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The AAPG/Datapages Combined Publications Database
The diagenesis of the Miocene Monterey Formation in the Pismo syncline can be described in five phases: (1) silica, (2) carbonate, (3) clay, (4) organics, and (5) seawater. During progressive diagenesis resulting from increasing temperatures due to burial, the reactions that characterize the four solid phases in contact with seawater are: (1) opal A ^rarr opal CT ^rarr quartz; (2) 2 CaCO3 + Mg++ ^rarr MgCa(CO3)2 + Ca++; (3) Mg++ + Na-clay ^rarr 2Na+ + Mg-clay, and 2 NH4+ + Mg-clay ^rarr Mg++ + 2 NH4-clay; and (4) CnH2n+1 COOH ^rarr Cn-H2n+2 + CO2, and CnH2n+2 ^rarr × CH
These reactions are not independent of one another, for example the opal A ^rarr opal CT ^rarr quartz reaction sequence is not strictly a function of temperature. In addition to temperature this reaction also appears to be influenced by at least the chemical potential of Mg++. Thus, the reaction sequence is highly sensitive to the presence of other phases that complete for Mg++.
Isotopic data suggest that most of the dolomitization of CaCO3 occurs in the presence of light CO2(^dgr13C = -13 to -17). The light CO2 is probably a result of decarboxylation reactions. The source of Mg++ during dolomitization appears to be concentrated subsurface fluids (seawater), with the rate controlling mechanism being dilution. The dilution in turn is a function of the opal A ^rarr opal CT ^rarr quartz reactions and the accompanying dewatering.
During early diagenesis the organics underwent both fermentation and sulphate reduction, but the most significant organic reactions were decarboxylation and cracking. The decarboxylation reactions appear to have been pervasive, whereas the cracking reactions have been documented only deep in the center of the syncline (>6,500 ft).
The clays, mainly smectites, were probably subjected to early cation exchange reactions and may have affected
both the formation of early dolomite concretions and the reaction opal A ^rarr opal CT. We have found no evidence to suggest a smectite ^rarr illite reaction, probably due to the low activity of K+ and the high activity of H4SiO4 during diagenesis.
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