Megatsunamis from Flank Failures in Hawaii: Determining Recurrence and Mapping Paleorunup from Tsunami Deposits
Abstract
McMurtry et al. (2004a) proposed that calcareous conglomerate deposits on Kohala volcano on Hawaii were tsunami deposits. Since Hawaii is known to be subsiding, other mechanisms for emplacement of these deposits are unlikely. Similar deposits on Lanai, Maui and Molokai have also been postulated to be tsunami deposits, but this idea is not totally accepted. If, as we have already demonstrated for Kohala, these deposits are demonstrated to be from tsunamis, we shall then have the information necessary to hindcast the mechanics of specific megatsunamis that have impacted the southeastern Hawaiian islands. Since our initial work we have collected more samples of in situ coral clasts from Kohala as well as from southern Lanai, Molokai, and West Maui. Our additional Kohala samples display two age extremes. One least-altered sample, with an apparent age of 112±2 ka, extends our initial linear trend of calcite abundance versus apparent age for Kohala. The other sample, with an apparent age of 170±3 ka, appears to date a much older reef that was brought up to the Kohala deposit site. We are convinced that southern Lanai experienced megatsunamis at ~120 ka and ~240 ka, thus reconciling Moore and Moore (1984) with Rubin et al. (2000). These tsunamis entrained calcareous material from the respective stage 6 and 8 apex aged reefs and left evidence for multiple waves; two for the 120 ka event and three for the 240 ka event. Runups on southern Lanai were at least to 180 m; using the 1.9 mm/yr subsidence rate of Campbell (1986) suggests a minimum runup to 408 m, or to 554 m using the 326-m site of Stearns (1978). These runups exceed even our largest simulation results for Lanai (McMurtry et al., 2004b). Another unexpected result of the new U-series dating is apparent ages of 111±1.2 ka and 107±0.9 ka for two generations of coral from a composite in situ clast on West Maui that are stratigraphically reversed. This result is not inconsistent with the relationship of decreasing apparent age with calcite abundance found for Kohala, however. One sample from Lanai, with an apparent age of only 100±2 ka and no detectible calcite, is harder to explain, although this sample had a distinct reddish color from fine included iron oxides that may have affected the apparent age. Strontium isotope analyses were performed on the Kohala samples to test if this method could delineate the origin of the carbonates in Units 2 and 3. The samples were subjected to a series of mild acetic acid leaches. We also included a sample of obvious caliche from a vein in a nearby Hawi andesite flow. The results show the relative seawater origin for the marine fossiliferous Unit 2, the rock origin for the caliche, and a mixture of seawater and terrestrial origins for the Sr in the carbonate matrix of afossiliferous Unit 3. Unit 3 is clearly a stratigraphically-older, carbonate- cemented basaltic conglomerate with palesol features in many places. The apparent complete lack of marine fossils suggests alteration, consistent with its greater age and with the interpretation of this deposit as a mixed seawater-terrestrial origin, or, more likely, an original seawater origin with subsequent terrestrial overprinting (soil formation). A working hypothesis is this massive, enigmatic deposit on Kohala represents the 240 ka megatsunami event found on Lanai and apparently exclusively on Molokai.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2007
- Bibcode:
- 2007AGUSM.V31A..05M
- Keywords:
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- 1040 Radiogenic isotope geochemistry;
- 1051 Sedimentary geochemistry;
- 3022 Marine sediments: processes and transport;
- 4564 Tsunamis and storm surges;
- 8488 Volcanic hazards and risks