Lacustrine hyperpycnal flow deposits after explosive volcanic eruptions, Cretaceous Beolkeum Member, Wido Island, Korea

The Cretaceous Beolkeum Member was formed in a lacustrine environment affected by explosive volcanic eruptions, and hyperpycnal flow deposits are well developed following the eruptions. Beds of hyperpycnal flow deposit are generally less than 25 cm thick and consist of an inversely graded and planar laminated lower part, a poorly-sorted and massive middle part, and a normally graded, planar laminated upper part. An internal erosional surface is common between the lower and middle parts. After explosive volcanic eruptions, subaerial drainage systems would be highly disturbed by deposition of fine-grained and loose pyroclasts. The pyroclasts can easily be remobilized by surface runoff, and the surface runoff evolved into sedimentladen floods with excess density to plunge into the lake, providing favorable conditions for the occurrence of the hyperpycnal flows. Compared with classic models of hyperpycnal flow deposits, predominant planar laminations in the lower and upper parts suggest high fallout rates of suspended sediments from the hyperpycnal flows during initial and late stages of deposition. This implies that the hyperpycnal flows were driven from relatively highly concentrated subaerial floods owing to erodible subaerial conditions following the eruptions. Relatively thinly bedded hyperpycnal flow deposits (< 25 cm thick) in comparison with the classic models (1 to 4 m thick) can be attributed to short-lived hyperpycnal flows, arising from the disturbed subaerial conditions following eruptions together with relatively small-scale drainage basins around the lake.