Leg2A - 1(Hilina slump)

K209: Papau seamount (M. Coombs ) Sept. 5, 2001

Results of Dive K209

Date: September 5, 2001
Place: Southwest side of the Papa'u Seamount, South of Kilauea
Observer: Michelle Coombs
Co-observers: Pete Lipman, Tom Sisson
Advisors: Eiichi Takahashi, Toshiya Kanamatsu

Purpose of Dive K209
Papa'u seamount is a NE-SW trending elongate ridge centered 15 km off the south coast of Hawaii, due south of Kilauea volcano. It is aligned with and helps define what is known as the transverse boundary, which marks the western edge of the Hilina slump. Several hypotheses exist regarding the origin of the seamount. One is that it is an anticlinal fold, composed of volcaniclastic sediments such as those of the lower bench (J. Morgan, unpublished manuscript). Based on bottom photographs, this rounded submarine topogrpahic high has also been interpreted as a massive sand-rubble landslide deposit derived from an embayment near the Kilauea shoreline (Fornari and Moore, 1979). There have been no dives to date on this enigmatic feature. The SW side of Papa'U is the steepest, providing ~1000 m of relief in <4km. For this reason, it is thought that this route will have the best chance of revealing sediment-free outcrop. We will observe the structure of the seamount, looking in particular for evidence of coherent, folded beds versus debris, as well as obtain samples for petrologic and geochemical study. This dive is being made in conjunction with two other dives on this leg: K-207, on a ridge below and to the south of Papa'u, and K-208, on a ridge ~15 km to the east.

Summary of dive
Kaiko landed on the bottom at 8:58 at a depth of 1679 m. The sea floor was covered in sediments and loose talus. Kaiko then began ascending, moving over mud and sand with varying amounts of talus. No outcrops were seen visually or with the sonar, despite an average slope of ~15?. There was very little microtopography, the slope being remarkably constant. Sampling of talus blocks occurred at four sites during this segment of the dive, at depths of 1715, 1639, 1534, and 1456 m. Of note were large boulders, many ~1m in diameter, that littered the slope amongst much more prevalent smaller debris. These boulders varied in roundness, perhaps becoming more rounded as depth decreased.
We first sighted in-place outcrop at 1404 m, in the form of a small spine of weakly indurated breccia. Two samples were taken (#s 5a and 5b) and a push core (red) of breccia matrix was obtained. The weak induration seen here may explain the absence of microtopography on the slope; the material is too weak to form large ridges or cliffs. The second unequivocal breccia outcrop was spotted at 1230 m, and we obtained two angular clasts from it (#s 8a and 8b).
The slope continued to be predominantly covered with mud and talus. We took two more talus samples at 1329 m (#s 6a and 6b). At 1286 m, a light-colored bedding plane (?) was seen at the top of a leftward-slopping decline. The fine-grained unit was sampled (#7) and placed in the lidded box for safe keeping. Later in the dive, at least two more outcrops of light-colored, fine-grained bands appeared, at 1035 m and 962 m. Clasts from just below the bands were sampled (#10 and #11, respectively). Unconformity, strike and dip.
Two very uniform, sediment-free talus slopes were crossed at 1172 and 1104 m. The upslope direction was right (NE) in both cases. These were almost identical in nature, consisting of angular, weathered blocks 20-40 cm in diameter. We sampled two blocks from the upper of the two slopes (#s 9a and 9b).
The upper 300 m of the dive covered mainly sediment and gravelly talus, with occasional large blocks. Many rounded, reddish (oxidized?) cobbles and boulders were recognized. Zooming in on several of these revealed that they appeared to be vesicular. Reddish loose clasts were collected at 889 m (#s 12a, 12b, and 12c). The topography in this area was less uniform than on the lower slopes. The dive track crossed numerous evenly spaced benches and gentle rises, at about 20 m spacing. These showed up very well on the sonar display. These trend NW-SE, parallel to the overall seamount structure.
The summit of the seamount was reached at 14:43 and 742 m, making this the shallowest dive on record for ROV Kaiko. We collected samples just below the summit (#s 13a, 13b, and 13c) before ascending at 14:57.
Twenty-five rock samples were collected during the course of the dive, as well as four piston cores. All of the rocks are basalt lavas. They vary in vesicularity, crystal content, and glass content. There was no systematic variation in clast type with elevation.

Video Highlights of K209

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