A. GUILCHER 1. BERTHOIS F. DOUMENGE A. MICHEL A. SAINT-REQUIER R. ARNOLD LES ’ RdiCI.FS El’ LAGONS CORbiLLPE,NS i i be -NIOPELRA’ ET DE BORA-BORA dLES DE -LA’ SOCBÉT~> : ‘owa DE LA RECHERCHE SCIENTIFIQUE ---_ ----_-~- ----~~-~ ---1------Y .--.__~---__“- ---LI_ --- - 1.--1_--ET TECHNIQUE“-- ---. c----- OUTRE-MER..-- ~---11_-_” -.--I--~-------1 ! OFFICEDE LA RECHERCHESGIENTIFIQUE ET TECHNIQUE OUTRE-MER CATALOGUE SOMMAIRE desPublications fl) DIFFUSION - VENTES Tant pour les abonnements aux revues periodiques que pour I’achat d’ouvrages ou de cartes, il convient d’adresser les commandes impersonnellement à : Service Centra1 de Documentation da I’O.R.S.T.O.M., 70-74, route d’Aulnay - 93- BONDY. Les paiements seront effectués par virements ou chèques postaux, au profit de : Regisseur des Recettes et Dépenses des S.S.C. de I’O.R.S.T.O.M., 70-74, route d’Aulnay, 93- BONDY. - C.C.P. 9182-54 Paris. Exceptionnellement, achat au comptant possible auprès de I’O.R.S.T.0.M. 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LES KÉCIFS ET LAGONS CORALLIENS DE MOPELPA ET’ DE BORA-BORA @LES DE LA SOCIÉTÉ) MÉMOIRES ORSTOM no 38 ANDRÉ GUILCHER - LÉOPOLD BERTHOIS FRANCOIS DOUMENGE - ALAIN MICHEL ANNE SAINT-REQUIER - RENÉ ARNOLD LES RÉCIFS ET LAGONS CORALLIENS DE MOPELIA El’ DE BORA-BORA @LES DE LA SOCIÉTÉ) et quelques autres récifb et lagons de comparaison (Tahiti, Scilly, Tuamotu occidentales) Mar-ho Zogìe,sédìmentalogìe, fonctìonnement by drologìpe ORSTOM PARIS * 9 6 9 English Summary The Society Islands and Tuamotu Islands are parts of volcanic chains in which the activity migrated gradually from Northwest to Southeast, SOthat the southeastern ends are younger, whereas the subsidente has usually caused the volcanoes to disappear under the corals in the Northwest. The Tuamotus are much older than the Society Islands: the former include only atolls except at Mangareva, while in the latter ali types of structures are found. Mopelia (alias Maupihaa) Atoll and Bora Bora barrier reef and lagoon were selected because they are most suitable from scientific and logistic points of view. A supplementary research was made in 1965 in Tahiti lagoon. Comparisons are made with some atolls of the Society Islands and the Western Tuamotus. The writers have not studied nor visited the Eastern Tuamotus. Mopelia and Bora Bora reefs bear low islands, which are located on the windward, eastern side of the structures; the southern and southwestern sides are devoid of islands. Both lagoons communicate with the open sea by deep passages (dva, in Polynesian) which can accommodate ships, and are situated on the leeward side. In both cases, the growth of corals is presently restricted to the outer slope and the lagoon; as a whole the reef flats of Mopelia and Bora Bora barrier are formed of an old ledge rising slightly (up to o. 80 m-1 .oo m) above present-time sea-leve1 in a number of places. A radiocarbon date was obtained for a sample in position of growth taken at Motu Manu, Mopelia Atoll, and another one for a sample from Bora Bora, probably in the same position. Both are Holocene, thus differing from other determinations previously made on emerged samples from the Western Tuamotus, which point to an interglacial, Riss-Wtirm, age. Ali these reefs and the Western Tuamotu atolls are capped by a pink outer Poro&~on ridge resembling very much the ridge in the Marshall Islands: its botanica1 constitution has been investigated in some detail. A well- developed spur-and-groove pattern is everywhere visible, merging into room-and-pillar structures inside the reefs, with PoroZ&on coatings tending to cover and to close the holes. The lagoon topography has been studied by echosounding profìles. Mopelia lagoon consists of two basins in which a host of pinnacles rise to variable Ievels. Corals live on parts of them. Bora Bora lagoon is more complex, being influenced in its shape by the submarine slopes of the voltano. Its basins are, however, much smoother than at Mopelia, as they bear practically no pinnacle. This contrast is not peculiar to these two lagoons, since the two types are found in other places in the Society Islands (e.g. Scilly apparently 8 MOPELIA ETBORA-BORA without pinnacles, Maupiti and Bellingshausen with pinnacles). SO far, no explanation is proposed in this respect. The atoll rims in this region are generally cut in places by shallow passages, boa, in Polynesian, through which the swash surges into the lagoons when surf is heavy. On other atolls, as Mopelia, the absence of islands on the southwestern sides of the rims allows the heavy swell generated from time to time in the roaring forties to feed easily the lagoons. Since the section of the deep passage of this atoll is too small to allow the water piled imo the lagoon to escape immediately, the water leve1 rises during periods of strong surf. A par-t of the water in excess flows over the leeward reefs on either side of the deep passage, forrning on the rim crescentic bars of sand and grave1 which face the lagoon. Scilly and Kaukura Atolls show the same features as Mopelia. Bora Bora lagoon is different in that the deep passage has a much larger section, and seems always able to admit ali the water in excess during periods of heavy swell. Consequently, no outwash crescentic bar is found on Bora Bora leeward reefs. On the other band, Bora Bora gives examples of different kinds of hoa (shallow passages) on its eastern side. Some of them are permanently open; others are closed during periods of moderate surf by spits built by the lagoon waves at their inner ends, and become taz?tia; stili others are attive only during exceptional storms, hurricanes, or tsunamis. Aprons of sand are prograded into Bora Bora lagoon through the hoa and tend to fili the lagoon. Hoa, tairua and protruding aprons in lagoons are common in the other islands of the Tuamotus and the Society Islands. The problem of the renewal of water in Mopelia and Bora Bora lagoons has been investigated on the basis of oxygen content titrations. Both lagoons have always solved oxygen down to the bottom, thus allowing many organisms (oysters, Worms, algae, etc.) to live at depth.