The process of mixing-zone dolomitization has received enough criticism in recent years to suggest that it no longer be considered viable for making large amounts of dolomite. This study proposes an alternative model that still requires fluid mixing. In this model, ascending flow of freshwater mixes with evaporated seawater, leading to extensive dolomitization. The new model is constrained by data from upper Miocene strata of southeast Spain, where dolomite yields fluid-inclusion Tm-ice data that range from −0.2 to −2.3 °C, indicating precipitation from fluids of 4 to 43 ppt seawater salt equivalent, thus confirming fluid mixing. The δ13C and δ18O of the dolomite ranges from −4.5 to +3.0‰ VPDB and +0.9 to +6.0‰ VPDB, respectively, and displays positive covariance. Although such positive covariation may represent fluid mixing, the mixing of two dolomites during sampling, or an originally homogeneous dolomite that has been partially altered (recrystallized), fluid-inclusion data and cathodoluminescence petrography disprove the mixture during sampling and partial-alteration hypotheses. Modeling of the most positive dolomite δ18O values indicates an evaporated seawater endmember of 43 ppt salinity, identical to that of the most saline fluid inclusions. Because paleotopography is preserved in the study area, the spatial variation in dolomite geochemistry can be mapped to evaluate if it is consistent with the hydrogeology of a typical mixing-zone model or an alternative. The data reveal that both downdip and updip areas were influenced by high-salinity and low-salinity fluid endmembers, and that stratigraphically lower units have more depleted δ13C and δ18O than upper units. These results are consistent with a model of upward flow of freshwater that provides the mechanism of mixing of freshwater and evaporated seawater. Upward flow and degassing of CO2 from loss of pressure contributed to nearly platform-wide dolomitization in this succession, and may have analogs in other ancient successions.