CHAOTIC DEPOSITION BY A GIANT WAVE, MOLOKAI, HAWAII

A coral-basalt breccia-conglomerate is exposed >60 m above present sea level and nearly 2 km inland from the present shoreline on the southw est side of East Molokai Volcano, Molokai. The conglomerate is preserv ed mantling interfluves between gullies and forms a blanket-like depos it 0.5 to 3 m thick. No evidence of a wavecut terrace either within th e blanket or at its upper or lower limits of exposure can be found. No coral was found in growth position. Unsorted basalt clasts as large a s boulder size are subrounded to angular, randomly oriented, and vary from 2-3 to >50 volume% of the deposit. The size of the largest basalt and coral fragments shows almost no systematic variation with elevati on, distance inland, or lateral position parallel to the shore. Two in distinct rock units are present in a few places, a lower carbonate-ric h unit that we interpret as an ''onwash facies'' and an upper basalt-r ich unit, an ''offwash facies.'' The carbonate-rich onwash facies is d ominated by biogenic debris, especially fragments of coral and beach r ock and gastropod and bivalve shells. The offwash facies contains 40% to >90% basalt cobbles and boulders in a matrix of finely broken bioge nic fragments. Where the distinct facies are lacking, the proportion o f basalt clasts increases crudely from the lower to the upper part of the conglomerate at a single place and from lower to higher elevation outcrops. Where the deposit overlies a cinder cone, several carbonate- bearing clastic dikes extend from the conglomerate as much as 10 m dow n into the basalt cinders. This deposit was apparently laid down by a giant wave that broke over an outer reef, similar to the present fring ing reef, and advanced as a turbulent bore over the back-reef flat, pi cking up a slurry of carbonate-rich debris and depositing it on the sl opes inland as the wave advanced. The offwash picked up loose basalt d ebris on its return flow. U-series dating of coral fragments indicates that the age of this deposit is 240-200 ka, indicating the wave is di stinct from that which produced a similar deposit on Lanai about 100 k a. This giant wave was most likely caused by one of the many large sub marine landslides that have been identified on the lower slopes of the major Hawaiian Islands.