PETROGRAPHIC EVIDENCE FOR AMORPHOUS SILICA PRECURSORS AND GEOMICROBIOLOGIC PROCESSES IN SILICIFIED AND PYRITIZED HOLOCENE WOOD

Decimeter to meter-sized petrified wood fragments in a Holocene alluvial aquifer in Macon Co., AL preserve evidence of anaerobic geomicrobiologic processes that led to the simultaneous degradation of organic matter and formation of authigenic silica and pyrite. The wood material apparently served as loci for sulfate reducing conditions within an overall Fe-reducing groundwater system. Textural evidence indicates a complex series of mineral dissolution-precipitation processes and synergistic and cyclic biogeochemical processes that led to the fossilization and preservation of original wood textures, authigenic mineral formation, and development of paragenetic indicators. Log fragments are locally silicified and pyritized, and surrounding detrital quartz grains are commonly replaced by biogenic pyrite (based on S-isotopes). Within the logs, textures indicate that silica originally was deposited as an amorphous phase (gel?). Similarly, original iron-sulfide phases were likely amorphous. Contraction fractures present in pyrite due to volume loss, probably from dehydration of iron-sulfide gel and formation of pyrite, are in-filled with silica. Apparent silica-gel precursors crystallized to form more thermodynamically stable chalcedony and quartz, yet preserved colliform and crustiform silica textures. This texture is remarkably similar to crystallized silica gels documented in Yellowstone National Park. Results indicate that the early dissolution of detrital quartz (solubility <10ppm) led to the subsequent precipitation of amorphous silica (solubility >100 ppm). For this to occur at 20° C implies that the organic by-products of cellulose-consuming fermentative bacteria played an important role in raising the solubility of silica to a concentration sufficient to precipitate amorphous silica.