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Three petrographically distinct types of dolomite occur in the upper member of the Mural Limestone, lower Albian, of southeastern Arizona and northeastern Sonora: (1) very fine to medium grained sucrosic dolomite occurs replacing lime mud fills of constructional reef cavities 5 cm to 2 m in diameter, emplaced along stylolites and in irregular patches of pressure-solved matrix in rudstones and floatstones, and replacing matrix in oolitic and skeletal packstones; (2) medium to extremely coarse ferroan baroque dolomite, characterized by curved cleavage and sweeping extinction, is an important void filling cement; and (3) euhedral, non-ferroan dolomite cement partly fills some late fractures.
Available evidence indicates that dolomitization occurred in four episodes. Petrographic and field evidence suggests that sucrosic dolomite in reef cavity fills and in coarse packstones preceded baroque dolomite cement. Baroque dolomite followed calcite rim cements and some blocky spar, and was followed by additional blocky spar. Fractures containing non-ferroan dolomite cement postdate all of these cements. Pressure solution-related dolomite formed still later, during middle to late burial diagenesis and tectonism.
Dedolomitization affected all dolomite types in the Mural Limestone. Nearly all sucrosic dolomite has been calcitized. The resultant fabric consists of calcite rhombs in a mass of anhedral calcite, stained by exsolved iron. Baroque dolomite cement is commonly partly calcitized. It alters to single crystals of "rusty" calcite in optical continuity with the dolomite, retaining the latter's curved cleavage and sweeping extinction.
Dedolomitization in the Mural is attributed to exposure to low-Mg fresh water under near-surface temperatures and pressures. These conditions have probably been in effect since the mid-Tertiary.
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