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Abstract

The periphery of the well exposed Miette buildup in the Front Ranges of the Rocky Mountains is heavily dolomitized. Dolomitization is most abundant in a narrow zone less than 1 km wide and decreases rapidly into the buildup interior. Detailed mapping, pétrographie analysis, and chemical studies facilitated the recognition of Five types of dolomites in the following paragenetic sequence: (1) diffuse microcrystalline dolomite, an early phase of microcrystalline dolomite in lime mudstones and wackestones; (2) ‘mosaic’ dolomite which replaced large volumes of precursor limestone matrix and forms at least 70 percent of the dolomite in the buildup; (3)pressure solution related dolomite, coarse to fine crystalline, associated with pressure solution interfaces; (4)white sparry dolomite which forms irregular, coarsely crystalline pods in the massive dolomite facies at the buildup margin; and (5) late stage dolomite cements composed of coarse, clear, scalenohedral dolomite crystals forming isopachous crusts which line leached void spaces and fractures associated with brecciation at the buildup margin. Vuggy and intercrystalhne porosity is associated with type 2 dolomites, but most reservoir porosity has been destroyed.

The Miette buildup was subaerially exposed only for brief periods and evaporites or evaporite-related features are absent. Pétrographie observations supported by geochemical and isotope data mdicate that pervasive dolomitization beginning with type 2 dolomites was a late diagenetic phenomenon postdating cementation and üthification of buildup interior deposits. Most of the dolomites in the buildup probably formed by a combination of three processes: (1) migration of brines from adjacent basin muds undergoing compaction, (2) pressure solution, and (3) mixing of near-surface fluids with deep burial brines along fracture controlled conduits. Most of the dolomitizing waters are beheved to have been derived from the de watering of basinal strata adjacent to the buildup and from strata underlying the buildup.

The deep burial regime has been underestimated as an environment conducive to massive dolomitization. Significant kinetic obstacles which hinder the formation of ordered dolomite at or near the surface may be largely overcome with increasing burial depths within a sedimentary basin.

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