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In the Monterey Formation of the Santa Barbara coastal area, porosity decreases with increased burial diagenesis from an average of 60% in diatomaceous (opal-A) rocks to an average of 10% in rocks bearing diagenetic quartz. In carbonate-free siliceous rocks, porosity was lost principally in two abrupt reductions of 10 to 30% porosity during the two silica phase transformations (opal-A to opal-CT and opal-CT to quartz). Large differences in porosity among interbedded rocks with different silica phases show that porosity losses resulted directly from the two silica phase transformations rather than from specific conditions of temperature and burial depth. Porosity reduction was thus probably due to brief loss of strength during solution-precipitation of silica in the rocks. Similar compositions and different thicknesses of sedimentary
units in adjacent stratigraphically equivalent sections with different silica phases show that most porosity reduction during these transformations resulted from compaction, not from addition of silica.
In calcite-bearing siliceous rocks, the pattern of porosity reduction was similar, and minor additional reduction was caused by silica filling of some foraminiferal tests. In calcite-bearing rocks, with moderate (10 to 40%) silica, only 5 to 10% of porosity was lost during each silica phase transformation, and additional gradual compaction occurred in the interval between the two transformations, presumably in response to increased burial load. Formation of disseminated dolomite, in contrast to the rarer formation of highly cemented dolomite beds and nodules, was accompanied by only moderate reduction in porosity (0 to 10% porosity).
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