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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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Sedimentary Structures (1)

Wavy bedforms are abundant throughout the study area. For example, ripples appear extensively in the camera images. There is a broad spectrum of ripple types, including symmetrical and asymmetrical forms (Figure 12). Crests commonly are sharp in profile, although they can be rounded, and in plan they are straight to curved and continuous to discontinuous. Some sections of the 3.5-kHz profiles show closely spaced hyperbolic diffractions (Figure 13) that can indicate the presence of rippled bedforms, which is confirmed where there are coincident camera images.

Bedforms larger than ripples are common, also. A local area of asymmetric, sharp-crested megaripples is apparent along part of one video transect and on the accompanying 3.5-kHz profile (Figure 13). The 3.5- kHz profiles show relatively large waves, here termed sand waves, on the sea floor at many places where ripples appear in the photographs, and also beyond the camera transects (Figure 14). The waves tend to have rounded crests and nonuniform sizes. Most have symmetrical profiles, but some are asymmetrical, most facing in a westerly direction on east-west tracklines. The waves typically are less than 3m high and up to 200 m wavelength. Many are underlain by a flat to wavy reflector, which distinguishes them from reef mounds. Linear features that represent the crests of wavy bedforms can be detected on the sidescan mosaic, both within the low- backscatter natural sediment and within the high-backscatter sediment adjacent to drowned reefs (Figure 15). Their trends are northwest to northeast.

Some ripples appear highly degraded, and others have crests that are short and disorganized. No inferences regarding current direction can be made for those ripples. However, many, if not most ripples are fresh in appearance, and a dominant crestal trend can be discerned. Ripple type and orientation can change significantly over short distances. Although there is a large variance, most crests trend within 45° of north; that is, they are more north-south than east-west (Figure 16). The majority of asymmetrical ripples are short-crested, have a lunate or linguoid shape, and face upslope or along slope in a westerly to northwesterly direction; few face downslope. The megaripples mentioned above face east, as do their superimposed ripples.


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