RÉSUMÉ
A new and unusual type of fossil, siliceous hot-spring deposit (sinter)-comprising monomictic, quartzose conglomerate encrusted with silicified microbial laminates-has been recognized in distal portions of Jurassic and Miocene paleo-geothermal fields of South and North America, respectively. The siliceous clasts are inferred to have originated as conduit-delivered hydrothermal silica gel, owing to their general plastic morphologies, which were then locally reworked and redistributed in geothermally influenced stream paleoenvironments. Today, hot-spring-fed streams and creeks, in places with silica-armored pavements, host microbial mats coating streambeds and/or growing over, and silicifying at, stream air-water interfaces, for example, in Yellowstone National Park (USA) and Waimangu Volcanic Valley (New Zealand). However, the modern deposits do not contain the plastically deformed silica cobbles evident in Mesozoic and Cenozoic examples described herein. Moreover, the fossil microbial laminates of this study are relatively dense and strongly coat the silica cobbles, suggesting the mats stabilized the clasts under fully submerged and hot, high-energy conditions. Thus, this new sinter facies, typically found a few kilometers from main spring-vent areas, is a perhaps unexpected extreme environment in which life took hold in hydrothermal-fluvial settings of the past, and may serve as an additional target in the search for fossil biosignatures of early Earth and possibly Mars.