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Mururoa and Fangataufa : sea-level changes, karstification and the atoll morphology

Mururoa and Fangataufa atolls are the emerged parts of islands born on the Pacific plate. Their edification has been governed by the structural discontinuities of the oceanic plate. Rift-zones have constituted the frame of the edifices. Elongate and parallel to the Austral fracture at Mururoa, the rift-zones are nearly radial at Fangataufa which is more typical of a classical sea-mount. The morphology of the volcanoes were different, at least two classical cone-shaped volcanoes at Mururoa and a single flat one at Fangataufa. The numerous wells which have been cored from both atolls, vertically under the rims and the lagoons and with a seaward deviation through the rim of Mururoa, have revealed a discontinuous buildup of the atolls. Within the volcanic basement of both atolls, carbonate beds of reefal origin occur and attest of gaps in the volcanic building. The distribution of these carbonates is related to the occurrence of the rift-zones which enabled the sedimentation when they were active. With a greater abundance at Mururoa and an occurrence both under the rim and the lagoon, these beds are less extended at Fangataufa. In both atolls they occur at the same depths. The sedimentary pile recovering the volcanoes is constituted with a basal volcano-sedimentary series and a carbonate cap which is composed of dolomites underlying limestones. The total thickness depends on the topography of the volcanoes, the thickest deposits (570 to 330 m) being located at the periphery and above volcanic valleys, while the thinnest ones (130 m to 230 m) are in the centre above the volcanic tops. Above these tops the volcano-sedimentary series is absent. The geometry of the dolomitic body was conditionned by the volcanic topography. With a top currently situated at the same depth from both atolls (-190/-210 m), the dolomitic series does not exist at the centre of Mururoa but is generalized to the whole edifice on Fangataufa. The deposition of the sedimentary pile began concurrently with the cessation of the volcanic activity, around 10,5 Ma at Mururoa and 9,5 Ma at Fangataufa. At both atolls the sedimentary series contains emersion surfaces and numerous karstic horizons, those ones extending laterally under a whole atoll from the periphery to the centre. At Mururoa, the dating informations ( 14 C, U/Th and magnetostratigraphy) from the rim vertically down to 300 m deep and laterally behind the reef wall, and the submarine observations of the slopes down to 1,200 m by a R.O.V., have supplied interesting data which increase the knowlegde about the structure of the atoll and support former ideas concerning the construction of the atoll. All of the results and the great similarities between the two atolls clearly prove that several emersions have occurred during the buildup of the atolls. The carbonates enclosed in both volcanoes indicate that the earliest emersions occurred when the volcanoes began their building around 10 to 11 Ma. Karstification currently found at different depths within the sedimentary series indicate several post-Miocene emersions. The sedimentary series deposited at the periphery of the subsiding volcanoes progressively extended above the whole edifices recovering the highest volcanic tops. Successive emersions modified the islands morphology. The final one, the atoll shape, is the consequence of a progressive differentiation between the centre and the periphery which occurred principally during the Pleistocene when emersions occurred.