Abstract

Studies of clay fractions of sedimentary rocks from the Ventura Basin revealed a compositional difference between shales and sandstones. Montmorillonite or illite dominates in shale whereas penninite is most abundant in sandstone. Penninite increases in abundance with stratigraphic depth of the sandstone, its growth probably being favored by increased temperatures and pressures due to burial. Compositions of the shales also vary with stratigraphic position, but less regularly than the sandstones, possibly resulting from variation in proportions of allogenic clays supplied to the basin combined with different degrees of diagenetic alteration after deposition.

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