The upper Miocene Stevens Sandstone deep-water turbidite

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in the Gulf Poloma oil field, San Joaquin Valley, is an immature, arkosic sandstone overlain and underlain by distal fan siliceous shales. Porosity development in about 100 ft (30 m) of Stevens Sandstone in well U36-28 (present burial depth ~ 11,000 ft, 3,350 m) has followed the stages: (1) reduction of primary porosity due to initial burial compaction to near zero in the finer grained rocks, (2) replacement of silicates and filling of remaining primary porosity with calcite cement reducing the porosity to near zero, (3) dissolution of most calcite cement to produce up to 15% secondary porosity, and (4) reduction of secondary porosity by growth of authigenic clay minerals, mostly chlorite and kaolinite. Thus, major porosity development is contingent on a source of calcium for the calc te cement. The source of the calcium is assumed to be from dissolution of calcite tests in the surrounding siliceous shales, though this is difficult to prove. Calcium is concentrated in stylolites and fractures within the siliceous shales, suggesting the mobilization of calcium. The siliceous shales, equivalent to the siliceous Monterey Formation, are more quartz-rich than the arkosic Stevens Sandstone. The source of the silica in the siliceous shales is inferred to be recrystallized diatom frustules. The < 1 µm quartz content in the sandstones generally increases toward the contacts with the siliceous shales suggesting invasion of silica from the shales into the sandstones.

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