March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
April 22, 2001: Leg 3; Day 2
Judith Connor writes: Today's dive was on the windward side of Molokai exploring a plunge pool in one of the submarine canyons. The dive started around 6:30 a.m. and lasted into the evening so I'm getting lots of time in annotating video and enjoying all the geological discussions around me!
Gary Greene writes: Dive 300 was located in a geomorphically well-defined plunge pool in a submarine canyon off of the northern coast of Molokai Island. The plunge pool is a nearly circular 90 m deep depression that is ~750 m in diameter within the axis of a submarine canyon. The plunge pool is at the base of a very steep (>45°) 350-m-high semi-circular headwall. The objectives of this dive were to determine the history of the flanks of Molokai, and more specifically the origin of the canyon, the developmental history of the plunge pool, and the present-day activity of this submarine canyon system. Thirty-five rock samples, 9 push cores, 4 scoop-bag samples, and 1 Ecuadorian King Cola can were collected. In addition, continuous sampling of the benthic boundary waters for radium isotopes was conducted.
This was a spectacular dive and an extensive data set was obtained. The dive transect started at 1562 m water depth on the backside, or down canyon flank of the plunge pool. Here a rubble field of angular to sub-rounded boulders of mainly basaltic rocks was encountered. Observations and cores taken along the transect up and over the sill (1540 m) of the plunge pool showed that the sill is draped with a silt and mud cover through which occasional boulders crop out, indicating that no recent deposition of talus has occurred. Boulders were again encountered at a depth of 1567 m and existed along the base of the sill to nearly the center of the depression at a depth of 1638 m. The presence of the boulders suggested that the sill of the depression is a constructional deposit formed by episodic landslides whereby materials fall from the steep landward scarp into the depression and piling up at the down canyon.
Observations and rock samples collected during the ascent of the headwall cliff revealed a remarkable island construction history. Vesicular basalts collected at the base of the cliff (1622 m) indicate subaerial deposition of lava. Some of the basalt layers appear to be interbedded with volcanoclastic layers that dip toward the east. A distinct angular unconformity occurs above this unit with a thick (~3 m) boulder conglomerate layer overlying the volcanic unit. The conglomerate contains well-rounded boulders and cobbles like those found in river deposits. Above the conglomerate unit lies a volcanic sandstone unit deposited in the marine environment. We speculate that these rocks were deposited close to a paleo-shoreline. However, most of the rocks on the canyons headwall scarp appear to have been deposited in a subaerial environment.
At 1496 m a vivid red-orange layer (~1-5 cm thick) was sampled and may represent a paleo-soil horizon. Both áá (1445 m) and pohoehoe (1292 and 1286 m) lava flows of vesicular basalt were observed above the paleo-soil horizon.
At a depth of 1290 m an unconformity was encountered which separated the massive black rocks below from white materials above. Samples of the black rock, which form the steep cliff, are vesicular basalts. Samples of the white rocks within a meter of the contact contained well-preserved corals. This contact was at a distinct change in the slope at the top of the headwall above the plunge pool and marks the final drowning of the islands flanks.
The surface of the canyon floor landward of the contact is very gentle. A transect across the channel that feeds the plunge pool revealed eroded side walls of approximately 30 m high comprised of well-layered marine mudstone. Near the center of this channel at a depth of 1288 m, a rubbly stringer of angular to sub-angular boulders, cobbles, pebbles, gravel and sand occur that probably represents a pulse of sediment transport down the channel. In this area scours, lee-side comet marks and gravel stringers indicate a strong bottom current direction of south to north, down canyon.
The flanks of the channel contained a stair case of exposed beds of sediments. The sediments became grayer as we passed through the first 5-10 m of section above the basal contact. This unit appeared to be a fining upward sequence, which is suggestive of suggestive of deposition in progressively deepening waters. In the deepest sections of the channel the surface of a lava flow deposit is exposed.
The final transect of the dive ascended the eastern wall of the channel, again composed of well-layered marine mudstone, and crossed the flat terrace-like platform to another steep scarp and back to the plunge pool head wall. The terrace surface was sediment covered and heavily bioturbated with the presence of many mounds, depressions and burrows. At the southern end of this terrace a 40 m high vertical cliff was encountered and comprised of a marine mudstone that exhibits differential erosion. The last leg of this transect descended the plunge pool headwall where the contact between the lower basalt and overlying carbonate deposit was confirmed to lie at a depth of 1281 m.
This dive provided insight into how the canyon was formed. There has been considerable shipboard discussion of the possibility that the canyon grew from its bottom upwards into the continental margin as a consequence of the progressive failure of the rock. These discussions involve the relative strength of the lava flows, which overlay the weaker volcanic breccias. Erosion of the volcanic breccia units may destabilize the overlying lava flows. These lava flow rocks periodically fail and travel down slope to further erode the canyon. Significant sediment transport down through the plunge pool does not appear to have occurred recently as the sill is covered with fine-grained sediment.
More to come as shore based analyses are conducted.