HR: 09:15h
AN: OS21E-06    [Abstracts]
TI: Reinterpretation of a Stage-11 21-m Sea Level High Stand Deposit on Bermuda: Megatsunami from a Giant Submarine Landslide?
AU: * McMurtry, G M
EM: garym@soest.hawaii.edu
AF: SOEST, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822 United States
AU: Tappin, D R
EM: drta@bgs.ac.uk
AF: British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG United Kingdom
AU: Sedwick, P N
EM: psedwick@bbsr.edu
AF: Bermuda Biological Station for Research, Biological Station, St George's, GE01 Bermuda
AB: A deposit of coral-bearing, marine shell conglomerate found at an elevation 21 m above sea level (asl) in a Bermuda cave has previously been interpreted as evidence of a towering sea level high stand during marine isotope stage 11. Hearty et al. (1999) U-series dated this deposit at 420 +/- 30 ka and correlated it with carbonate sand deposits at up to 21 m asl in the Bahamas; they attributed this putative sea level high stand to a 20% decrease in Antarctic ice volume and suggested direct geologic evidence for ice sheet instability. Although stage 11 was an extended interglacial period similar to the Holocene, few marine isotope sea level curves support a high stand greater than that of stage 5e over the past million years, and those that do suggest a maximum height of no more than about 9 m asl. The sea level evidence is primarily from Bermuda, where no subsidence has been identified, and Oahu, Hawaii, where the rate of uplift is generally known and the Kaena high stand, containing corals U-series dated at 400 to 550 ka , is exposed at up to 30 m asl. On Oahu, the uplift for the island is recorded by a nearly linear (r = 0.999; n = 7) emergence rate of 0.069 +/- 0.003 mm/kyr, using isotopically-dated, uplifted marine terraces back to nearly 500 ka BP. This uplift rate is supported by identical stage 7 comparisons with Bermuda, Papua New Guinea and Barbados (Jones, 1993). When this rate is applied to the "best" U-series age for stage 11 (430 ka) at the maximum elevation of 30 m asl, we arrive at a maximum sea level height for stage 11 that is no greater than the present sea level. This evidence, together with: (1) a lack of layering in archived samples of the 21-m Bermuda conglomerate; (2) its reported field relations; and (3) the occurrence of stage 11 deposits at only 5 +/- 3 m asl elsewhere on Bermuda (Hearty et al., 1992), suggest an alternative deposition mechanism for this deposit. We hypothesize that this conglomerate was rapidly deposited by a "megatsunami" during stage 11, an interpretation that is consistent with the climate correlation of McMurtry et al. (2004). Likely causes for such a megatsunami are a flank collapse of an Atlantic oceanic island volcano, such as the Canary or Azores Islands, or a giant submarine landslide on the Atlantic continental margin.
DE: 9325 Atlantic Ocean
DE: 4267 Paleoceanography
DE: 4546 Nearshore processes
DE: 4556 Sea level variations
DE: 4564 Tsunamis and storm surges
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting