of Ca > alkalinity whereas in more recent times, the evaporite mineralogy has developed with alkalinity > Ca. The shallow depth cores are divided into distinct geochemical units based on elemental ratios. Units I and III reflect high chemical weathering whereas the middle unit, II, indicates relatively low weathering and enrichment in evaporites.