Leg2A - 1(Hilina slump)

K207: Western part of Hilina slump (T. Sisson)
Sept. 3, 2001

Breccia consitutes nearly the only rock type exposed in the basins. Most exposed breccias are clast supported with highly angular blocks and little matrix. Rare, sub-horizontal sandy beds in the breccias are at a high angle to the slope and show that the breccia exposures are not slope mantle materials. Talus slopes range from 60% sediment covered to nearly sediment free (common). The sparse sediment cover on many talus slopes indicates recent tectonism in the area. Potentially primary volcanic deposits were restricted to a single exposure of poor pillow lava at -2657m near the top of the lower basin (samples R16, 17). This may have been pillow breccia.

Kaiko collected clasts from breccias throughout the dive, commonly with multiple samples from single exposures to check for homogeneous or heterogeneous sediment sources. Shipboard examinination shows that clasts from the lower basin are dominantly poorly-sorted, well-indurated glassy volcaniclastic rocks (11 volcaniclastic vs. 4 basalt samples). Clasts from the upper basin were judged during the dive to be mostly pillow lava fragments, and shipboard examination confirms this (2 volcaniclastic rocks vs. 8 basalts). Breccias in the upper basin are also coarser than those in the lower basin. Sampling of weakly indurated breccia matrix was attempted at 2 sites by push corer, but only one recovered material (green corer PC-3, -2808m). One sample was collected by push corer of a sand layer interbedded with breccia (blue corer PC-4, -2547m).

Surficial unconsolidated sediment overlies breccia unconformably, and local slumps near the top of the dive exposed internal stratification of alternating thin (cm) white (pelagic) and black (basaltic glass) sands sampled by yellow push corer (PC-2, -2062m) at the end of the dive. Kaiko left bottom at -2028m, 19o 2.8f N, 155o 12.5f W having collected 28 rocks, 4 push core samples, plus matrix debris in the sample basket for two sites.

Significance
Dive K207 found weakly indurated breccia almost to the complete exclusion of other rock types. Bedded, well-indurated volcanic glass sandstones, common on the frontal scarp of the Hilina bench, are completely lacking. Indurated volcaniclastic sandstone, gritstone, and fine breccia are common as clasts in poorly indurated breccias in the lower basin, and these are probably fragments of Hilina bench-type bedded deposits, resedimented during younger landsliding or slumping. The volcaniclastic clasts generally haved coarser consitituent grains than do frontal scarp sandstones, but are otherwise similar. These volcaniclastic rocks may have been more proximal equivalents to the frontal scarp sandstones, prior to being disrupted and resedimented. The upper basin breeccias are coarse, matrix-poor, and have clasts almost exclusively of pillow basalts. These breccias could be proximal deposits from landslides off of ancestral Kilauea, or they could be younger and derived either from the transitional basalt pillow lavas that overlie the Hilina bench sedimentary section, or from shoreline crossing subaerial Kilauea tholeiites (less likely). Chemical analyses of pillow glasses and whole rocks will help to answer these questions. The abundant almost completely uncemented breccias, as well as the young talus fields encountered during the dive show that the area southeast of Papafu Seamount is undergoing frequent tectonism. This may be due to uplift of the Seamount, faulting along the southwest margin of the Seamount, or internal deformation in the Hilina Bench.

Sample log

Dive log

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