Sea-floor geology of a part of Mamala Bay, Hawaii

Pacific Science
By: , and 

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Abstract

We surveyed the sea-floor geology within a 200-km2 area of Mamala Bay, off Honolulu, Hawaii by collecting and analyzing sidescan sonar images, 3.5-kHz profiles, video and still visual images, and box-core samples. The study area extends from 20-m water depth on the insular shelf to 600-m water depth in a southeast-trending trough. The sidescan images depict three principal types of sea-floor material: low-backscatter natural sediment, high-backscatter drowned carbonate reef, and intermediate-backscatter dredged-material deposits. Cores indicate that the natural sediment is muddy sand, composed of carbonate reef and microfauna debris with some volcanic grains. Vague areal trends in composition are evident. The dredged material comprises poorly sorted, cobble- to clay-size mixtures of reef, volcanic, and man-made debris, up to 35 cm thick. Dredged-material deposits are not evident in the 3.5-kHz profiles. In the sidescan images they appear as isolated, circular to subcircular imprints, apparently formed by individual drops, around the periphery of their occurrence, but they overlap and coalesce to a nearly continuous, intermediate-backscatter blanket toward the center of three disposal sites investigated.


We did not observe significant currents during our camera surveys, but there is abundant evidence of sediment reworking: symmetrical and asymmetrical ripples in the visual images, sand waves in the 3.5-kHz profiles and side-scan images, moats around the reefs in 3.5-kHz profiles, winnowed dredged material in the visual images, and burial of dredged material by natural sediment in cores. Most current indicators imply a westerly to northwesterly transport direction, along contours or up-slope, although there are a few areas of easterly indicators. Internal waves probably drive the transport; their possible existence is implied by measured water-column density gradients.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Sea-floor geology of a part of Mamala Bay, Hawaii
Series title Pacific Science
Volume 51
Issue 1
Year Published 1997
Language English
Publisher University of Hawaii Press
Contributing office(s) Coastal and Marine Geology Program
Description 22 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Pacific Science
First page 54
Last page 75
Country United States
State Hawaii