The Hawaiian megatsunami of 110±10 ka: the use of microfossils in detection

Abstract. INTRODUCTION

McMurtry et al. (2004) described a thin (c. 20–50 cm) bioclastic, carbonate gravel from the NW coast of Hawaii, on the flanks of the extinct Kohala volcano (Fig. 1). This unit is found between modern altitudes of c. 1.5–61 m above sea-level. The deposit is sandwiched between a fossil soil below and a modern soil above in Keawe’ula Bay (Fig. 2). Dating of coral fragments from within the deposit indicate an age of 110±10 ka (McMurtry et al., 2004). Given rates of subsidence on Hawaii, this would place the deposit at an original palaeo-altitude up to 491 m. The deposit contains a range of bioclasts including bivalves, gastropods, corals, bryozoans and foraminifera, largely representing assemblages from a reef flat. The geological setting of the unit, coupled with the evidence from the contained marine fossils, indicate a megatsunami genesis, probably linked to the collapse of the submarine Alika Slide at about 120 ka and with a run-up in excess of 400 m and at least 6 km inland (McMurtry et al., 2004).

FOSSILS PRESERVED IN THE TSUNAMI DEPOSIT

Fossils of the tsunami, and adjacent deposits, have been collected at 14 sites (material is deposited in the collections of the British Geological Survey, registered as MPA51883–51891, 51893–51897). Further information is available in BGS archives (see report IR/02/197R, available through the BGS library at: http://www.bgs.ac.uk and http://geolib.bgs.ac.uk).

The tsunami deposit on the flank of Kohala volcano contains a range of macrofossil debris and prolific microfauna (Fig. 3). The . . .