ABSTRACT: Diagenetic Alteration of Biotite in the Great Valley Group of Central California
CLAEYS, PHILIPPE, University of California at Davis, Davis, CA
The samples described in this study were selected from wells drilled in the Forbes sandstone of the Sacramento and northern San Joaquin Valleys of central and northern California. The Forbes sandstones are composed mainly of feldspathic and lithic arenite with minor occurrences of lithic wackes. Most biotite grains contain fine-scale phyllosilicate intergrowths of kaolinite and chlorite. These phyllosilicates appear to be the result of layer-by-layer replacement rather than displacive growth within the host biotites. The replacement is clearly diagenetic in origin because the delicate morphology, as well as the extrusion of phyllosilicate layers at grain termination, are unlikely to have survived transport. Kaolinite is the most abundant phyllosilicate intergrowth formed in the detrit l biotite of the Forbes, especially at shallow depths (<1500 m). The kaolinite/chlorite ratio within altered biotites decreases with depth. Kaolinite intergrowths are still present at great depth (>3000 m), but biotite transformation to chlorite apparently is the dominant alteration process below 2000 m. No petrographic or geochemical evidence supporting kaolinite-chlorite transformations was detected. Below 2000 m, the detrital biotite is almost completely replaced except where the rock was cemented by early poikilotopic calcite. Step-scanning microprobe analyses from the unaltered part of a biotite grain to the kaolinitic or chloritic parts show transitional stages of mineral composition indicated by a progressive loss of K and Ti. Kaolinite replacement of biotite has been interp eted to be an early diagenetic process occurring under high [H+/K+] conditions and high fluid flow. Alteration to chlorite occurs preferentially under reducing conditions, less acidic pH, and fluid enriched in Fe and Mg.