HR: 1330h
AN: T12B-0454    [PDF]
TI: The Age of Large Landslides From the Koolau Volcano, Oahu, From Coring on the Northern Hawaiian Arch
AU: * Acton, G
EM: acton@geology.ucdavis.edu
AF: University of California, Davis, Department of Geology, Davis, CA 95616 United States
AU: Duncan, R
EM: rduncan@coas.oregonstate.edu
AF: Oregon State University, Oceanic and Atmospheric Sciences, Corvallis, OR 97331 United States
AU: Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: University of Hawaii, Hawaii Institute of Geophysics and Planetology, Honolulu, HI 96882 United States
AU: Sherman, S B
EM: bean@higp.hawaii.edu
AF: University of Hawaii, Department of Geology and Geophysics, Honolulu, HI 96882 United States
AU: Garcia, M
EM: garcia@soest.hawaii.edu
AF: University of Hawaii, Department of Geology and Geophysics, Honolulu, HI 96882 United States
AB: Large landslides, although infrequent on human time scales, occur off the flanks of most oceanic islands and have the potential to cause considerable devastation. To understand future hazards related to large landslides, we need to know the number, age, and size of past landslide events as well as their geologic setting. With this in mind, Ocean Drilling Program (ODP) Leg 200 cored Site 1223 on the northern Hawaiian Arch with the goal of recovering a record of the distal portion of the giant Nuuanu Landslide. This catastrophic event or series of events removed $\sim$40% (3000 to 4000 km$^{3}$) of the Koolau Volcano on the Island of Oahu. The distal landslide deposits cored at Site 1223 (4235 m water depth) must have traveled over 100 km, traversing the Hawaiian Moat (4800-5500 m water depth) and then over 0.5 km uphill before reaching the arch. Leg 200 results showed that multiple landslide events have occurred. The landslide-related deposits recovered include at least 8 turbidites $>$10-cm thick in the upper 7 m of sediments, an unconsolidated black sand unit $>$2-m thick at 7.9-10.8 meters below seafloor (mbsf), and two lithified vitric tuff units at 12.7-15.1 and 32.0-37.0 mbsf. The timing of these events, particularly those in the upper 7 m, can be constrained well from a detailed magnetostratigraphy, with the aid of independent age constraints. In particular, 40Ar-39Ar dates indicate that both the unconsolidated black sand unit and the upper vitric tuff have estimated ages of $\sim$3 Ma or $<$4 Ma including 95% uncertainties, which is consistent with these units being derived from the Koolau Volcanic series. Furthermore, this agrees with petrological and geochemical observations, which indicate that the source was subaerial and has Koolau affinities. The biostratigraphy analyses indicated that reworked Eocene radiolaria were most abundant, although some Neogene species have been identified. Given these constraints, the sequence of normal and reversed polarity intervals from the upper 7.2 m of core most likely span the middle to upper portion of the Brunhes Chron (0-0.78 Ma) to the old end of the Olduvai Chron (Chron C2n; 1.77-1.95 Ma). The units below 7.2 mbsf are interpreted to have reversed polarity and to have been deposited during Chron 2r (1.95-2.581 Ma).
DE: 1035 Geochronology
DE: 1520 Magnetostratigraphy
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3022 Marine sediments--processes and transport
DE: 9355 Pacific Ocean
SC: Tectonophysics [T]
MN: 2003 Fall Meeting