Inclusion-rich echinoderm-syntaxial cements comprise up to 38% of the intergranular cements in Mississippian skeletal calcarenites of the Lake Valley Fm., N.M. These cements have crystal morphology, substrate selectivity, and luminescence characteristics identical to previously interpreted fresh water phreatic cements, yet contain microdolomite inclusions, implying that they were originally Mg calcite. These petrographic characteristics, plus the restriction of the microdolomite-rich syntaxial cements to the lower part of the Lake Valley Fm., argue for their having precipitated in the zone of mixing between meteoric and marine phreatic groundwaters. Petrography of these cements, their association with interpreted fresh water phreatic cements, and their regional restriction to the south, suggest that the mixing zone cements were formed mainly daring the slowing down stages of a late Mississippian global sea level drop. These cements plus other microdolomite-rich calcites reported on elsewhere indicate that coarse crystals of Mg calcite commonly act as closed or semi-closed systems during re-equilibration to low-Mg calcite, rather than undergoing complete loss of Mg through conventional incongruent dissolution.