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Coral reefs of the New Hebrides, Melanesia, with particular reference to open-sea, not fringing, reefs
Guilcher, A. (1974). Coral reefs of the New Hebrides, Melanesia, with particular reference to open-sea, not fringing, reefs, in: Proceedings of the Second International Coral Reef Symposium. pp. 523-535
In: (1974). Proceedings of the Second International Coral Reef Symposium. Great Barrier Reef Committee: Brisbane, more

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Keywords
    Coral reefs; Fossils; Geography; Geology; Tectonics; Marine

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  • Guilcher, A.

Abstract
    The New Hebrides island arc is divided into 2 island chains and an inter-arc basin, preceded by a deep trench on the western side of the structure. This location of the trench means a reversed polarity of the system, the normal polarity including, in the westernPpacific, a trench on the eastern side. The polarity was probably normal before the mid-Miocene, and reversed later on. The reversal was accompanied by a migration of the volcanic activity from the western island chain to the eastern chain, where it is now found. Tectonic disturbance continues to be active in the New Hebrides. On the islands, bearing the lst generation of volcanoes, as Erromango, Efate, Malekula and Santo, raised terraces of Plio-Pleistocene reef limestones are beautifully exposed. In places, as at Efate, several sets of coral terraces are found; they may result either from a general uplift associated with alternations of transgressions and regressions, or from faulting. The terraces may have remained horizontal or have been tilted. The lowest terrace lying at 4 m in Erromango Island around Cook Bay may be related to the last interglacial high sealevel. All these old reefs are fringing reefs, and grew either around volcanos or around previously raised fringing reefs. Present-day fringing reefs, often accompanied by sand cays, exist around all these islands, 1 of the finest being at Aneitvum. 2 reefs, however, belong to another type. They are very low structures in which the basement is concealed as in atolls, although they deserve to be called orientated coral banks rather than true atolls. The smaller one is Cook Reef, lying between Efate and Epi in the centre of the archipelago. It faces South-South-East, includes a shallow lagoon, and does not bear an island which is uncovered at high tide. It was observed only from aircraft at low altitude and was not investigated in the field. The larger 1 is Reef Island, in the Banks Group at the northern end of the archipelago; it was studied in the field by the writer and F.Doumenge in July, 1971. Reef Island, which will be described here in some detail, faces East, that is towards the trade wind swell. As a whole, it is not a present-time reef, but consists of slightly emerged corals in the position of growth, up to high spring tides; these have been dated by radiocarbon at between 4150 and 6640 yrs (6 samples: CNRS Lab., Gif, France).A cemented rampart includes coral blocks of older age which were not found in situ. The reef is being now corroded by coastal weathering. The distribution of surficial features shows, form East to West, a windward row of islands made of emerged reef and rampart; a medium row of sandy islands with some beach-rock; a shallow lagoon coated with sand and turtle grass and a leeward sand cay with recurved sand spits and a pond in which some mangrove trees live. Reef Island was inhabited until 1939, and then abandoned after a hurricane which caused seas to wash over it.

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