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.