Leg1B (West to southwest Oahu)

K205: Waianae landslide (M. Coombs)
Aug. 29, 2001

Results of Dive K205

Date: August 29, 2001
Place: Farthest block of Waianae landslide, 54 km SW of Oahu
Observer: Michelle Coombs
Co-observers: Jim Moore, Dave Clague
Advisors: Eiichi Takahashi, Jiro Naka

Purpose of Dive K205
Waianae landslide is a slump-type landslide, consisting of several coherent blocks spaced 5-20 km apart, separated by transverse scarps (Moore et al. 1989). Only a small tongue of exposed hummocky terrain projects SW from the furthest slump block. The bulk of the slump is believed to consist of material from Waianae volcanoe, although the 130-km-wide slump may comprise material from Kaena ridge to the west and Penguin bank to the east (Moore et al., 1989). We proposed to dive on central portion of the steep toe of the slump (the outer face of the outermost block), which meets the seafloor abruptly at a depth of ~4700m.

The purposes of this dive were twofold. The first is to combine geologic observations made during the dive with single channel seismic data from Leg 1 to better understand the structure and emplacement of the Waianae landslide blocks. The second purpose was to collect samples from the block, in order to determine their lithology and composition, and possibly age. This deep site potentially provides access to very early Waianae volcano, and thus may provide information regarding the geochemical and geologic evolution of Waianae, in much the same way that volcaniclastic rocks from the Hilina slump are derived from early Kilauea (e.g., Lipman et al. 2001). If the parallel to Hilina/Kilauea holds true, the rocks collected during this dive may be from the alkalic stage of the volcano's history, and thus dateable. The results of this dive may be used in conjunction with a dive on an upper slump block (158?28'10" W, 21?23'15", -2560 to -1920 m), performed earlier this summer by Dave Clague at MBARI, to provide a more complete picture of the evolution of Waianae.

Dive Plan
The dive track was planned to ascend the outer scarp face of the outer block from Waianae landslide, 54 km SW of Oahu. This block is BLANK m high at its peak, which descends to a flat bench at approximately 4200 m depth. Our dive ascended from the base of the block and nearly reached the basin. Dive site location was severely limited due to the presence of numerous telecomm cables in the area. At least 18 cables run westward from Oahu in this region, many of which lie over steep parts of the outward block. Within the small acceptable area available to us, we selected a portion of block's outer scarp with ~400 m vertical rise and ~21? slope. JAMSTEC regulations for the use of the ROV require that the dive site be the distance of one water depth from cables. The base of the block of interest is ~4700m. Therefore, the dive track was kept 4.7 to 4.2 km away from cables.

Dive Summary
We encountered only volcaniclastic rock outcrops during the dive, with the exception of one possible pillow lava outcrop near the top of the ridge (see below). Kaiko landed on a muddy slope littered with small loose rock chips; a push core was taken in this sediment. The first 80 m of upslope travel encountered mud and sand covered slopes with increasing amounts of blocks, some grouped to form vague outcrop-like areas. Eight samples at four locations were sampled during this portion of the dive. They range from volcanic breccias to mudstone.
At a depth of 4557 m, we encountered the first probable outcrop. At 4543, coherent outcrop was seen and sampled (sample K205-06). From this depth to about 4300m, jointed and fractured outcrop alternated with talus.
Above this depth, slope-mantling sediment predominated. In addition, rocks were severely cemented with MnO coating. In places, the slope mantle itself was indurated, and coated with ~2mm of MnO. Along this upper section, one significant outcrop was encountered, at 4291m. This was a ~30 m long band of pillow lava. The outcrop differed significantly from those further down: its appearance was lobate, with several apparent squeeze-out buds at the ends of larger pillows. Unfortunately, we were not able to sample at this outcrop due to exteme Mn-cementing. This was the only possible lava outcrop encountered during the dive.
Soon after, at 4237 m, the terrain flattened as we reached the top of the ridge. Because we had reached this position earlier than expected (12:39), and the remainder of the planned course covered flat ground, we changed course to the SE, in order to make another SE ascent approximately 500 SE. At the end of this second ascent, two high-reflectivity ridges appeared on the sonar. Both were outcrops of mudstone containing clasts of lithified volcanic breccia (sample sites 10 and 11). It was difficult to tell if these two outcrops contained "primary" outcrop or indurated slope mantle. Finally, a push core was taken in muddy sediment, and then the dive ended at 14:39 at a depth of 4316m.

Geologic observations
1) As mentioned above, all samples are volcaniclastic in origin. The majority are well-lithified volcanic breccias. These breccias contain clasts of basalt as well as clasts of older breccia and sandstone.
At least some of the collected samples appear to be clasts in younger, unlithified breccias. It is difficult to determine whether this is the case, or whether the samples were taken from fractured and jointed rock. If they were unconsolidated breccias, the amount of matrix was very low. The emplacement history of these samples is obvious complex, recording as many as three generation of sedimentation and erosion.

2) The rock outcrop on the main portion of the scarp face is highly fractured and undergoing continuous sloughing, as evidenced by relatively sediment-free talus at the base of the steeper outcrops. Whether this is simply a function of slope steepness, or whether it indicates active deformation, is unclear. As the slope decreased towards the top of the dive, MnO coating apparently increased in thickness, as rocks became extremely cemented. Slope mantling material also bore MnO coating, indicating that very little erosion (or sedimentation) was occurring there.

3) The apparent presence of pillow basalt near the top of the scarp is somewhat puzzling, as volcaniclastic rocks are in place above and below the outcrop. It is unfortunate that we wre not able to sample the pillows.

Video Highlights

Reference
Lipman, PW, Sisson TW, Ui T, Naka J, Smith JR, 2001. Ancestral submarine growth of Kilauea volcano and instability of its south flank. In AGU Monograph, in press.
Moore JG, Clague DA, Holcomb RT, Lipman PW, Normark WR, Torresan ME, 1989. Prodigious submarine landslides on the Hawaiian ridge. Jour Geophys Res 94, 17465-17484.

Dive log

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