The Hulopoe Gravel, Lanai, Hawaii: New sedimentological data and their bearing on the "giant wave" (mega-tsunami) emplacement hypothesis

Recognition that many oceanic islands are shaped by giant landslides has hi ghlighted claims that the Hulopoe Gravel on south Lanai, Hawaii, was deposi ted by giant waves (mega-tsunami) generated by such a landslide. This inter pretation is controversial. Resolution of the controversy has global implic ations because mass wasting of oceanic islands has been a common process fo r as long as hot spot volcanism has affected the ocean basins. Thus, if meg a-tsunami are attendant upon the mass wasting process, their effect on eart h surface processes should be discernible for much of geological time and m ay be comparable to that resulting from bolide impacts that form astrobleme s. Detailed facies analysis of the pebble, cobble and boulder gravels that for m the Hulopoe Gravel type section shows that the gravels are composed predo minantly of basalt clasts with appreciable amounts of limestone clasts in 8 of the 14 beds present. Deposition was not continuous: eight disconformiti es are recognized in the 9.2 m type section, three of which are associated with truncated paleosols. The Hulopoe Gravel was not deposited by a single tsunami at 105 ka, as has been proposed. One bed is clearly an alluvial dep osit. The origins of others are unclear but the facies data do not exclude tsunami as one of the processes that deposited individual beds within the H ulopoe Gravel, either above or below sea level.