LAUSR

Abstract In French Polynesia, low-lying, reef islands, composed of unconsolidated skeletal material rest on cemented conglomerate platforms. These are regarded as mainly generated by storm activity during the mid-late Holocene. Field observations and coral sampling were carried out on conglomerates from three atolls located in the north-western Tuamotu, namely, Takapoto, Takaroa and Fakarava. There, fifty-seven samples were dated using the uranium‑thorium (U/Th) method. The lateral age distribution of clasts indicates that conglomerate platforms have accreted from lagoon sides seawards. In addition to U/Th dating, the coral clast-age datasets include eighteen calibrated radiocarbon ages from Takopoto and other Tuamotu atolls – Takapoto, Mataiva, Rangiroa, Arutua, Apataki, Marokau, Hao, Pukarua, Nukutavake. As a matter of comparison, another seventy-seven, previously published radiometric conglomerate ages from various French Polynesian islands (Society, Tuamotu, Gambier) were incorporated in the present study. It appears that the formation of conglomerate platforms has taken place principally during a 2000-year interval, between 3000 and 1000 yr BP when sea level dropped from about +0.60 m to its present position. Before and after these periods, lower amounts of coral detritus have been provided to conglomerates, possibly due to clast deposition in adjacent lagoons and to trapping into reef rim-islands respectively. Age discrepancies of most coral clasts collected from multiple stacked layers at Takapoto and Fakarava do not exceed 500 years, strongly suggesting limited clast reworking before final stabilization. The superimposition of two to four firmly cemented conglomeratic layers at the same place gives evidence that marine cementation has operated shortly after clast deposition. This event is regarded as having taken place within both phreatic and vadose environments. Rubble sheets found seawards at some atoll-rim sites are interpreted to be conglomerates in the making, emplaced over the last centuries. Dating of atoll-islet material indicates clearly that, over the 2500–1000 yr-BP span, the building of conglomerates and overlying islets has interplayed. The former have evolved within subtidal to supratidal zones while the latter have been accreted subaerially. This may explain the firmly consolidated state of platforms and the unconsolidated state of islets. Due to their firmness, conglomerate platforms are assumed to be key structures for the maintenance of atoll islands subjected to sea level rise and increasing storminess in the coming centuries.