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 flattopped 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).
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
714
MURUROA ATOLL
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 flattopped 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).
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
714
MURUROA ATOLL