ABSTRACT

Critical assessment of dolomite abundance throughout the Phanerozoic suggests that most large-scale dolomite has had an origin related to salinity-elevated seawaters. The distribution of such dolomite is not directly related to periods of major continental flooding but rather to periods when extensive peritidal sequences or large-scale evaporite basins/lagoons existed. The development of such depositional systems is controlled largely by eustasy, paleogeographic/tectonic setting, and paleoclimate.

Extensive peritidal sequences are characteristic of mature, passive-margin carbonate platforms that formed during long periods of Earth history generally devoid of major continental glaciation (e.g., late Precambrian through early Paleozoic, and Mesozoic). Sea-level fluctuations of relatively low amplitude during such periods allowed extensive, cyclic peritidal-dominated platforms to develop. Such platforms typically lack raised rims, had depositional surfaces that stayed near sea level for tens of millions of years, and show little evidence of major drops in sea level that would have exposed the shelf for long periods. This would have allowed repeated replenishment of pore waters by Mg-rich marine fluids over very extensive supratidal surfaces. Also, since the cycles are very thin, t e downward-moving brines could penetrate several cycles on the inner platform, causing massive dolomitization. In contrast, during major glacial periods (e.g., Middle to Late Mississippian to Early Permian and late Cenozoic) higher-amplitude, ice-driven fluctuations in sea level led to extensive meteoric-water flushing of shallow platforms, but related dolomitization has been of minor significance. Under conditions of elevated Mg/Ca ratio in hypersaline brines, however, large-scale dolomitization could occur in response to the formation of extensive evaporite basins/lagoons. Suitable climatic and paleogeographic/tectonic conditions have allowed such dolomitization at various times during the Phanerozoic, regardless of specific styles of eustatic sea-level change. Lack of massive dolomiti ation in Holocene carbonates may be because neither extensive peritidal sequences nor large-scale evaporite basins/lagoons are prevalent in modern carbonate environments.