coral terraces

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coral terraces A coral terrace is a special type of marine terrace, comprising a staircase-like set of subhorizontal terrace surfaces, each separated by a former sea cliff. The base of each sea cliff, defined by the inner landward edge of the terrace surface, is a good marker of past sea level, to the extent that the location and elevation of these former shorelines record sea-level and tectonic fluctuations during Quaternary times. The age of emergence of a coral terrace can be determined by uranium-series dating of uncrystallized coral samples and other dating methods. By using these data, former sea-level elevations and the magnitudes of past changes in sea level can be quantified.

The formation of coral terraces is interpreted as the product of approximately uniform long-term uplift superimposed on eustatic changes in sea level. Coral terraces are thus well developed on tectonically active coastlines close to convergent plate boundaries, such as at the Huon Peninsula of Papua New Guinea, Barbados Island in the West Indies, and Ryukyu Islands of south-western Japan. Past sea-level elevations deduced from coral terraces at Huon Peninsula have been combined with oxygen isotope data to establish global sea-level histories that are widely used for Quaternary studies. Furthermore, rates of uplift estimated from coral-terrace data may reveal significant regional and local differences in tectonic behaviour. For example, the coral terrace corresponding to the last interglacial maximum of about 125 ka ago (oxygen isotope stage 5e) occurs at an elevation of 400 m at Huon Peninsula and at 220 m at Kikai Island near Ryukyu trench. This difference indicates greater tectonic uplift in the Huon Peninsula area, although both areas are characterized by relatively rapid uplift. At both Huon Peninsula and Kikai island, successive former sea levels younger than isotope stage 5e (stages 5c, 5a, and 3) are recorded as a series of lower terraces. This enables rates of uplift in both areas to be tracked through time.

Holocene coral terraces fringing the older terraces along rapidly uplifting coasts are usually composite features, each comprising a series of small terraces. At Huon Peninsula, for example, the Holocene coral terrace reaches up to 25 m in elevation but it is subdivided into a maximum of seven discrete steps, indicating rapid but intermittent uplift over a short time-span. Such small terraces can also be found in late Pleistocene terrace profiles at Huon Peninsula, indicating that a similar tendency operated in earlier times. These multiple small terraces are interpreted as evidence for repeated major jerks of the coast during earthquakes, and can thus be used for reconstructions of palaeoseismicity.

Yoko Ota

Bibliography

Chappell, J. (1974) Geology of coral terraces at Huon Peninsula, Papua New Guinea; a study of Quaternary tectonic movements and sea level changes. Geological Society of America Bulletin, 85, 553–70.
Chappell, J. and and Shackleton, N. J. (1986) Oxygen isotopes and sea level. Nature, 324, 137–40.
Chappell, J.,, Omura, A.,, Esat, T.,, McCulloch, M.,, Pandolfi, J.,, Ota, Y.,, and and Pillaus, B. (1996) Reconciliation of late Quaternary sea levels derived from coral terraces at Huon Peninsula with deep sea oxygen isotope records. Earth and Planetary Science Letters, 141, 227–36.
Ota, Y. and and Omura, A. (1992) Contrasting styles and rates of tectonic uplift of coral reef terraces at Ryukyu and Daito Islands, southwestern Japan. Quaternary International, 15/16, 17–29.
Ota, Y. and and Chappell, J. (1996) Late Quaternary coseismic uplift events at Huon Peninsula, Papua New Guinea, deduced from coral terrace data. Journal of Geophysical Research, 101B, 6071–82.

terraces, coral see coral terraces