The Espiritu Santo Formation of the Mississippian Arroyo Penasco Group represents the oldest Paleozoic stratigraphic unit preserved in north-central New Mexico. The Espiritu Santo Formation is a diagenetically complex carbonate unit that exhibits a well-developed cement stratigraphy reflecting changes from meteoric fresh to marine-phreatic environments. Recrystallization of the algal-laminated sediments occurred during subaerial exposure of the overlying Macho Member of the Tererro Formation, a collapse breccia produced by the dissolution and removal of gypsum. The breccia and recrystallized limestone are indicative of broad, low-relief topography and shallow water table.

Cathodolumninescent petrography reveals the presence of pseudomorphs of dolomite and gypsum throughout the Espiritu Santo carbonates. Typical dedolomite morphologies are: inclusion-rich cores surrounded by limpid rims; corroded Mn-rich rhombs within calcite pseudospar; highly zoned rhombs; and uniformly luminescent rhombs enclosed in gypsum pseudomorphs. Calcitized gypsum, occurring as bladed to hexagonal crystals and nodules, varies from highly zoned to uniformly luminescent crystals. The varying luminosity is a possible relict of the original trace-element distribution and/or the diagenetic environment.

Meteoric waters migrating from the Macho Member were enriched, but undersaturated, in dissolved CaSO₄ and have low Mg/Ca ratio. Thus these pore fluids within the Espiritu Santo carbonates dissolved gypsum and dolomite. The solution, supersaturated with respect to CaCO₃, precipitated calcite. Therefore, the dissolution of gypsum and dolomite and the precipitation of calcite occurred simultaneously during diagenesis. The reaction terminated once the supply of gypsum was exhausted.

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