The late Miocene Stevens Sandstone deep-water turbidite in

End_Page 456------------------------------

the Paloma oil field, San Joaquin Valley, is an 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; 3,352 m) has followed these stages: (1) reduction of primary porosity due to initial burial compaction; (2) formation of authigenic quartz and smectite; (3) replacement of silicates and filling of much of the remaining primary porosity with calcite cement reducing the porosity to near zero; (4) dissolution of most calcite cement to produce up to 15% secondary porosity; and (5) 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 calcite 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. Thin beds of ankerite containing diatom frustules attest to the presence of a siliceous-calcareous microfossil component in the siliceous shales. The siliceous shales, stratigraphically equivalent to the siliceous Monterey Formation, and more quartz-rich than the arkosic Stevens Sandstone. The source of the silica in the siliceous shales is inferred to be recrystallized diatom frustules. Mobilization of silica within the shales is suggested by the presence of stylolites, which are depleted in silica relative to the amount of silica in the non-stylolite zones. Abundant quartz overgrowths within the sandstone may have formed in part from silica derived from the surrounding siliceous shales.

End_of_Article - Last_Page 457------------