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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 (1)

The sidescan mosaic displays three primary signatures that indicate different types of sea-floor material (Figure 2). The majority of the mosaic has a dark tone (low backscatter) that samples and visual images confirm is granular sediment. The light-toned (high-backscatter), irregular and sharply bounded shapes that extend primarily throughout the southwestern part of the mosaic are exposed carbonate reef and associated sediment. The areas of intermediate tone centered on, but extending well beyond, the disposal sites are the imprint of dredged-material deposits. Acoustic profiles presented by Gregory and Kroenke (1982) indicate a maximum sediment thickness of about 100 m over volcanic basement.

The low-backscatter sediment (here termed natural sediment), as confirmed by core samples, is predominantly tan-colored carbonate muddy sand and gravel. The grain size of several samples is shown in Figure 5A. The coarse fraction (>0.062 mm) is composed mostly of skeletal carbonate fragments of unidentifiable taxon, with lesser amounts of planktonic foraminifera tests and basalt fragments. Up to a few percent echinoderm and bryozoa fragments, benthonic foraminifera tests, volcanic mineral grains, and small mollusk shells also are present. Clay minerals are composed of poorly crystallized montmorillonite and halloysite/kaolinite. The amount of acid-insoluble residue, composed mostly of volcanic rock and mineral grains, varies from less than 1% to 35% (Figure 6A).

The composition of the natural sediment is areally variable, but only vague patterns of certain components are evident at our sampling density, as shown in Figure 7. There is a group of samples in the central part of the study area, with a northwest-southeast trend, that has a relatively high percentage of volcanic grains (10% to 50%). Several samples in the western part of the area have an orange color, evidently due to iron staining, and the deepest-water samples tend to have large amounts of planktonic foraminifera (20% to 50%). Samples taken just seaward of a reef exposure at the edge of the insular shelf contain abundant gray to black carbonate fragments, giving the sediment a distinctive gray cast. A sample of the nearby reef contains large fragments of this material in addition to pieces of white coral.

The drowned reefs appear as isolated knobs or extensive rough sea floor in the profiles (Figure 8A) and as tan, craggy pinnacles or ledges in the camera images (Figure 8B). Local, small reef exposures occur outside the southwestern area. Many are visible on the sidescan mosaic and also on the profiles, for example, east of the old Pearl Harbor site. Several small reef knobs appear on the video images that are not apparent on the sidescan mosaic. A sample from one of the reefs consists of large coral fragments and carbonate sand. The distribution and history of the reefs were discussed by Gregory and Kroenke (1982), who concluded that they exist mainly on elevated areas of the volcanic basement and formed before and during the initial stages of subsidence.


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