La karstification de l'île haute carbonatée de Makatea (Polynésie française) et les cycles eustatiques et climatiques quaternaires

Résumé En Fr

Located in the Central Pacific, in the northwestern part of the Tuamotu Archipelago, Makatea island (148°15 ' W -15°50' S) is an uplifted, karstic, carbo¬ nate construction of Early Miocene age which reaches 113 m in height. From 1906 to 1966, phosphate deposits were exploited on Makatea island. These phosphate deposits (apatite ) overlaid the miocene series and filled the karstic cavities in the higher regions of the island. Several traces of ancient shorelines can be observed on Makatea : -three different reef formations which reach about + 27 m, + 7 m, + lm above the present mean sea level and respectively dated 400,000 ± 100,000 yr BP, 240,000 ± 30,000 yr BP, between 4,470 ± 150 yr BP and 3,710 ± 130 yr BP ; -four distinct marine notch lines on the Early Miocene cliff at about + lm, + 7m, + 27 m and + 56 m (or + 47 m on the west coast caused by tilt) above the present mean sea level ; -two exposed marine platforms respectively at + 29 m and + 7 m above the present mean sea level. The ages of the former Makatean shores are inferred by using : (1) the Pacific gla-cio-eustatic sea-level curve for the last 140,000 yr BP, (2) the Pacific oxygen iso¬ tope curve for the last 900,000 yr BP, and (3) a constant uplift rate during the Pleistocene. In this way, according to their age and elevation, the sea-level indicators at about + 1 m + 7 m and + 27 m (+ 29 m) above the present mean sea level can be respectively related to the Holocene transgression (Flandrian) dated between 6,000 and 1,500 yr BP, to the last Pleistocene interglacial period (Sangamon) dated between about 130,000 and 110,000 yr BP, and to a Middle Pleistocene intergla-cial period (Yarmouth ) dated between about 315,000 and 485,000 yr BP. If we assume that a sea level similar to the present occurred during the Yarmouth inter-glacial period, the uplift rate is valued at 0.085 mm/yr to 0.056 mm/yr. Thus the sea-level associated with the marine notch at about + 56 m (+ 47 m) may be about 650,000 yr to 1 m.y. old and can be associated with another Pleistocene interglacial period (Aftonian). Consequently, as indicated by the former shores, the sea level fluctuations can be related to the major glacio-eusta-tic quaternary events. This climatic and eustatic setting is used to explain the karst observed on the Makatea island. So, carbonate dissolution and essentially vertical karstogenesis were the result of the superposition of several cycles. Each cycle was initially composed of a dissolution of the carbonates during an interglacial period, followed by a drainage of the saturated solutions during the marine regression associated with the consecutive glacial period . Nevertheless, this scheme is not enough to explain the specific morphology of the Makatean karstic cavities and we suggest using insular phosphatization to explain this karstogenesis. It is generally accepted that phosphate rock deposits on coral reef islands are the result of chemical reaction between seabird guano and reef limestone. Furthermore, pétrographie and stable isotope studies suggest several genera¬ tions of phosphorite formation and reworking episodes in the history of these deposits. The primary deposition of phosphates must have begun during a glacial period . This deposition was followed by some redistribution of phosphorites during the interglacial period and by additional precipitation of apatite from meteoric waters. This assumed process of phosphogenesis is consistent with both the field observations and the geodynamic evolution of Makatea. Thus, the particular morphology of the Makatean karst can be the result of the dissolution of the carbonates caused by phosphoric acid etching. This acid is derived from the evolution of the phosphorites during the pleistocene intergla-cial periods.

Située sur la bordure nord-ouest de l'archipel des Tuamotu (Polynésie Française), à 245 km au nord-est de Tahiti, par 148°15' de longitude ouest et 15°50' de latitude sud, Makatea constitue une île haute carbonatée soulevée. Elle se distingue par sa surface supérieure à la morphologie karstique très irrégulière mise en évidence à la suite de l'exploita¬ tion de la couverture phosphatée. Les traces d'anciens rivages observées à j la périphérie de l'île de Makatea permettent de reconnaître les variations du niveau marin enregistrées depuis le début de l'émersion de l'île actuelle, il y a environ -2 millions d'années, jusqu'à nos jours. L'analyse chronologique de ces paléorivages permet de proposer des épisodes climatiques et eustatiques propices à la dissolution et à la karstification des formations carbonatées. Ainsi, la karstification des formations de l'île de Makatea est-elle caractérisée par une dissolution à composante principalement verticale qui a pour origine la superposition de plusieurs cycles. Chaque cycle est constitué, d'abord par une mise en solution des carbonates lors d'une période interglaciaire quaternaire, puis par l'évacuation des solutions saturées au cours de la régression associée à la période glaciaire consécutive. Compte tenu des données dont on dispose à ce jour, le processus envisagé pour la formation des phosphates insulaires s'intègre dans ce schéma d'évolution et a pu contribuer à la karstification de l'île de Makatea. Ainsi, la morphologie particulière du karst de Makatea aurait pour origine la dissolution des formations carbonatées sous l'action de l'acide phosphorique libéré lors de l'évolution des dépôts phosphatés au cours des périodes interglaciaires quaternaires.