ADDENDUM: NUUANU LANDSLIDE
Drilling at the Nuuanu Landslide Site NU-1A was added to the ODP Operations Schedule in September 2001, after the Leg 200 Prospectus had been completed. This addendum explains the science planning, background, objectives, and drilling plan for coring that may occur at Site NU-1A.The ODP Science and Operations Committees indicated that Site NU-1A should only be cored during Leg 200 if the JOIDES Resolution is ready to leave port at least one day early. Currently, Leg 200 is scheduled for a five day port call in Honolulu, with port call activities associated with Leg 200 beginning at about 8:00 AM on 17 December 2001. Thus, if the ship is underway prior to roughly 8:00 AM on 21 December, Leg 200 will core at Site NU-1A. Otherwise the ship will cruise directly to Site H2O.
Background
The rapid growth and enormous size of Hawaiian volcanoes cause them to be gravitationally unstable and collapse. The collapse of Hawaiian volcanoes has generated some of the largest landslides on Earth and undoubtedly produced large tsunami waves (Moore, 1964; Moore and Moore, 1988; Moore et al., 1989). Deposits from dozens of major landslides, some with lengths of 200 km and volumes greater than a thousand cubic meters, have been recognized along the Hawaiian Ridge (Moore and Normark, 1994). Large landslides have also been recognized on the flanks of other ocean volcanoes such as at Reunion Island (Lenat et al., 1989) and Canaries (Carracedo, 1990).
The Nuuanu Landslide, which broke away from the northeast flank of Koolau Volcano on the Island of Oahu, is the largest Hawaiian landslide. It is a debris avalanche that contains enormous blocks such as the Tuscaloosa Seamount, which is ~30 km long, 17 km wide, and at least 2 km tall. The landslide is spread over a 23000-km2 area (Normark et al., 1993; Naka et al., 2000), with distal portions extending up the Hawaiian Arch. To reach the upper portion of the arch (the target site for drilling), the landslide would have had to traverse the deep moat on the northeast side of Oahu and travel over 100 km uphill (Fig. 17).
Reaching the landslide deposit by gravity or piston coring has proven difficult because the deposit is overlain by a carapace of younger debris (turbidites and associated deposits). Thus, the thickness and depositional history of the landslide are poorly known. The distal portion of the landslide deposit is thought to be <10-m thick, but estimates vary from 1 to 100 m (Rees et al., 1993; Naka et al., in press). Similarly the age of the landslide is poorly constrained, though it apparently occurred near the end or after the formation of the Koolau Volcano, which has surface flows that are 1.8-2.6 Ma based on K-Ar dating by Doell and Dalrymple (1973).
Scientific Objectives
If Nuuanu Landslide Site NU-1A is cored, the core material will be used to:
Drilling Strategy
Site NU-1A is located along the transit route out of Honolulu to the H2O site, so no additional transit time is incurred by operations at this site. Coring operations are limited to 1.8 days with no logging. The plan is to double APC/XCB core at Site NU-1A, with the coring to ~100 mbsf in the first hole. Assuming the base of the landslide deposit has been recovered by 100 mbsf, we will core a second hole to 50 mbsf to ensure complete recovery of the sedimentary section roughly down to the top of the landslide deposit. Complete recovery of the section overlying the landslide deposit is important as it is from magnetostratigraphic and biostratigraphic studies of these sediments that a minimum age for the landslide will be established, with the minimum age likely being very close to the true age. Coring gaps are common in single cored sites and could result in large uncertainties in dating the deposit. If we have not recovered the base of the landslide deposit by 100 mbsf in the first hole, we plan to continue coring deeper until the base is recovered or the allotted time for coring at Site NU-1A has been expended. This could mean that no time will be available for a second hole. A sedimentologist will be on the catwalk monitoring cores as they are recovered to establish whether there is a need for penetration beyond 100 m.