Hot spring waters flowing from Porkchop Geyser, Norris Geyser Basin, Yellowstone National Park, USA. enter a shallow wetland basin and precipitate opal-A silica particulate. Particulate formation by chemical (rather than biochemical) and colloidal mechanisms is suggested by floc- and shard-like particle morphologies comprising opal-A silica nanospheres and microspheres of colloidal dimensions and precipitation from waters with opaque milky-blue colouration, indicative of aqueous silica-sol conditions. Sediment accumulates in the wetland at a rate of ca. 20–25 mm/year, is unconsolidated, and massive to diffusely bedded to laminated. Post depositional features include soft sediment deformation and scouring, and in drying conditions, relatively deep desiccation. Establishment of geochemically dominated wetland sedimentation is favoured where alkali-chloride hot spring fluids of circum neutral to basic pH and high silica concentration discharge to and cool (to < ca. 35 °C) within topographic depressions that receive only small volumes of non-hot spring water. Local wetland vegetation, which is composed of hydrophytes, halophytes, and alkali-tolerant species more typical of coastal wetlands, colonizes the soft wetland substrate and may be relatively quickly buried by rapid sediment accumulation. Prior to the evolution of the diatom silica-sink, geothermal wetlands containing geochemically precipitated silica sediments may have been much more common and widespread. Rhizoliths, chert nodules with organic cores, scour fabrics, soft sediment deformation, desiccation cracks, and massive to diffuse bedding preserved in Palaeozoic geothermal environments may all be evidence of ancient unconsolidated geochemical sediments and geothermal wetland conditions.

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