Modern Mineral Formation in the Thermal Lake Fumarolnoe (Uzon Caldera, Kamchatka) as a Key to Paleoreconstruction

Resumo

Paleoreconstruction of events involved in the bottom sedimentation has been carried out on the basis of mineral composition of the section of the minor lake IV (Lake Fumarolnoe, Kamchatka) bottom sediments. The lower sediments are composed mainly of smectite in association with gypsum and jarosite; they are overlain by smectite–kaolinite with a large amount of framboidal pyrite and diatom shells. There is also a layer of sinter opal–pyrite units and calcite. It is concluded that initially the minor lake IV was a mud pit with sulfide-bearing solutions. Then, after the lowering of the water level and the partial exposure of the bottom sediments on the surface, the latter were oxidized. The subsequent phreatic explosion provided the covering of oxidized sediments with the reduced substance from the lower horizons of the pit. Further setting of sediment accumulation corresponded to a source with cyanobacterial community developing around the gryphon, and then it changed to that of a shallow-water lake similar to a contemporary one. Sediments also include two layers of pyroclastic material originated from eruptions of neighboring volcanoes.