Latest Megatsunami Deposits from Giant Submarine Landslides Found at Ka Lae (South Point), Hawaii

Megatsunami deposits in the Hawaiian Islands have remained controversial because of their astounding implications of gigantic wave heights and runup (>100 m) and their confusion by some investigators with raised reef deposits on land. We have discovered evidence for very late Pleistocene megatsunamis in deposits at Ka Lae (South Point) on the Island of Hawaii. Hawaii island, like all the islands located southeast of Oahu, has undergone subsidence over the past 500 k.y., so upraised reefs are not possible. Here we report several layers of reworked Pahala Ash containing basalt and calcareous sand and shell, with rounded to subangular basalt cobbles and boulders to >1 m in diameter. Exposures facing the fossil -150 m low-stand reef on the Mauna Loa SW Rift contain abundant coral clasts and marine shell, and contain at least four main layers representing separate wave events. Total deposit thickness is >2 m, with inland extent to >100 m from the present coast. The nearest likely reef source is located about 4 km offshore. Calculated runups are >150 m from sea level at the event time, which is between 12 and 14 ka BP based upon radiocarbon dating of the youngest overlying Pahala Ash and the most recent U-series age of corals recovered by submersible from the drowned low-stand reef offshore. Dark gray soot-containing ash overlies the tsunami deposits in places, which may result from wildfires induced by hot ash falls after the megatsunami event. If the ash falls result from magma depressurization, then the suggested Kilauea source has been influenced by giant submarine landslides on the flanks of adjacent Mauna Loa, in this case the Ka Lae East or West giant submarine landslides immediately offshore. The timing of this event and an earlier one at Ka Lae suggested by large boulders buried under the Pahala Ash and resting upon the Kahuku Basalt cliff (Marques, Hildenbrand and McMurtry, Trans. Am. Geophys. Un., in press) reinforce the suggestion that a changing climate may trigger these giant landslides and their resulting megatsunami.