Rapid facies changes in Holocene fissure ridge hot spring travertines, Rapolano Terme, Italy

Holocene hot water travertine continues to form at Terms San Giovanni, near Rapolano Terme, central Italy, although artificial diversion of the water has reduced deposition. Mesothermal water (approximate to 38-39 degrees C) emerging from fault-controlled vents located on a hilltop has created a lin ear fissure ridge 240 m long and up to 10 m high. Active parts of the ridge crest are covered by small cones; inactive parts are locally neotectonical ly fissured and have small pools. Ridge deposits include crystalline crust, paper-thin raft and shrub lithotypes. The ridge has both smooth and terrac ed marginal slopes, dominated by crystalline crusts with small shrubs in te rrace pools. At the base of the ridge, there is a rapid transition to later al flats and depressions, where water from the ridge collects and deposits shrub, irregular pisoid, reed, paper-thin raft and fine-grained and organic -rich travertines. Water channelled to nearby valley sides deposits thick c rystalline crusts on valley slopes and waterfall overhangs, locally with sm all pools filled by smooth spherical pisoids. On the valley floor, mixing o f waters forms varied stream-fill deposits that include micritic reed, pape r-thin raft and coated bubble travertines. The diversity of travertine faci es observed results from the location of the Terme San Giovanni hot springs on a hill crest, thus providing a wide array of downslope locations for fu rther deposition. The abrupt facies transitions observed are characteristic of hot spring carbonates and result from a combination of rapid decrease i n precipitation away from vents, variations in local surface topography and the feedback effect of travertine deposition itself, which darns and diver ts water now.