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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
Discussion, 2


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Sea-floor Materials (2)

The 3.5-kHz profiles show that the sea floor is high on the east side of most reef outcrops, and there is a topopgraphic low, or moat, on the west side (Figure 8A). Extensive high-backscatter sediment covers the sea floor on the west side of the large reef outcrops along the southwestern side of the study area. A single sample reveals the sediment to be coarser grained than the low-backscatter natural sediment, and it is composed carbonate sand with shell fragments a few mm in size and some larger pieces of coral.

The dredged-material deposits appear in the mosaic as isolated, circular to subcircular imprints, apparently formed by individual disposal drops, around the periphery of their occurrence. The imprints overlap and coalesce to a nearly continuous, moderate-backscatter blanket toward the center of the three disposal sites (Figure 2). In core samples, the dredged material typically is cohesive and olive green to black in color. It has a heterogenous composition and texture, with particles from clay to cobble size. It is typically muddier and contains larger-size gravel than the natural sediment. The grain size of several samples is shown in Figure 5B. The coarsest particles include pieces of reef carbonate, calcareous worm tubes, shells and shell fragments, volcanic clasts, mud balls, and man-made debris. The finer fragments include comminuted skeletal carbonate, basalt (some oxidized to a rust color), plant material, cemented clastic carbonate, echinoderm shells, pellets, coral, and rusted pieces of metal. The amount of acid-insoluble residue measured in 24 samples varies from 7% to 44% (Figure 6B). The greatest recovered thickness is about 35 cm (Figure 9). Nowhere is there convincing evidence of the dredged material in the 3.5- kHz profiles. The dredged material in the Honolulu Harbor site typically has larger maximum-size particles than in the other two sites.

A few cores have a thin layer of natural sediment that covers dredged material (Figure 10). In the camera images, the dredged material appears to either completely cover the sea floor in a single frame or be patchily interspersed with natural sediment, very sparsely in some places. There is a continuum between sparse and complete coverage (Figure 11A,B).

The camera images also show a multitude of man-made objects on the sea floor. Most numerous are vehicle tires, wire rope, metal structures, beverage cans, and ordnance.


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