Support for the Giant Wave (Mega-Tsunami) Hypothesis: Evidence From Submerged Terraces off Lanai, Hawaii.

The origin of subaerial coral conglomerate deposits on the Hawaiian islands of Lanai and Molokai continues to be controversial, primarily because these deposits are difficult to interpret and the tectonic history of these islands is poorly understood. These deposits were first interpreted as highstand deposits, then mega-tsunami deposits, then re-interpreted as highstand shoreline features deposited during the last two interglacials. We present a different approach to this problem that focuses on an analysis of new bathymetry, remotely operated vehicle (ROV) and submersible observations, and sedimentary and radiocarbon age data from carbonate deposits recovered from two submerged terraces at -150 m (T1) and -230 m (T2) off Lanai, Hawaii [Webster, et al., 2005]. The tops of the terraces are veneered by relatively thin (<5 m) in situ accumulations of coralline algal nodule, coralgal nodule, and Halimeda facies deposited in intermediate (30-60 m) to deep fore-reef slope settings (60-120 m). The data are used to develop a sedimentary facies model that is consistent with eustatic sea-level variations over the last 30 ka. Both nodule facies on T1 and T2 initiated growth 30-29 ka following a fall in sea level of ~50 m and increase in bottom currents during the transition from Marine Isotope Stage 3 to 2. The nodules accreted slowly throughout the Last Glacial Maximum when sea-level was relatively stable. Drowning occurred during the early deglaciation (17-16 ka) and was marked by the complete drowning of coralline algal nodules facies on T2 and incipient drowning of coralgal facies on T1. Abrupt sea-level rise during the middle deglaciation, perhaps associated with global meltwater pulse 1A (14-15 ka), finally drowned the coralgal facies on T1, which in turn was overlain by a deep-water Halimeda facies. Our data indicates that Lanai has experienced relatively little vertical tectonic movement over the last 30 ka. Using internally consistent age vs. depth relationships, reconstructed paleowater depths and published sea-level data, we estimate that Lanai has experienced maximum rates of uplift of 0.1 m/kyr or subsidence of 0.4 m/kyr over this period, consistent with published geophysical, numerical modeling and recent tide data. Taken together these data support the interpretation that coral conglomerates at elevations higher than +35 m on Lanai are tsunami deposits representing a minimum wave run up of 170 m, rather than typical shoreline deposits formed during the last two interglacials, then uplifted to their present elevations. Webster, J. M., et al. (2005), Drowned coralline algal dominated deposits off Lanai, Hawaii; carbonate accretion and vertical tectonics over the last 30 ka, Marine Geology (in press).