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The AAPG/Datapages Combined Publications Database
Four discrete phases of dolomite are present within rocks which compose the south margin of the small, isolated Miette buildup. Detailed mapping, petrographic analysis, and chemical studies facilitate distinction of the following dolomite types: (1) an "early" phase of microcrystalline dolomite occurring as a diffuse matrix component of micrite limestones; (2) a pervasive phase of saccharoidal dolomite exhibiting a variety of features "inherited" from precursor limestones; the degree of dolomitization associated with this phase ranges from partial recrystallization of limestones from all depositional facies to complete dolomite mosaics which modify most primary textures and depositional features in the buildup margin; (3) a relatively minor phase of subhedral to anhedral icrocrystalline to microcrystalline dolomite confined to the buildup interior and associated with bitumen, clay, and iron residues along stylolites, and with characteristic green microcrystalline carbonate accumulations along solution surfaces; (4) a late phase of coarse, white dolomite associated with coarse calcite spar, which is related to brecciation of phase 2 dolomites of the buildup margin.
Paragenetic relations between the first three phases are best observed in partially dolomitized micritic limestones from the interior of the buildup. Vuggy porosity is associated with the pervasive dolomites of the buildup margin.
Petrographic observations, supported by geochemical and isotopic data, suggest different origins for these different phases of dolomite. Pervasive dolomitization postdates cementation and lithification of back-reef deposits; furthermore, there is little evidence of subaerial exposure, and evaporite-related solution features are absent. Alkaline, magnesium-rich fluids derived from adjacent and underlying basinal strata may have been responsible for the major phase of pervasive dolomitization.
Before dolomitization models can be applied to specific localities and to rocks displaying specific facies relations, careful petrographic, geochemical, and stratigraphic analyses are essential.
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