Southern California Miocene sands are cemented with porcelaneous cement, a mixture of opal-A, opal-CT, microquartz and montmorillonite clay. This cement was observed filling interparticle pore space in sandstones from the Beta and Wilmington Fields in the Los Angeles Basin. The silica minerals result from recrystallization of siliceous organisms, such as diatom frustules and sponge spicules, which are incorporated into the turbidite sands. The montmorillonite sources include detrital clay and alteration of unstable grains. About 70% of the bulk volume of the cement is micropore space. This microporosity constitutes from one-fifth to four-fifths of the total porosity in these sandstones. Rocks with large amounts of microporosity have low permeabilities, since fluid flow through the micropore system is restricted. Recrystallization of the porcelaneous cement with further burial would result in a chert-cemented sandstone. It is suggested that biogenic silica may be one source of chert cements observed in ancient turbidites.