GEYSERNAYA RIVER ALLUVIUM (KAMCHATKA): COMPOSITION AND FEATURES OF FORMATION<a href="#FN2"><sup>2</sup></a>

Abstract

A comprehensive compositional analysis of the alluvium from the river with active gas-hydrothermal manifestations was carried out for the first time. Geysernaya river alluvium is characterized by: poor roundness of boulders and pebbles (grades 1–2), poorly sorted fine-clastic component, abundance of rock fragments and intergrown minerals even in the fine sand fraction. All these indicate the sediment weak disintegration with significant amount of slope material in it, including redeposited by mudflows. The high content of smectite-zeolite and other newly formed (secondary) minerals aggregates (up to 70% of the light fraction 0.1–0.25 mm in size) and altered rock fragments (up to 70–80% in the pebble fraction of alluvium) indicate that a gas-hydrothermal activity significantly impact the bedrock and alluvium weathering within the thermal fields. The material accumulates predominant in the areas of: (1) active delivery of slope material (temporary dams), (2) the longitudinal profile flattening in dammed reservoirs, and (3) mudflow material removal. Formation of two dammed reservoirs over the past 15 years, as well as layers of fine sand found in the low terrace’s sediments, indicate that such short-lived basins form periodically in the Geysernaya river valley. The conditions of the fine sand deposition in sections of a single-branch channel and within dammed reservoirs are differ, which affects the heavy fraction leading minerals ratio. The well-rounded gravel and abundance of coarse sand, and their decrepit appearance indicates that, along with the processing of incoming slope and mudflow material large volumes the Geyzernaya river continues to cut in and erodes the ancient fluvial sediments.