Geomorphology and Late Quaternary development of Middleton and Elizabeth Reefs

Middleton and Elizabeth Reefs are two mid-latitude, annular reefs within the Lord Howe linear chain of volcanic islands and seamounts in the southwestern Pacific Ocean. Drilling, vibrocoring, seismic profiling, and dating indicate that each has a rim of Holocene reef framework, enclosing a lagoon partly filled by prograding sand sheets composed of fragments of coral, coralline algae, foraminifers, and other skeletal debris. The reefs lie close to the latitudinal limits for coral growth and the reef framework is very porous, dominated by branching rather than massive corals. Coralline algae are the principal binding agent in the upper reef framework. Holocene reef growth began on a foundation of Pleistocene reefal limestone encountered at a depth of 8 m in cores on the windward side of Middleton Reef. Holocene corals became established on this foundation around 6,700 radiocarbon yr B.P., implying little if any lag after inundation of the platform by the post-glacial sea-level rise. Windward reef growth tracked sea-level rise (keep-up mode), and a prominent reef crest was established on both reefs by 5,000 yr B.P. Leeward margins appear to have been characterized by catch-up growth. Development of cays is limited, and has been restricted by the paucity of coarse coralline debris or cemented conglomerate on which islands could become established. The morphology and development of Middleton and Elizabeth Reefs has been similar to that of tropical atolls, although the rate of subsidence appears to have been relatively slow reflecting their position on the margin of the foundered continental crust of the Lord Howe Rise.