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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Observations from petrography, paleontology, and geochemistry indicate the varied carbonate mineralogy of Holocene sediments may not be representative of pre-Carboniferous rocks.
Petrographic examination of Phanerozoic oolites reveals that ooids with calcitic relict textures are characteristic of pre-Carboniferous carbonate rocks, whereas ooids with aragonitic relict textures are dominant in younger rocks. Marine invertebrates that secreted magnesian calcite or aragonite hard parts characterize
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post-Permian fossil assemblages, whereas calcitic forms decline or become extinct. These observations imply a post-Carboniferous change in the chemistry of the earth's surface environment.
To determine the relative importance of factors controlling magnesian calcite solubilities in seawater, saturometry experiments were performed. It is important to recognize from these experiments that the composition of a magnesian calcite precipitating at saturation from seawater can be treated as a function of atmospheric CO2 pressure, at an essentially constant seawater Mg/Ca ratio. Thus, observed trends in the textures of ooids and their interpreted mineralogy through the Phanerozoic may be due to changes in atmospheric CO2 levels, not necessarily to a major change in the Mg/Ca ratio of seawater. It is possible that pre-Carboniferous CO2 levels were high favoring precipitation of calcitic ooids and skeletal parts; after the Carboniferous, CO2 levels fell and aragonite and high magnesian calcites increased in abundance as precipitates.
Diagenetic implications of the time variance in CO2 pressure are many. Diagenetic patterns based on Holocene models may not be valid for the Paleozoic; solution-reprecipitation and inversion would be less common than recrystallization prior to the Carboniferous. Reservoir targets of intraparticle porosity in relict aragonite oolitic sands would be less important in pre-Carboniferous rocks. Higher atmospheric CO2 pressure favors dolomite formation. Finally, the CO2 content of an environment rather than the Mg/Ca ratio may be the important parameter to consider in many carbonate diagenesis studies.
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