Inclusions in synsedimentary prismatic calcites of the Mississippi Lake Valley Formation bioherms, New Mexico, are 1-10-micron subhedral to euhedral "microdolomites" which occur in crystallographic continuity with the host calcites. These microdolomite inclusions occur exclusively in echinoderm grains, in the cloudy prismatic calcites, and in rare cloudy syntaxial overgrowths on echinoderms; they do not occur in adjacent mud nor in other skeletal grains. Cathodoluminescence of the cloudy dolomite-rich prismatic calcites reveals a nonluminescing, radial fibrous component, and an interfiber luminescing calcite. The radial fibrous mosaic, whose fibers are length slow, reflects the fabric of the cement precursor to the cloudy radiaxial prismatic calcites; whereas the luminescing calcite is interpreted as younger syntaxial interfiber pore filling. The microdolomite inclusions are restricted dominantly to the nonluminescing portions. This restricted distribution of the microdolomite inclusions argues for an autochthonous source for the Mg on a scale of single grains or crystals, and thus indicates a high-Mg calcite precursor to the cloudy calcites. Infilling of the interfiber pores of the radial fibrous mosaic with the luminescent, low-Mg calcite effectively isolated the metastable high-Mg calcites from conventional diagenetic stabilization. The Mg was redistributed within individual grains and fibers into microdolomite inclusions and low-Mg calcites by either an incongruent-dissolution or solid-state exsolution process. The presence of the microdolomite inclusions in cloudy calcites is prime evidence for a high-Mg calcite precursor, and, therefore, for a marine precipitational environment. We suggest this criterion is widely applicable to coarse prismatic fabrics throughout the geologic column, and is of particular importance considering that crystal fabric alone is generally inadequate in distinguishing coarse prismatic cements precipitated in marine waters from those precipitated in fresh meteoric waters.