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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 (2)

Poorly developed ripples with disorganized crests occur where natural sediment is interspersed with dredged material (Figure 11B). However, ripples do not occur at all where dredged material continuously covers the sea floor, probably because the material is too cohesive and because there is too much coarse sediment. Disorganized ripples are common near, but outside, the dredged-material deposits, also. Although some sort of bedforms are present most places along the camera transects, the sea floor appears exceptionally smooth in natural sediment near the eastern end of some.

Because the wavy bedforms described above are formed by currents and have implications about sediment dynamics, it is significant that during the camera surveys (10-15 May 1994) we did not detect any bedload sediment movement and only a few instances of suspended sediment transport, where weak currents apparently swept grains off nearby elevated reef outcrops. Also, when the camera sled occasionally impacted the sea floor, the cloud of displaced sediment typically showed little or no deflection by currents as it collapsed. Currents were weak to nonexistent during our surveys, but the fresh appearance of many ripples suggests recent stronger currents in the area.

X-radiographs show that most samples are extensively bioturbated with some degree of subtle, original stratification. Unambiguous cross lamination is rare (Figure 17). Lebensspuren appear from place-to-place in the camera images, particularly in the northeastern part of the area. A few instances of small fish interacting with the sea floor and infauna emerging from burrows were seen, but overall, evidence of animal- sediment interaction at the sea floor was uncommon.

The sidescan images show enigmatic high-backscatter linear to curvilinear trains of short, parallel lines superimposed on the natural sediment in the southern and southwestern parts of the mapped area (Figures 2 and 18). They may or may not be bedforms. Individual lines are 50 to 100 m long and spaced at about 25 to 50 m, and individual trains of lines are up to 1 km long. No indications of these features appear in the other data types.


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