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Western Coastal & Marine Geology

Hampton, Monty A., Torresan, Michael E., and Barber, Jr., John H., 1997, Sea-floor geology of a part of Mamala Bay, Hawaii: Pacific Science, v. 51, n. 1, p. 54-75. Reproduced by permission of the University of Hawaii Press.

  Materials, 1
  Materials, 2
  Structures, 1
  Structures, 2
Discussion, 1
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The areal distribution of insoluble residue indicates sediment transport not only in a westerly direction, but also to the south. Two lines of samples, one extending west and the other extending south from the South Oahu site, both show a generally decreasing amount of insoluble residue away from the site (Figure 19), which can be explained if dredged material (relatively high original insoluble residue content) is transported from its original depositional site and mixed into natural sediment (relatively low original insoluble residue content). In contrast, samples approaching the Honolulu Harbor site from the east consistently have moderately small contents of insoluble residue, probably close to the original amounts. The amount of insoluble residue in samples far from the disposal sites in any direction generally is small, also. The smallest amounts, less than 1%, occur in samples in shallow water at the steep head of the trough. These are the samples that also contain the gray carbonate fragments apparently derived from the nearby reef at the edge of the insular shelf.

A likely explanation of this distribution is that, during lowered sea level, volcanic rock and mineral grains, which constitute most of the insoluble residue, were transported from the island across the emergent reef and mixed in small to moderate amounts with carbonate fragments, to be deposited in the deeper trough as natural sediment. Presently, dredged material supplies an enriched amount of insoluble residue, which is remobilized and mixed with natural sediment by westerly and southerly bottom currents, yielding the observed gradients away from the sites and the lack of a gradient approaching from the east. Sediment in samples from the steep head of the trough recently was transported over the edge of the insular shelf and has only small amounts of insoluble residue because most land-derived sediment is trapped in the estuaries and harbors during high stands of sea level.

Allen and Moberly (1977a) measured currents of up to 50 cm/s velocity and net transport direction to the southwest within the Pearl Harbor disposal site, coincidently taken at the same time of year (May) as our 1994 survey. Shortly after a seven-week period of dredged-material disposal, they determined a southwesterly sediment dispersal pattern on the sea floor. The distribution became significanly more widespread and omnidirectional six months after the disposal ceased (Allen and Moberly 1977b).

Allen and Moberly (1977b) determined that the measured currents were not principally the result of surface tides and mentioned internal waves as a possibility. We favor that suggestion, in light of strong vertical density gradients measured in the area (Neighbor Island Consultants, 1977). Internal waves can mold large and small wavy bedforms of the types we observed (Southard and Cacchione, 1972; Karl et al., 1986; Cacchione et al., 1988). Transport of sediment in an upslope direction by internal waves has been reported within bedform fields on Horizon Guyot (Lonsdale et al., 1972; Cacchione et al., 1988) and in large submarine canyons of the eastern Bering Sea (Karl et al., 1986). The return flow from breaking internal waves can transport sediment in the opposite direction of wave advance (Southard and Cacchione, 1972), which perhaps accounts for the local area of easterly-directed megaripples.


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