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In marine, pre-Tertiary, carbonate-platform deposits, secondary dolomite is commonly associated either with primary dolomite or with evidence of subaerial exposure. “Primary” dolomite is used here to mean dolomite formed by penecontemporaneous replacement of calcareous sediment essentially coincident with deposition. Primary dolomite preserves microcrystalline textures and fabrics indicative of inner-platform sedimentation. “Secondary” dolomite is used here to indicate postdepositional replacement of limestone or calcareous sediment by the progressive slow growth and coalescence of discrete dolomite crystals. Secondary dolomite is generally characterized by sugary textures, which may be formed at different diagenetic stages as well as by hydrothermal processes. The boundaries of secondary dolomites may cross stratification surfaces, thereby making depositional and paleogeographic interpretation difficult.

Primary and secondary dolomites occurring in the Great Basin show a distinctive and recurrent pattern of spatial relationships. In ascending order, marine hmestone, which may be of any facies, is supplanted by sugary secondary dolomite, which is then stratigraphically overlain by primary dolomite. The boundary between the unaltered limestone and secondary dolomite is commonly a zone of mottled limestone and dolomite in which nodules or beds of secondary chert occur. The boundary between primary and secondary dolomite is abrupt and, in some cases, is an unconformity of regional extent.

The regionally coextensive occurrence of secondary dolomite either with primary dolomite or with evidence of exposure, as illustrated by examples from the Ordovician-Silurian, Silurian-Devonian, and Triassic of the Great Basin, and as observed by other workers elsewhere, indicates that secondary dolomitization results from processes related to its upper surface or to the sediment deposited upon this surface. It is thus eogenetic. Given the wide range of possible scenarios affecting the water chemistry of carbonate platforms, either the brine-reflux or mixed-water hypotheses could be adapted to explain these relationships between eogenetic secondary dolomites. For example, depositional sites of primary dolomite could either be associated with the concentration of magnesium or, through exposure, be the conduit through which meteoric water is introduced into the system.

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