by

Monty A. Hampton¹, Charles T. Blay², Christopher Murray³, Laura Z. Torresan¹, Cathy S. Frazee1, Bruce M. Richmond1, and Charles H. Fletcher⁴

¹U.S. Geological Survey, Menlo Park, CA
²TEOK Investigations, Poipu, HI
³Battelle Pacific Northwest National Laboratory, Richland, WA
⁴University of Hawaii, Honolulu
 

Introduction

This Open-File Report presents data and derivative products from an investigation of carbonate sediment deposits on the reef front in four areas around the island of Oahu, Hawaii - in Kailua Bay off Oahu's windward (east) side, off the leeward (west) coast from Makua to Kahe Point, off the north coast from Waimea to Camp Erdman, and off the south coast around Waikiki (Figure 1). The primary purpose of the investigation was to assess the resource potential of the deposits, particularly as a source of sand for beach nourishment. This work builds on previous studies by researchers from the University of Hawaii (Moberly et al., 1975; Coulbourn et al., 1988; Barry, 1995). The field program included collection of high-resolution acoustic-reflection profiles and vibracore sediment samples in Kailua Bay and off the leeward and north coasts. In a related project, in collaboration with the Hawaii State Department of Land and Natural Resources and the University of Hawaii, sidescan images and vibracores were collected in the Halekulani channel and on the adjacent Makua Terrace off Waikiki along the south coast.

The submerged terraces on which the sediment deposits rest are Quaternary lowstand insular shelves (Stearns, 1935; Coulbourn et al., 1974). The deposits are elongate parallel to the reef front and connect landward with the sediment fill of drowned river channels that cross the modern insular shelf. From a geological perspective, the deposits are attractive because of their predominantly carbonate composition, similar to existing beaches, and because of their great total volume (4 x 10⁸ m³), about an order of magnitude larger than the volume of all beaches on the major Hawaiian islands (Chamberlain, 1968; Moberly, 1968; Dollar, 1979).

The data is most comprehensive in Kailua Bay where the reef-front deposit has a relatively simple and basic form: it has a single depocenter seaward of a reef-platform channel and extends with an arcuate form for about 4 km parallel to the reef edge (Figure 2). The deposit thins irregularly in the offshore direction and more smoothly parallel to the reef front. The deposit apparently is confined entirely within the Kailua embayment. Along the leeward and north coasts, deposits issue from several channels and coalesce over substantial distances along the reef front, so they are compound features. There are many analogous deposits around Oahu, essentially encircling the entire island (Moberly et al., 1975; Coulbourn et al., 1988; Barry, 1995).