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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.

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  Structures, 1
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Discussion, 1
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Our sidescan images, acoustic-reflection profiles, photographs, and sediment samples provide information about the sea-floor morphology, the distribution and composition of geological materials, and the sediment transport in Mamala Bay. We surmise that the natural sediment that covers most of the sea floor was deposited during Pleistocene low stands of sea level, derived mostly from the emergent reefs but with a small to moderate contribution eroded from the island's volcanoes. The sediment was deposited in a broad, southeast-trending trough, on and around drowned reefs, some of which remain exposed at the sea floor. Reef-derived sediment (e.g., the gray-colored sediment samples) apparently is now being deposited over the edge of the insular shelf, on the steep headwall of the trough. Most volcanic grains presently are trapped in coastal estuaries and harbors. We cannot discount the occasional occurrence of sediment gravity flows into deeper water, as has been documented off Kahe Point to the west (Tsutsui et al., 1987), but we saw no evidence of such activity.

Deposits of sediment dredged from Honolulu and Pearl Harbors, more than 30-cm maximum thickness, cover relict natural sediment and reefs over much of the study area and can be detected in all data sets except the 3.5- kHz profiles. Episodic bottom currents, most likely from internal waves, affect the sea floor, forming ripples and larger wavy bedforms over extensive areas and depressions (moats) on the downcurrent sides of the larger exposed reefs. The currents are oscillatory but transport dredged material and natural sediment primarily in a net westerly to northwesterly direction, although there is evidence of southward transport, also. Natural sediment is encroaching over and burying the dredged material in some places.


We are grateful to the following U.S. Geological Survey colleagues for their technical assistance and discussions: David Cacchione, Thomas Chase, Marguerite Gowen, Gretchen Luepke, Larry Phillips, Paula Quinterno, Bruce Richmond, and Florence Wong. The crew of the R/V Kila provided excellent support to help us achieve our scientific goals. This study was supported by the U.S. Army Corps of Engineers and the Environmental Protection Agency. We are grateful to Michael Lee and Kathleen Dadey (USACE) and Allan Ota (EPA) for their encouragement and cooperation. Kathleen Dadey, Bruce Richmond, and David Twichell made helpful comments on the manuscript.


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