Paper No. 57-7
Presentation Time: 3:35 PM
ELUCIDATING TSUNAMI EVENTS PRESERVED IN THE ROCK AND DEPOSIT RECORD AT KA LAE (SOUTH POINT) AND OTHER SITES IN THE HAWAIIAN ISLANDS: PALEOMAGNETIC, ROCK MAGNETIC AND RADIOMETRIC CONSTRAINTS
Deposits in the Hawaiian Islands preserve the record of past tsunami events. These range in size from <10 m runup to megatsunami class (>100 m runup) depending upon deposit location and size of ocean displacement. We propose to place better constraints upon the origin and timing of the events that are recorded in the rock and sedimentary deposit record in Hawaii, mainly using collected samples from sites identified in previous work. Presently lacking is further analytical work that would provide an improved interpretation of these events based upon more complete dating and deposit sedimentological analysis. At Ka Lae (South Point) on the Island of Hawaii, we discovered very late Pleistocene megatsunami deposits that included large, few-m diameter boulders resting upon 10-m cliffs that range in age from >93 ka (U-series age on abutting palesol) to the 120 ka age of the underlying Kahuku Basalt (Jicha et al., 2012; McMurtry et al., 2012). We studied the Natural Remanent Magnetization of in-situ lavas and these boulders. Whilst in-situ lavas were characterized by values agreeable with the Geocentric Axial Dipole formula, loose lava blocks showed a total disagreement with the expected inclination, which indicates that they deposited by an event such as a megatsunami. Near present sea level, we discovered >2 m thick layers of reworked Pahala Ash containing basalt and calcareous sand and shell, basalt cobbles and boulders to >1 m in diameter, all with inland extent >100 m from the present coast. Ages of included corals range from 246 to 978 years BP (n=10). These deposits could be further evidence for the 1425- 1665 AD tsunami event found in sinkhole deposits on the Island of Kauai (Butler et al., 2014) or are older corals reworked during local seismic tsunami events in either 1868 or 1975. At elevations of up to 27 m ASL, sections within the Pahala Ash also contain similar deposits that presently cannot be constrained in age beyond 12 to 93 ka. These likely record another large tsunami event there, with runup to >30 m.