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The diagenesis of carbonate sediments can be ascribed conveniently and naturally to pene-depositional and post-depositional processes. Pene-depositional effects
occur at or a short distance below the sediment-water interface; post-depositional effects result largely from solution alteration accompanying subaerial exposure and (or) increasing lithostatic pressure. The thinness of Recent carbonate accumulations precludes solution alteration due to lithostatic pressure; consequently the present discussion is confined to the remaining diagenetic processes.
Within the marine environment pene-depositional diagenesis affects both sedimentary structures and grain textures. The most obvious, and perhaps most important, structural change is the destruction of stratification in carbonate mud deposits by a vagile infauna. Indeed, this absence of stratification is so commonplace that the occasional occurrence of laminated carbonate mud deposits heralds unusual environmental conditions.
The most important change in grain texture results from the recrystallization of various grain types, both skeletal and non-skeletal, to cryptocrystalline carbonate. Recrystallization is effected by the replacement of aragonite by aragonite or by the replacement of high magnesium calcite by high magnesium calcite. The only obvious change accompanying this replacement is the obliteration of the pre-existing crystal fabric of the affected grain. In fact, there seems to be no decrease in solubility accompanying the replacement. These data coupled with the observed replacement of organic matter by cryptocrystalline carbonate suggest that decomposition of the contained organic matter in certain carbonate grain types may indirectly effect recrystallization to cryptocrystalline carbonate.Subaerial exposure of unconsolidated carbonate deposits results in carbonate solution, replacement, and precipitation in that order. High magnesium calcite and aragonite are dissolved preferentially by meteoric water until the percolating solutions become saturated with respect to low magnesium calcite. At this point the more soluble high magnesium calcite and aragonite constituents are replaced by less soluble low magnesium calcite. Pore space precipitation of low magnesium calcite occurs when calcite saturated solutions lose water or carbon dioxide. Precipitation and (or) replacement transform the unconsolidated carbonate deposit into a limestone.
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