Physical and chemical depositional processes when volcanoes meet lacustrine environments: the Cretaceous Imjado Volcanics, Jeungdo, southwestern Korea

A Cretaceous volcano-sedimentary succession (Imjado Volcanics, Jeungdo, SW Korea) was ana-lysed to understand volcanic influences on physical and chemical depositional processes of a shallow alkaline lake during and after explosive eruptions. The succession is composed of primary and resedimented volcaniclas-tic deposits interbedded with fine-grained sediments and a bedded chert. The primary volcaniclastic deposits are characterized by two end-members: thick (20 m), welded lapilli tuff and thin (0.9 m), planar stratified tuff depos-its. The first member deposits were accumulated by steady pyroclastic density currents that displaced the lake water from the shoreline. The second end-member deposits were accumulated by unsteady pyroclastic density currents that rapidly disintegrated at the shoreline and transformed into turbidity currents. The reverse to normally graded volcaniclastic sandstone is one of the resedimented volcaniclastic deposits accumulated by hyperpycnal flows originating from subaerial discharge. On top of this deposit, a bedded chert is exposed and composed of microcrystalline texture without biogenic remains. The microscopic analysis and stratigraphic relationship suggest that the chert bed was formed by chemical precipitation as a result of changes in the hydrochemistry of lake water by interaction between fresh water (hyperpycnal flows) and the alkaline bottom water of the lake.