Diagenetic Albitization, Zeolitization, and Replacement in Miocene Sandstones, Sites 796, 797, and 799, Japan Sea

Abstract

Framework grains and matrix in sandstones from ODP Sites 796, 797, and 799 have been altered to various degrees owing to chemical diagenetic processes. Petrographic and geochemical data show that albitization of Plagioclase feldspars and replacement of matrix, glass shards, volcanic rock fragments, and feldspars by zeolites, carbonates, clay minerals, sulfates, and sulfides were the dominant diagenetic processes.Feldspars in the volcanic lithic sandstones at Site 796, which were buried to shallow depths and subjected to relatively low diagenetic temperatures (40°-65°C), have not been albitized. On the other hand, zeolites have replaced many of the glass shards, and calcite has extensively replaced mud matrix and framework grains in many sandstones at this site. Most Plagioclase feldspars in the volcanic lithic sandstones at Site 797, where diagenetic temperatures may have exceeded 100°C, have been altered to albite. In addition, zeolites, mixed-layer clay minerals, anhydrite, barite, and calcite commonly replace glass shards, volcanic rock fragments, and feldspars. Many Plagioclase feldspars in the feldspathic sandstones (arkoses) at Site 799, which were subjected to diagenetic temperatures of approximately 100°C, have also been albitized. Zeolites are absent in Site 799 sandstones, but kaolinite occurs as a cement and as a replacement mineral for matrix and feldspars. Carbonate minerals, particularly calcite, are common replacement minerals for matrix, Plagioclase, orthoclase, and volcanic rock fragments at Site 799.Because albitization has altered the original compositions of Site 797 and 799 Plagioclase feldspars, these feldspars are not reliable indicators of the compositions of the parent source rocks. Total replacement of some framework grains in Site 796, 797, and 799 sandstones by zeolites or carbonates has also reduced the provenance information available from the framework grains.