Formation of terraces in a river valley with active gas-hydrothermal manifestations (the Geysernaya River valley, Kamchatka Peninsula as an example)

The morphology, structure and composition of sediments at low terraces which occur in the form of non-extended fragments in the Geysernaya River valley have been studied. Coarse, poorly sorted and weakly rounded debris flow material of different age generations are dominated in the sections. Layered sand and gravel deposits that accumulated under dammed reservoir conditions were exposed in some areas. Alluvial deposits are represented by thin layers of pebbles with boulders of better roundness and sorting with sand and gravel filler, underlying and/or overlying debris flow deposits. Some fragments of terrace-like surfaces are characterized by a smaller slope compared to the longitudinal profile of the river: apparently, they represent areas of former debris flow – landslide dams. Sediments of modern debris flows can be traced from 0 up to 50 m and of ancient once from 0.5 to 12 m above the river, which indicates the absence of a direct dependence of the age of sediments from the level of their occurrence. The change in loose material is due to the proximity and activity of thermal manifestations of the Geysernoe thermal field. Gas-hydrothermal processes lead to a significant transformation of the composition and properties of the analyzed sediments – mainly to their cementation, which makes it difficult to determine the time of sediment formation. The structure of the studied sections indicates the repeated occurrence of debris flows along the valley and the formation of temporary dammed reservoirs there as a result of the landslides and debris flow dams. The active supply of material from the slopes and its redeposition by debris flows causes poor rounding and sorting of sediment, and its weak disintegration. Among the rock-forming minerals of the fine sand fraction, magnetite and pyroxenes dominate with the participation of ilmenite. The light fraction is represented mainly by opal-smectite-zeolite aggregates, and to a lesser extent by geyserite. In the mineralogical spectra of sediments accumulated in dammed lake conditions, the set of secondary minerals and aggregates is expanding. In the alluvium units underlying the mudflow material there are signs of redeposition of ancient well-rounded sediments.