March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
April 30, 2001: Leg 3; Day 10
Gary Greene writes: Dives 313 and 314 were located on two sloping terrace surfaces located offshore of southern Oahu Island, southwest of Pearl Harbor. The objectives of these dives were to examine and sample the surfaces and sides of the terraces to determine their composition and origin. These terraces appear in the bathymetry to have been one terrace that has broken apart and slid down slope. Our intent was to determine if these features are in place or have been displaced. Of particular interest is in the possibility that these terraces could have caused a tsunami. If indeed they broke apart and instantaneously slid down the slope as block glides, they could have generated at least a small tsunami. This set of dives were weather alternates and were undertaken because weather conditions on the north side of Oahu were not favorable for diving on first priority sites there.
Twenty-nine rock samples and 3 scoop bag samples were collected on Dive 313 and 312 were obtained on Dive 314. Push cores were not tried, as no thick sediments exist that could be penetrated with the corers. Continuous water samples were collected on both dives for radium analyses.
Dive 313 started at the top of the lower terrace at a depth of 458 m. Here the sea floor is very hard and slabby with no sediment cover and strongly swept by currents. Quite a few live gorgonian and dead finger corals were observed attached to high mounds that projected above the flat, smooth bedrock surface. Although no old coral-like structures were observed, many corals were collected from the top of the terrace surface.
The transect down the western face of the lower terrace revealed a hard smooth sculptured and fluted bedrock surface that alternated with a pitted, grooved and rubble-like surface. Black coated rock was locally present and in many places appeared like the material was under the remaining black surface had been removed elsewhere.
At 520 m the sea floor character once again changed to the flat surface of the channel floor that separates the terraces. This surface experiences strong currents as it is armored with a non-cemented cobble-pebble pavement. Hard smooth, grooved and jointed (oriented NW-SE) bedrock is exposed locally within the channel and in some localities thin (~10 cm amplitude) long period gravel ripples occur, often with exposed bedrock in their troughs. In the immediate center of the channel, at a depth of 529 m, a large conical bedrock hill is present. The base of this hill is of gently sloping rubbly-looking bedrock. Locally the bedrock surface is pitted and fluted and boulders are scattered over this surface. In one locality (at 519 m) a scree pile of broken finger corals were found and sampled. The sides of the hill appear to be composed of the same type of material that forms the lower terrace walls and were near some poorly developed, but nearly slope parallel bedding. The top of the hill was found at a depth of 499 m and contained ancient corals. Here the surface looked like it was frosted with a cemented cap. The texture appeared almost as if a congealing liquid cover flowed of the surface and dripped down the sides, not unlike what you would find when a cake is frosted with a warm liquid substance.
The transect up the eastern wall of the lower terrace, starting at about 529 m also appeared to represent similar type of structure as the bedrock found at the lower terrace. The bedrock appeared as a smooth surfaced grainstone or sandstone, well cemented and heavily grooved, fluted, pitted and with occasional N-S oriented joints. Potholes, similar to those found in bedrock of active streams on land, were observed and contained gravel, pebbles and cobbles. Isolated patches of black-coated bedrock were also found on this wall. Many overhangs occur which offer refugia to some fish. In many places a rugged, rubbly look is exhibited in the bedrock. Samples were hard to come by, but some were broken from the outcrop.
The top of the upper terrace was reached at 437 m where a smooth and highly fractured bedrock surface exists. Fractures crisscross the surface and many pits have formed along the fractures.
Dive 314 started at a depth of 570 m where smooth but fractured hard grainstone or sandstone outcrop was found. Many small erosional pinnacles were rising above an otherwise smooth bedrock surface. In many place black fracture fillings and bedding were observed and sampled. Most of the wall of this terrace was smooth and somewhat fluted, but did not resemble all of the features observed in the lower terrace wall. At the top of the upper terrace, at 461 m water depth, the bedrock was very smooth with little overhangs, slabs or other surfaces occur. No sediment cover existed and few rock samples were collected. However, several ancient coral samples were collected along the wall.
The tentative conclusion made from observations and sampling of the two terraces offshore of southern Oahu is that the terraces are part of an ancient carbonate platform complex. However, the two different terraces lie at separate elevations. The geometry of the two terraces suggests that they were once together and since spread apart. Since there is an apparent similarity between the southern wall of the upper terrace and the northern wall of the lower terrace, and an apparent difference in the leading edge or lower wall of the lower terrace, we suspect that these features are block glides that slid down hill producing the channel that separates them.