Reef-Top Sediment Bodies: Windward O‘ahu, Hawai‘i

Hawaiian fringing reefs display sand bodies on their surfaces that are potentially important components of littoral sediment budgets. This work provides a regional survey of modern reef-top sediment storage and investigated geologic controls on sediment storage potential. Sand bodies are formed when sediment accumulates in topographic depressions that are the result of meteoric water eroding the emerged carbonate reef platform during periods of lower sea level. The relief of some depressions may be modified by Holocene reef accretion. Depression morphology exerts a strong control on volume and internal distribution of sediment. In this study a total of 205 jet probe thickness measurements was collected from 54 major sand bodies on the fringing reef (0–20 m depth) adjacent to 22 km of Southeast O‘ahu coastline (Kailua, Lanikai, and Waimānalo). Volumes were determined and synthesized with previous volume estimates of coastal subaerial and deeper submarine sediment bodies (20–200 m depth), giving the total sediment storage within the coastal system. Sand bodies range from 50 to 2,800 m from shore. Measured thickness varied from 0 to greater than 3.0 m with a mean of 0.95 m. For this study sand bodies were classified into three dominate morphologies: channel, field, and karst depression. The volume of sediment stored in channels was 58,253 ± 618 × 103 m3, fields contained 171 ± 6 × 103 m3, and karst depressions contained 1,332 ± 248 × 103 m3. Correlation of sediment body distribution with reef and coastal plain morphology revealed potential geologic controls on sand body formation in this region. Meteoric runoff and reef slope are important controls on spatial distribution of sand bodies.