Authigenic clay minerals in lacustrine mudstones
Substantial amounts of authigenic clay minerals can accumulate in terrestrial mudstones where the following conditions are met: surface or pore waters are alkaline, aqueous silica activity is high, at least some dissolved magnesium (Mg) is present, and detrital input is relatively low. Availability of Al- or Fe-rich detrital clays likely leads to Mg-rich smectite formation, whereas sepiolite or kerolite is favored in environments with no detrital substrates. Surface waters of the Ngorongoro Crater, Tanzania, provide a good example of alteration of incoming detrital clay minerals into authigenic sediments with strong partitioning of Mg into silicate rather than carbonate phases. Quaternary deposits of Olduvai Gorge (Tanzania) and the Olorgesailie Basin (Kenya) provide end members for comparing processes in highly saline and alkaline settings (Olduvai) versus those in diatomaceous fresher water environments (Olorgesailie). Authigenic clays from around the world suggest that illitization and octahedral alterations are decoupled, and therefore indicative of different processes, emphasizing the need to supplement basal layer X-ray diffraction analyses with analyses of hkl reflections and geochemistry of purified phases. In general, authigenic clay minerals are more common in underfilled lake basins, usually associated with evaporitic basins with siliceous input from volcaniclastics or hydrothermal discharge.