The influence of the Mg/Ca ratio and salinity on dolomitization in evaporite basins

ABSTRACT The recent understanding of the role of salinity reduction in promoting dolomitization has led to a departure from older theories of dolomitization by brines generated in evaporite basins. However, extensive precipitation of gypsum (or anhydrite) by these evaporative brines would have raised their Mg/Ca ratios to levels that more than compensate for their increased salinity. These brines would retain a dolomitizing potential even after mixing with large amounts of normal marine water. Brines that do not evaporate much beyond the point of initial gypsum precipitation before replenishment with normal marine water are incapable of dolomitization because their early salinity increase is not parallelled by a significant Mg/Ca ratio increase. This type of early marine replenishment might lead to the anomalous undolomitized limestone and anhydrite varves that are observed in some evaporite sequences.