Drowned reefs as indicators of the rate of subsidence of the Island of Hawaii

Abstract

A major submerged terrace whose seaward edge is at about 150 m depth was investigated in 1983 during 10 dives with the Hawaii Undersea Research Laboratory (HURL) submersible Makali'i off Kealakekua Bay, Hawaii. The seaward termination of the terrace is a steep wall of reef limestone that extends from 150 to 250 m below sea level. Three samples of limestone collected from the reef face at depths of 204-219 m yield a weighted average age of 13,250 (S.D. 380) years before present. The -150 m terrace as well as two deeper terraces off northwestern Hawaii at depths of and apparently were formed by the interaction of glacioeustatic sea-level changes and island subsidence. The best fit of depth, age, and subsidence data indicates that the northwestern coast of Hawaii has subsided at an absolute rate of 1.8 to 3 + mm/yr and that the rate of subsidence has generally increased over the past 0.3 m.y. Each reef terrace grew intermittently for nearly 0.1 m.y.; the -150 m reef terrace was drowned about 13,000 years ago, the -390 m terrace about 145,000 years ago, and the -580 m terrace about 255,000 years ago. Warping of the two older terraces may have resulted from crustal loading caused by the major growth period of Mauna Loa volcano.