Calcite lilypads and ledges at Lorusio hot springs, Kenya Rift Valley: travertine precipitation at the air-water interface

Travertine forming at Lorusio Hot Springs in the northern Kenya Rift is con structed mainly by lilypads and ledges. The lilypads are flat, accretionary structures rooted to the substrate that are composed mostly of platy calci te crystals. They grow outward from a nucleus, subparallel to the water sur face, at or just below the air-water interface. Precipitation results from rapid degassing of CO2. Ledges, which have a similar morphology and interna l structure, are attached to the margin of a spring pool or outflow channel . As they grow laterally, lilypads and ledges may coalesce with their neigh bours to produce thin (1-3 cm) beds of travertine, examples of which are ex posed in subfossil deposits at the site. Once established, lilypads and led ges modify the outflow and can act as substrates for precipitation of other minerals and colonization by microbes on their cooler subaerial surfaces. Pore fluids are drawn upward through the lilypads by capillary evaporation. Amorphous silica then precipitates as surficial crusts upon microbial mats or forms spicular microstromatolites, some of which also contain calcite l aminae. Efflorescent Na-CO3 salts commonly encrust the drier central platfo rms of the exposed lilypad. The unusual abundance of lilypads and ledges at Lorusio reflects (i) the low-relief setting and the hydrostatic head, whic h limit terrace development, and (ii) the high temperature (>75 degrees C) of the waters, which inhibits colonization by microbial mats at crystal gro wth sites. Similar structures form in cave pools, evaporating brines, and f reezing water at sites where precipitation is induced by several processes active at the air-water interface.