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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.

Abstract
Introduction
Methods
Results
  Bathymetry
  Materials, 1
  Materials, 2
  Structures, 1
  Structures, 2
Discussion, 1
Discussion, 2
Conclusions
References

Figures

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DISCUSSION (1)

The sidescan mosaic and the box cores clearly distinguish three primary material types on the sea floor of Mamala Bay, two natural (natural clastic sediment and carbonate reef) and one artificially emplaced (dredged material). The distinction between material types on the sidescan mosaic is made on the basis of backscatter strength, which evidently is primarily related to sea-floor hardness and/or roughness. Locally derived sediment is strewn around large reef outcrops, predominantly on the west side, and it is difficult to demarcate using backscatter strength alone. However, the 3.5-kHz profiles usually can be used to ascertain, on the basis of sea-floor morphology, the extent of actual reef outcrop, versus the high-backscatter sediment, along tracklines within the highest backscatter areas.

Although the dredged material can be distinguished easily from natural sediment in sidescan images, cores, and photographs, it cannot be reliably differentiated or mapped using the 3.5-kHz profiles. The reflectivity contrast, thickness, and relief of the dredged-material deposits are too small to appear distinctively on the profiles.

The camera images show extensive small ripples whose roughness apparently does not have a significant effect on the acoustic backscatter. Areas of smooth sediment, seen in camera images, do not differ visibly in sidescan backscatter strength from rippled sediment. The 3.5-kHz profiles record sand waves, and they appear in certain places on the sidescan images. Notably, they appear on some images but might be absent on the adjacent one, implying that survey conditions (e.g., sea state, transit direction) affected the basic image quality.

It is obvious that sea-floor sediment is at least occasionally remobilized by currents throughout most of the study area and that it has been reworked significantly since dredged material has been dumped in the area. The evidence comes from the extensive occurrence of wavy bedforms, the differences in sea-floor elevation on opposite sides of reef outcrops, and the local deposition of natural sediment on top of dredged material. The common symmetrical profiles of both the large and small wavy bedforms suggest that oscillatory currents are an important, probably dominant, reworking agent. Oscillatory currents also can produce asymmetrical bedforms if sufficient velcoity asymmetry exists (Clifton, 1976). The sediment reworking must be episodic because we detected no bedload transport during our surveys and very little evidence of currents. A similar situation of ripples formed by episodic currents was noted on the flank of Eniwetok Atoll by Shipek (1962).

Our data, both the long-term indicators such as the sea-floor morphology around reefs and the short-term indicators such as asymmetrical ripples, indicate predominantly west to northwest transport, either parallel to isobaths or upslope. Asymmetrical megaripples and their superimposed ripples indicate one particular area of eastward transport, and other more local instances were recorded.

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