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Submarine diffused seepage (from 5 to 15 m depth) and intertidal focused gasohydrothermal venting take place on the West shore of the Bahía Concepción Bay, on Baja California, Mexico. The intertidal venting consists of a cluster of hot springs that occur a few meters offshore, with vent temperatures up to 62 °C and a pH of 6.68. Two irregularly shaped patches of silica-carbonate hot spring deposits occur around the main intertidal vent areas. In addition, a fossil bed of silica-carbonate hot spring deposits of about 75 m long crops out along a cliff next to the active vent area. Both fossil and modern silica-carbonate deposits are finely laminated, and form columnar, bulbous and smooth undulating microstromatolites up to 10 cm thick. Noncrystalline opal-A is the only silica phase present in the modern and fossil hot spring deposits and occurs as microspheres up to 300 nm in diameter forming porous aggregates and …