Petrology and Diagenesis of the Eocene Butano Sandstone, La Honda Basin, California

Abstract

The lower and middle Eocene Butano Sandstone, at least 1500 m and possibly as much as 3000 m thick, was deposited as a submarine fan in a narrow restricted continental-borderland setting. Its quartzofeldspathic composition suggests a local provenance from plutonic and metamorphic rocks of the Salinian terrane. It crops out in three belts, of which the middle belt in the central part of the basin was buried the deepest and underwent the most significant diagenetic changes. The central belt, made up of middle-fan deposits, contains higher amounts of quartz and lower amounts of feldspar than the more proximal southern inner-fan deposits and more distal northern outer-fan deposits. The presence of authigenic albite, laumontite, and pore-filling kaolinite and chlorite suggests that decreases of feldspars (mainly plagioclasej are related mainly to dissolution of plagioclase. It is possible that at least 20-25% of the plagioclase with respect to total feldspar has been diagenetically dissolved in the Butano Sand...