The Aleman Formation, New Mexico, is primarily dolomite with ribbons of chert. Stromatolites, relict evaporite nodules, and other features indicate an intertidal to shallow subtidal depositional environment. Stromatolitic layers are preferentially silicified with dissolution of opaline sponge spicules providing the main source of silica. Silicification was penecontemporaneous with accumulation of the sediment and concomitant with dolomitization. This is indicated by the preservation in silicified form of original anhydrite lath morphology, central cavities in chert nodules partially filled with geopetal matrix dolomite, and matrix dolomite immediately adjacent to conchoidal fractures of silicified carbonate grainstones. The latter relationship indicates that the chert was rigid and fractured before the carbonate host-sediments were lithified.--Modified journal abstract.