CALDERA KSUDACH’ LAKES (KAMCHATKA): COASTAL PROCESSES AND THE LEVEL FLUCTUATION

Abstract

The Late Pleistocene-Holocene history of the caldera complex is characterized by large fluctuations of the lakes’ level, which were repeated probability at each eruption. This led to the formation of lake terraces, as well as the effect on the character of the development of intracaldera river network. Large amounts of volcanic material supplied on the calderas slopes are subsequently subjected to erosion with the formation of lahars and debris fans in the lakes area. Our research in 2016 showed that abrasion is the dominated process on the shores of Klyuchevoye and Shtyubel lakes, accumulation prevails only near river mouths where there is an active removal of debris from the caldera slopes. Four levels of terraces can be traced on the shores of the lakes, as a rule, they are erosion. Three lower terraces (3–5, 7–8, 12–15 m above lakes level) are usually correlated with extensive proluvial fans and at these places they may be accumulative. Formation of these levels occurred after the eruption of the young Shtyubel cone in 1907. Probably it was caused by partitioning of the Teplaya river source by explosive deposits and its subsequent gradual incision. Formation of the terrace level of 25–30 m height, which is fixed near edges of the youngest caldera, probably due to the accumulation and subsequent redeposition of pyroclastic of another Shtyubel cone explosion after last caldera eruption 1725 years BP. The presence of well-defined abrasion niches in strong andesites near the modern lake Klyuchevoye level shows that the level was stable for enough long time with fluctuations up to 1 m. The cemented well-sorted coastal sediments were found at the level of +1 m above the present shoreline of lakes in the valleys of some intracaldera streams. They were accumulated in the period between the formation of 4 and 3 terrace levels when hydrothermal activity in the lakes was higher than at present.