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Abstract

Two alternative explanations still compete regarding the formation of atoll lagoons: the classic Darwinian theory and the karstic-saucer theory. Although the deep drillings at Mururoa Atoll, French Polynesia, certainly favor the first alternative, the discussion remains open elsewhere. Progress in knowledge may come from investigations of coral pinnacles or knolls, which dot lagoons in various quantities.

Data on sets of pinnacles in the Tuamotu, Society, and Gambier Islands are discussed. The recent endo-upwelling theory has tried to explain the formation of these features by an ascent of deep oceanic water rich in nutrients through the pervious mass of atolls. Shallow-core borings were made in 1988 into one pinnacle and the adjoining rim of Tikehau Atoll, Tuamotu Islands. Studies of nutrients in the interstitial waters support the endo-upwelling theory. However, preliminary examination of the cores shows the presence of a highly dolomitized Tertiary reef underlying the Holocene coating without intervening Pleistocene rocks. Tikehau thus appears as a rather special structure and the same type of investigations should be continued in other atolls in the Tuamotus.

Finally, the discussion is extended to the Gambier almost-atoll, a structure akin to Truk, Carolines, described in 1970 by Francis Shepard. Here the question of pinnacle origin also exists but is complicated by peculiarities associated with the shape and subsidence of the volcanic basement, differential tilting of the barrier, and absence of a deep passage into it. Thus, the discussion needs to be enlarged to consider diverse parameters of coral reef history. Results expected from multiple borings are a good approach to understanding atoll formation, still in question in spite of various proposals since the middle of the 19th century.

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