Landward fining in onshore gravel as evidence for a late Pleistocene tsunami on Molokai, Hawaii

Abstract

A conglomerate on the south coast of Molokai, Hawaii, has simple trends in carbonate-clast abundance and size that are explained better by tsunami deposition than by shoreline uplift. The conglomerate extends as much as 2 km inland, and to 72 m above present sea level. It consists mainly of basalt boulders derived from underlying bedrock but also contains carbonate pebbles and cobbles derived from coral reefs. Whereas basalt clasts display no vertical or lateral trend in size, carbonate clasts decrease in abundance and get smaller with elevation and distance from shore. The origin of the conglomerate is controversial; it may have been produced by either a single tsunami, or by wave action at multiple higher sea levels on Molokai. The deposit is best explained as the result of a single tsunami because only such a tsunami could move large grains so far inland and produce a simple trend in carbonate clasts while imparting no trend to the basalt clasts.