Syneruptive and intereruptive lithofacies in lacustrine environments: The Cretaceous Beolkeum Member, Wido Island, Korea

The Cretaceous Beolkeum Member was deposited in a lacustrine environment affected by explosive volcanism, and a number of syneruptive lithofacies are intercalated with lacustrine mudstones. The syneruptive lithofacies were formed by primary volcanic processes and resedimented depositional processes during and after eruptions and exhibit unique depositional features, which can be grouped into four syneruptive lithofacies assemblages (SLA-1-to SLA-4, from bottom to top).Primary syneruptive lithofacies are represented by welded massive lapilli tuff in SLA-1 and a normally graded lapilli tuff in SLA-2. In SLA-1, sustained pyroclastic density currents were able to displace lake water and rarely interacted with lake water, resulting in the formation of a welded texture and columnar joints. In SLA-2, unsteady pyroclastic density currents rapidly disintegrated, mixed with ambient water, and transformed into high-density turbidity currents, depositing thick normally graded lapilli tuff.Resedimented syneruptive lithofacies were primarily controlled by volcaniclastic sediment supply after eruptions. When volcaniclastic sediments were continuously supplied into the lake by debris flows, a volcaniclastic fan can be formed (SLA-1), resulting in a coarsening-upward trend and progradational geometry. In case of relatively small amounts of volcaniclastic sediment supply, turbidity currents would be a main depositional process, depositing a series of normally graded tuff on the primary syneruptive lithofacies (SLA-2), showing a fining-upward trend. In SLA-3 and SLA-4, there are only a few cm thick, normally graded tuff, reflecting minor volcaniclastic sediment supply. However, overlying normally graded sandstones in SLA-3, showing a coarsening-upward trend, and a thick normally graded sandstone in SLA-4 suggest favorable conditions for the generation of the sediment gravity flows, probably due to an increase in volcaniclastic sediment supply after eruptions.