March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
April 23, 2001: Leg 3; Day 3
Tiburon Dive 301 - Plunge Pool, Kohala
Gary Greene writes: Dive 301 was located in a canyon and plunge pool that lies offshore of Kohala on the big island of Hawaii. This is the most northern of the many canyons that notch a flat sea floor terrace at the 1000 meter isobath. Here the canyon deeply cuts back into the terrace and appears to receive sediment from a subtle flat-floored channel that is associated with a series of small canyon heads near the shore of the island. The cuspate-like morphology of the canyon head and the well-defined plunge pool are unusual and suggest an erosion process initiated lower down on the slope.
The objective of this dive was to study the origin and active processes of canyon and plunge pool development and to investigate the island building stratigraphy. Forty-eight rock samples, 4 push cores, 1 scoop bag and 2 Goflow bottle water samples were collected. In addition, continuous collection and filtering of the benthic boundary layer waters to strip the radium was conducted.
This was a very interesting dive in that it appears that the canyon and plunge pool formed in the marine environment and that canyon sediment transport processes are periodic but still active. We started the dive at a depth of 2355 m, on the backside or down-canyon slope of the plunge pool dam. The floor is composed of rippled dark sands with black sand forming the crest of the ripples. Scours around boulders and gravel stringers indicate strong bottom currents flow from south to north, down canyon. Further up on the back slope of the plunge pool dam and sill we encountered considerable rock debris and rubble comprised of angular to sub-rounded basalt and coral fragments.
The crest of the dam, or down canyon sill to the plunge pool, was located at a depth of 1128 m and was sand covered. In contrast, the dam flank on the plunge poolside consists of extensive amounts of very angular boulders precariously perched on a steep (~45°) slope at a depth of 1189 m. The descent into the plunge pool revealed a flat rippled sand-covered floor with small rubble of angular pebbles and boulders scattered about.
At about 1197 m we encountered a rubble apron of angular basalt and carbonate boulders interspersed with rippled sands at the base of the landward wall of the plunge pool. A very steep rock outcrop wall, undercut at its base, was found at a depth of 1170 m. Above the rubble apron a steep (generally 70° to 90°) differentially eroded bedrock cliff rises above the plunge pool. A beautiful volcanic, sedimentary and carbonate sequence exposed along this wall occurs between 1146 m and 997 m. This sequence consists of volcano-clastic sediments containing rounded cobbles and pebbles, carbonates horizons containing coral fragments and marine laid sandstones, lava flows and a massive 40 m thick coral bearing reef. The beds of volcano-clastics sediments are more easily eroded and facilitate the formation of caves, overhangs and ledges with the adjacent more competent layers.
Sculptured and fluted volcano-clastic rocks were often encountered during the ascent up the landward wall of the plunge pool. These features may have been formed from sediment transport events that plunged over the wall to the depression below. Only an occasional sessile organism was found indicating that energetic sediment transport events still may occasionally occur.
Both submarine and terrestrially deposited lava flows are represented in the stratigraphy examined during this dive. The presence of these deposits along with the shoreline sediment and coral rubble indicate that the area we investigated formed at or above sea level and since subsided. Much of the rubble deposits found in the canyon and associated with the plunge pool appear to represent the rock types found in the head wall of the canyon. We speculate that head-ward or retrogressive slope failure may be a primary process in the formation of this canyon.
Because of forecasts of deteriorating weather conditions, we commenced 24-hour operations. Within 2 hours of recovering the ROV from this dive, we were back in the water. Thus, we spent the entire night in dive operations. This has resulted in a blurry-eyed shipboard party.
Tiburon Dive 302 - Plunge Pool 2, Kohala
Dive 302 was located approximately 2 km south of the previous canyon investigated during Tiburon Dive 301. This canyon deeply notches the extensive 1000 m terrace off the Kohala coastline of the big island of Hawaii. Three depressions or plunge pools, here referred to as lower, middle and upper plunge pools, are located along the canyon axis within the 2 km long prominent canyon. Investigation of this canyon was particularly important because of the number of plunge pools it contains and their possible relationship to structure and lithology. The purpose of this dive was to further investigate canyon and plunge pool development.
Dive 302 was made along a ~2 km long transect. In a similar manner to the previous plunge pool dive investigations, Dive 302 started on the backside or down canyon flank of a plunge pool dam in 1348 m of water. Here, as well as on the dam of the middle plunge pool, angular to sub-rounded boulder rubble was located on both the inner and outer flanks with sand predominating on the crests. Both the lower and middle plunge pools were ~10 m deep with the floors of the depressions composed of cross-rippled sands. Push cores taken in the center of the depressions could not penetrate deeper than 10 cm into the sediment.
The upper plunge pool is deeper (~36 m deep) than the other plunge pools in this canyon and its dam is composed almost exclusively of boulder rubble and does not have sand covering its crest. The presence of fresh rubble suggests that the plunge pool is still active.
The channel flats between the plunge pools exhibit bedforms of cuspate and longitudinal ripples, scours around boulders, and gravel trails that indicate strong down canyon currents. At a depth of 1238 m the ripple depressions contained concentrations of small marble-size organic balls that may be comprised of diatoms, similar to the "benthic tumbleweeds" found in depressions along the central California continental slope. Basaltic and carbonate rubble is randomly scattered along the flat rippled channel floor and in some localities, near the walls of the canyon, nicely sculptured and fluted volcano-clastic rocks are exposed. In one location where the thalweg narrows, in 1194 m of water, soft volcano-clastic rocks are fluted and produce a gentle chute through which sediment travels.
A very interesting discovery made during this dive is that the headwalls of all three of the plunge pools exhibit similar stratigraphy. The headwall of the lower plunge pool is only ~20 m high and heavily fluted and appears to be entirely formed of volcano-clastic sediment. The headwalls of the 50 m high middle plunge pool and the 150 m high upper plunge pool are primarily composed of a nearly vertical outcrops of alternating volcano-clastic and volcanic flow layers with occasional local undercuts in the volcanoclastic units.
The top unit exposed in the headwall of the upper plunge pool is a 30 m thick carbonate reefal complex. The upper surface of this unit appears to be the floor of the extensive 1000 m deep platform that is cut by the canyons. Many prime specimens of corals were collected from the coralline rubble pile located at the base of the carbonate bed. This platform is locally capped with coarse-grained black sand cemented to the carbonates. These samples will be very useful in dating the reef and in determining the environmental conditions that existed during the time the reef was growing.
The presence of a thick reefal sequence at 1003 m unequivocally indicates that the islands flanks have subsided ~1000 m. The presence of coarse rounded boulders, thin carbonate horizons and vesicular basalts suggest that the underlying material were either near shore or subaerial.
Our initial interpretation of the data collected during this dive is that the canyon is being formed by retrogressive erosion or headward advancement. The canyon is also being deepened by the differential erosion of the volcanic rocks under the headwalls of the plunge pools. Undercutting of the headwalls also produce periodic rockfalls and debris flows that scoured the pools and built the pool dams. Progressive advancement of the erosion along the weaker volcano-clastic beds has produced a stair-step configuration to the canyon. This erosional process is both lengthening and deepening the canyon.