This study provides an example of early dolomitization of basinal limestone associated with a giant evaporite deposit. The Middle Devonian Ratner carbonate is extensively to completely replaced by microcrystalline to finely crystalline dolomite in the southern and central part of southern Saskatchewan in the Elk Point Basin. In the northern part, however, individual cored intervals of the Ratner laminite are either completely dolomitized or contain no microcrystalline to finely crystalline dolomite. Stratigraphic, petrographical and Sr-isotopic data suggest that the Ratner dolomite has formed penecontemporaneously from dense, evaporitic seawater that percolated downward. The distribution of the Ratner dolomite is highly localized in the northern part of the study area, suggesting that the downward flow of Mg2+-rich fluids was restricted by interbedded anhydrite, but where the fluids penetrated, the underlying carbonate deposits were dolomitized. The depletion in 18O isotope (varying from −4.5 to −6.6‰ PDB), and the coarser and inhomogeneous textures of the Ratner dolomite are considered to be the result of recrystallization, which is interpreted to occur at elevated temperatures related to a thermal anomaly during the Late Devonian and Carboniferous burial.