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.
Bathymetry and Sea-floor Morphology
The bathymetry and regional sea-floor topography are shown in Figures 1 and 3. The bathymetric depth points were obtained from our soundings and from the U.S. National Oceanographic and Atmospheric Administration's GEODAS archive. Water depths in the surveyed area vary from 20 to 600 m. The outer part of the Mamala Bay insular shelf appears along the northern edge of Figure 3. The shelf is less than about 50 m deep and extends a minimum distance of 600 m from shore off Diamond Head to a maximum of nearly 5,000 m east of Barber's Point. An extensive prominent step exists at the seaward edge of the insular shelf, at water depths to about 100 m. It is the Mamala shelf (Ruhe et al., 1965), thought to have formed during the latest Pleistocene low stand of sea level, either by wave erosion or by reef construction (Stearns, 1974; Gregory and Kroenke, 1982). Our 3.5-kHz profiles indicate that the step typically is a planar notch cut into the otherwise steeper slope (Figure 4). Stearns (1978) observed a drowned reef on the Mamala shelf during submersible dives.
Seaward of the insular shelf is a broad trough that slopes gently at an average of about 1° in a south to southeast direction. Most of the study area is contained within this trough. The head of the trough, from about 100 to 300 m water depth, is steep (~12°), cuspate shape, and has some local steps. The bathymetry map shows that the floor of the trough has slightly irregular topography generally of less than a few meters relief, although outcrops of drowned reefs approach heights of 50 m. The trough is bounded on the southwest by a southeast-trending platform that is underlain by a drowned reef (Gregory and Kroenke, 1982) and dissected by a canyon. The platform extends seaward from the widest part of the insular shelf (Figure 3). The trough is bounded on the northeast by the steep slope that leads up to Diamond Head. The entire region greater than about 200 to 300 m deep is part of the Lualualei shelf (Stearns, 1961), also known as the 500-meter shelf (Kroenke and Wollard, 1966). It is thought to have been sculpted into basaltic basement rocks by wave action, then locally overgrown by reefs, during subsidence to its present depth (Gregory and Kroenke, 1982).