Modern mineral formation in the thermal lake Fumarolnoe (Uson caldera, Kamchatka) is the key to paleoreconstruction

Paleoreconstruction of events involved in the bottom sedimentation has been carried out on the base of mineral composition of a section of bottom sediments in the Lake IV (the Lake Fumarolnoe). The lower levels are composed there mainly of smectite in association with gypsum and jarosite; they are overlapped by smectite-kaolinite depositions with the large amount of diatom algae shells and the fraboidal pyrite. There is also a layer of sinter opal-pyrite structures and calcite. It is assumed that initially the lake IV was a mud pit with sulfide-bearing solutions, then, after the lowering of water level and the partial exposure of bottom sediments of the surface, the substance has been oxidized. The subsequent phreatic explosion has provided the covering of oxidized substance by the matter of lower layers of the pit. The further situation corresponded to the source, with the cyano-bacterial community developing around the griffin, and it was replaced by the site of a shallow-water lake similar to the contemporary one. The studied sedimentary formation includes two layers of pyroclastic material originated from neighbouring eruptions.