Definition

The atoll of Mururoa, also known as Moruroa or Aopuni, lies in the south-eastern end of the Tuamotu Island Group (21°50′S, 138°53′W) in the central Pacific. Politically speaking, this island is part of the French Polynesia.

Geological setting

Mururoa is 28 km long and 11 km wide with a reef rim not exceeding 15 km2 in surface area. It falls into the open-atoll type with a large natural pass (4,500 km wide, 1–9 m deep) on its leeward side. Its volcanic basement has originated from the Pitcairn hotspot at present operating about 70 km to the south-east of Pitcairn Island (Gillot et al., 1992). The cessation of volcanic activity from about 11 to 10.5 million years favored prominent reef building. Subsidence of the volcanic shield at an average rate of 7–8 mm per millennium has been accompanied by deposition of a sedimentary pile varying between 130 and 570 m thickness (Buigues, 1997). From the end of the 1960s, in relation to nuclear testing, intensive biological, geological, and geophysical surveys were carried out on the modern reef and its carbonate foundations.

Modern reef morphology and zonation

The reef zonation, the composition and distribution of major reef-building biological assemblages have been described by Chevalier et al. (1969), Bablet et al. (1995) and Camoin et al. (2001). Up to 50 coral species have been described from the fore-reef to lagoonal areas.

The outer reef rim

The reef rim, 3 m in maximum elevation, consists of successive islets (« motus ») that form a continuous 400-m wide line along the windward side, and a discontinuous line, 1,100 m wide on the leeward side. The rim is made up of coral rubble mixed with skeletal sand, locally resting on the modern reef flat or overlying emergent, firmly cemented coral conglomerates of mid-to late Holocene age. The reef-flat zone is subdivided into three subzones. The outermost subzone is a typical algal ridge, up to 0.50 m high, 20–50 m wide, mostly made up of coralline algal crusts (Hydrolithon onkodes, Chevaliericrusta polynesiae) associated with encrusting foraminifera (Homotrema, Acervulina) and scarce robust branching corals (Acropora humilis, Pocillopora verrucosa). Behind the algal ridge, there is a coral-dominated subzone inhabited by stunted branching, massive, and encrusting colonies (A. humilis group, Pocillopora brevicornis, Favia stelligera, Montipora caliculata, Porites lichen). Behind the islets to the lagoon, there is an inner reef-flat subzone mainly covered by loose detritus and a few corals (Acropora muricata, A. corymbosa, A. microphthalma, A. tenuis, and Alveopora allingi).

The fore-reef zone can be delineated into three subzones. The upper subzone (to about 10 m deep) is gently dipping (less than 15°) and dominantly colonized by robust branching and massive corals (Acropora robusta group, A. humilis group, P. verrucosa, P. eydouxi, F. stelligera, Platygyra daedalea). Between the depth of 10 and 40–50 m, the intermediate fore-reef subzone becomes increasingly steeper (30–45°). The coral cover rate decreases; the dominating coral species include Acropora nasuta, Astreopora myriophthalma, Leptastrea transversa, Echinopora gemmacea, Leptoria phrygia, Pocillopora elegans, together with Montipora and Cyphastrea. The lower fore-reef subzone below 40–50 m forms a sub-vertical drop-off, locally covered by encrusting, tabular, and foliaceous corals (agariciids mainly) associated with coralline algae and encrusting foraminifera (acervulinids). In addition, the fore-reef zone exhibits terraces at 10, 20, 40, 55, and 65 m and cave-like features at 80, 90, 100, and 150 m deep (Buigues, 1997). The terraces were interpreted as former tops of successive reef units deposited during the late Pliocene and the Pleistocene, while the caves may have been the result of dissolution controlled by colder, interstitial reef waters or by meteoric waters during former low sea stands.

Locally, brackish ponds form in shallow reef-rim depressions (Trichet, 1969) and contain cyanobacterial communities. These develop as mats (matolites or « kopara » according to Défarge et al., 1994) in which calcareous microbialites and phosphorites are deposited (see Richert et al., 2006 for review). These processes are thought to be promoted by nutrient-rich, thermally driven convective currents within the carbonate pile (i.e., the endo-upwelling concept of Rougerie and Wauthy, 1993).

The lagoon

It averages 30–40 m depth and contains numerous reef patches. Coral communities on both lagoonal floor and patches consist chiefly of arborescent A. muricata group, A. pulchra, massive Porites lobata, Leptastrea purpurea, Favia speciosa, and branching P. verrucosa.

Lithology and structure of the sedimentary pile

During the building of the volcanic shield, submarine eruptive events close to the sea surface have alternated with episodes of coral settlement, resulting in coral-rich layers embedded into the volcanic rocks. The deepest occurrence of carbonate deposits is about 950 m beneath the modern rim surface. The volcanic rocks are overlain by a sedimentary pile that comprises two distinct series: a basal volcaniclastic series and a carbonate cap. The entire pile display a number of seismic reflectors identified in cores as unconformity surfaces at the top of different diagenetic units (Buigues, 1997).

The volcaniclastic series

It ranges between 100 and 0 m in thickness from the periphery to the center of the atoll. The series is typified by the superimposition of transgressive (retrograding) depositional sequences. At base the sequences are usually composed of volcanic conglomerates. Progressively upwards, the volcaniclastics decrease in grain size and thickness to thin sand beds. Correlatively, coral-derived material increases in volume, locally forming massive buildups trapped into the volcaniclastics. The younger volcanoclastics are found at around 300 m below the modern reef rim.

The carbonate cap

It occurs between 300–500 and 120–220 m beneath the reef rim and the center of the atoll respectively. An array of reef-related facies has been identified and includes coralgal framestones and bindstones, with skeletal rudstones to grainstones typical of high-energy, outer rim environments; coral bafflestones associated with skeletal floatstones to wackestones, characteristic of low-energy environments; coral platestones to sheetstones embedded into wackestones to mudstones, typical of sheltered, lagoon-like environments. The spatial distribution of these facies reveals that the carbonate system has evolved in terms of morphology and architecture over time. The development history of the carbonate cap has been reconstructed on the basis of age determinations, i.e., radiocarbon, uranium-series, and magnetostratigraphy (Buigues, 1997). The settlement of fringing and barrier reefs is likely to have been discontinuous throughout the Miocene, primarily controlled by volcanic activity and, from about 10.5 million years by residual volcanic topography, tectonics, and sea-level changes. The entire volcanic basement has been capped by an extensive flat-topped carbonate platform probably not prior to the Pliocene. The classical atoll morphology is thought to have been acquired during the late Pliocene to Pleistocene. This results from both dissolution in the central platform areas and lateral reef accretion at the periphery in response to changes in sea level (Perrin, 1990; Buigues, 1997; Camoin et al., 2001). From top to base, the carbonate cap is composed of four stratigraphical units. The Holocene reef unit ranges between about 4 and 20 m in thickness. The Pleistocene reef unit is about 50–150 m thick. It displays major unconformity surfaces at 10, 30, 65, 80, and 90 m below present reef surface, separating successive reef generations. The Pliocene reef unit ranges from about 70 to up to 150 m in thickness. The oldest unit (Late Miocene) locally exceeds 150 m in thickness (Figure 1).

Mururoa Atoll. Figure 1
figure 1_116

Lithology and structure of the carbonate cap with special reference to the outer reef rim over the last 300 ka, Mururoa Atoll, French Polynesia. HO Holocene unit, PLE Pleistocene unit, PLIO Pliocene unit, MIO Miocene unit. Numbers 1–12 refer to Marine Isotope Stages (MIS). Modified from Buigues (1997) and Camoin et al. (2001).

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Carbonate diagenesis

The carbonate rocks from both the volcaniclastic series and carbonate cap have suffered severe diagenetic alteration (replacement by low-magnesium calcite and dolomite, dissolution, karstification) probably controlled by freshwater aquifers in relation to changes in sea level (Aissaoui et al., 1986). There is a significant increase especially, both in calcite and dolomite diagenesis towards the periphery of the carbonate cap. Apart from the upper 20 m (Holocene) composed of metastable minerals (aragonite, high-magnesium calcite), the cap contains low-magnesium calcite or dolomite. The dolomite unit probably of late Miocene Age, occupies the lower two-thirds of the carbonate cap (Figure 1). Dissolution is one of the most prominent diagenetic features in the carbonate cap. The whole series appears to be severely karstified. Karst starts at 90–100 m and extends down to 150 m within the Pleistocene carbonates, both beneath the rim and the lagoon. Large-scale karstic surfaces also occur 180–200 m deep at the Pliocene–Miocene transition and 220–280 m and 300–350 m deep at the base of the Miocene carbonates.

Pleistocene reef growth and sea-level changes

Uranium-series age determinations of the successive reefs units drilled through the outer rim at Mururoa provide new constraints on reef physiography and variations in sea level during the past 300 ka (1 ka = 1,000 years) (Perrin, 1990; Camoin et al., 2001). The outer rim has developed seawards in the form of a series of overlapping fringing reefs (Figure 1). Estimates of former sea-levels are based on the age-depth relationship of selected corals with correction of the present depth for both thermal subsidence of the atoll and habitat-depth range of the corals. Prominent reef units primarily made up of coralgal boundstones have developed during four episodes of high sea stands (MIS stages 1, 5, 7, and 9). The data indicate that sea level was between 17 and 23 m below present sea level (bpsl) at 9 ka. During MIS stage 5 e (around 125 ka), sea level is estimated to have been 6–10 m above present reef surface. The paleo-sea levels at 212 ka (stage 7 c) and 332 ka (stage 9 c) were 11–17 m 26–33 m bpsl respectively. Low sea stands are typified by limited reef growth and large skeletal deposition and include MIS stages 2, 4, and 8. During stage 2 (Last Glacial Maximum), sea level was around 135–140 m bpsl within the 23–17 ka interval. The paleo-sea level during stage 4 at around 60 ka is assumed to have been between 76 and 91 m bpsl, while it was at 79–94 m bpsl during stage 8 d at about 270 ka.

Summary

Mururoa is an open atoll and exhibits low coral diversity (about 50 species). Drilling investigations revealed that the maximal thickness of the carbonate pile ranges between 500 and 220 m. The lowermost part of the pile is late Miocene in age. The typical atoll morphology is likely to have been acquired not before the late Pliocene. During the Pleistocene, and especially the last 300 ka, the outer rim has accreted seawards through a series of stacked fringing-like reef bodies. The carbonate deposits have experienced intense diagenetic alteration by fresh water during successive low sea stands.

Cross-references

Atolls

Atoll Islands (Motu)

Conglomerates

Diagenesis

Dolomitization

Geomorphic Zonation

Holocene Reefs: Thickness and Characteristics

Lagoons

Oceanic Hotspots

Pacific Coral Reefs: An Introduction

Sea Level Change and Its Effect on Reef Growth

Stromatolites