Chert layers associated with the Middle Ordovician Deicke, Millibrig, and V-7 K-bentonite beds have been examined in thin-section to determine their origin. Textural evidence indicate that the chert layers are inorganic in origin. Complex diagenetic features include abundant chalcedony veinlets, replacement textures, and preservation of primary CO[sub 3] cements. The model the authors propose for the formation of these chert layers incorporates both silica release associated with the devitrification of volcanic glass and silica precipitation from a migrating regional brine. Precipitation from a brine is the dominant mechanism of silicification based on stoichiometric calculations of silica release during illite/smectite (I/S) transformations, petrography of the chert layers, and work by previous authors. However, multiple generations of silica observed in the rock suggest that other contributing small scale sources of silica may also exist. Brine derived silica preferentially replaces low Mg calcite that has high surface area, such as micrite and fossil debris. Some dolomite rhombs preserved in the chert may have formed during primary sedimentary dolomitization yet textural comparisons indicate this is unlikely for most samples. Birdseye and brachiopod cements are typically well preserved in the cherts. Cherts that have been subjected to elevated burial temperatures (150--200 C) have significant amountsmore » of authigenic clay minerals. Pyrite mineralization exhibits cross cutting relationships with chert and micrite. Clay and pyrite mineralization suggest the presence of significant porosity early in the history of formation of the chert layers.« less

Weathering profiles from representative cores of the Coweeta Group, schist, and Tallulah Falls Fm, gneiss, in the Blue Ridge Mountains of North Carolina were examined. The predominant alteration minerals, which formed from the partial alteration of biotite and plagioclase, include kaolinite, chlorite, vermiculite interstratified with biotite, gibbsite. These mineral phases were identified using petrographic observation, SEM, X-ray diffraction, and selected electron microprobe analysis. These minerals commonly represent a weathering profile (developed from surface to depth on a single source rock) ranging from mature (gibbsite), to intermediate (kaolinite + interlayered biotite/vermiculite), to immature (plagioclase + biotite) at depth. In this study, however, there is no clear vertical zonation of the weathering profile. This indicates a more selective weathering process than would typically be assumed. This nonsystematic weathering profile may reflect variations in bulk composition of the parent rock and/or variations in the composition of ground water. The presence and abundance of gibbsite in these weathering profiles is unusual because it is normally associated with bauxite in which the Al[sub 2]O[sub 3] content is > 80 wt.%. In this study the Al[sub 2]O[sub 3] content of the regolith is approximately 20 wt.%. The presence of gibbsite emphasizes the importance of solution-reprecipitation ofmore » Al-rich phases as an independent process in temperature climates, and suggests the activity of silica is more critical than regional climatological effects in controlling regolith formation.« less