History of coral reefs and lagoons
The development of the New Caledonian fringing and barrier reefs and the nature and distribution of their biofacies
were documented by several studies performed on cores
made mainly around the Grande Terre (Coudray, 1976;
Fontes et al., 1977; Cabioch et al., 1995, 2008; Cabioch,
2003). These cores are composed of alternating layers of
coral and coralline algal frameworks, detrital facies, sands,
and gravels. Moreover, these cores revealed that the fringing reefs are characterized by the succession of two reef
generations, while the barrier reefs by several reef or carbonate unit generations, all separated by unconformities
resulting of emersion during the low sea stands. It appears
that the reef growth results from the combination of both
tectonic and climatic factors. The history of the lagoons
is not well known, but recent bathymetric and seismic surveys documented our knowledge on the structure of the
southwest lagoon.
Fringing reefs
In the fringing reefs of the Grande Terre, coral dating in 37
cores, drilled along perpendicular or parallel transects on
the reef flat, shows that the initiation growth stage is generally missing and that the reefs started to grow very
quickly and suddenly (Figure 2) (Cabioch et al., 1995).
In the south, the growth started prior to 5 ka BP upon
old reef karstic formations and in the north after 4.2 ka
BP, generally upon non-carbonate rocks, mainly metamorphic rock (Cabioch et al., 1995; Cabioch, 2003). These
differences in initiation age would be explained by
unavailable suitable foundations and also as resulting
from colonization of various types of substrates, the
karstic limestone surfaces being probably the most suitable substrate for coral-larval and attachment recruitment.
Core analysis also shows that framework, generally, dominates over skeletal debris in the higher energy reefs, while
skeletal debris are the main components in the lower
energy reefs (Cabioch, 2003). Moreover, the branching
and tabular coral assemblages (acroporids) are mainly
developed during the early stages of reef growth, whereas
the domal (favids, poritids) and/or robust-branching
(acroporids) assemblages appeared later (Figure 2).
Unfortunately, the reef slope and front edge zones were
not investigated by coring so that two interpretations were
proposed to explain this observation (Cabioch, 2003):
a progressive succession from low-energy to high-energy
water environments interpreted as a progressive increase
of water energy as the reef approaches the sea surface, typical of a catch-up reef growth mode (Growth classification
and sea level); or alternatively, a transition from lagoonal
deposits typified by coral and skeletal debris to reef
coralgal biofacies interpreted as a reef initiation,
retrograded over its back-reef. Examination of the distribution of reef facies from reef to reef also provides additional information and shows a strong relationship with
the antecedent topography (Antecedent platforms) and
the local water-energy conditions. The high water energy
reefs characterized by steeply dipping antecedent platforms are dominated by frameworks while medium water
energy reefs by abundant reef detritus.
Barrier reefs
The history and the development of barrier reefs in New
Caledonia are less known because it is more difficult to
drill in the barrier reef tracts strongly exposed to the oceanic conditions. One of the first studies was performed
by Coudray in 1976 at Ténia, on the west coast
(Figure 1). Analysis of the 226 m-long core revealed the
occurrence of several episodes of reef development during
several interglacial quaternary stages. More recently, cores
recovered at Amédée in the southwest and Kendec in the
northwest (Figure 1) (Cabioch et al., 2008) showed that
the New Caledonian barrier reefs are composed of
a succession of depositional units resulting from
a combination of glacio-eustatic sea level variations and
subsidence (Antecedent platforms). In both areas, 11 carbonate units deposited during the high sea stands are
observed, separated by unconformities resulting from
emersion during the glacial periods (low sea stands). It
appears that the reef growth would have begun around
400 ka in marine isotope stage 11, considered as
a particularly favorable period for coral development.
Prior to this period, the carbonate units are not characterized by biofacies typical of coral reefs sensu stricto
although scarce isolated corals can be found.
Lagoon
Lagoons are the transition between the two morphological
features of the fringing and barrier reefs, but their history
is still little known. Nevertheless, a recent high resolution
seismic and multibeam surveys of the South-West lagoon
(Figure 1) has revealed several depositional sequences
bounded by erosional unconformities, indicating two
periods of deposition and erosion in the past 200 ka (Leroy
et al., 2008). Two generations of incised deep fluvial valleys, connected by passes to the continental slope, were
identified. These data would indicate that the lagoonal history does not extend beyond 200 ka. In addition, the analysis
of morphology of this network of drainage suggests a progressive south-westward tilt of the outer lagoon expressing
the role of tectonics in the formation of the lagoons.
Climatic and/or oceanographic factors affecting
the reef growth and the coral development
The distribution and the growth of modern fringing and
barrier reefs are dependent not only on the tectonic context
but also on the climatic variations. The climatic variations
at the scale of the last 100,000 years (astronomical theory),
marked by sea-level variations of the glacial and interglacial periods, influenced the internal structure and reef succession in the barrier reef tracts (see above the history of
Barrier reefs). At the scale of the Holocene, the oceanographic factors and climatic variations influenced on the
initiation and growth of both barrier and fringing reefs.
NEW CALEDONIA
719
The development of the New Caledonian fringing and barrier reefs and the nature and distribution of their biofacies
were documented by several studies performed on cores
made mainly around the Grande Terre (Coudray, 1976;
Fontes et al., 1977; Cabioch et al., 1995, 2008; Cabioch,
2003). These cores are composed of alternating layers of
coral and coralline algal frameworks, detrital facies, sands,
and gravels. Moreover, these cores revealed that the fringing reefs are characterized by the succession of two reef
generations, while the barrier reefs by several reef or carbonate unit generations, all separated by unconformities
resulting of emersion during the low sea stands. It appears
that the reef growth results from the combination of both
tectonic and climatic factors. The history of the lagoons
is not well known, but recent bathymetric and seismic surveys documented our knowledge on the structure of the
southwest lagoon.
Fringing reefs
In the fringing reefs of the Grande Terre, coral dating in 37
cores, drilled along perpendicular or parallel transects on
the reef flat, shows that the initiation growth stage is generally missing and that the reefs started to grow very
quickly and suddenly (Figure 2) (Cabioch et al., 1995).
In the south, the growth started prior to 5 ka BP upon
old reef karstic formations and in the north after 4.2 ka
BP, generally upon non-carbonate rocks, mainly metamorphic rock (Cabioch et al., 1995; Cabioch, 2003). These
differences in initiation age would be explained by
unavailable suitable foundations and also as resulting
from colonization of various types of substrates, the
karstic limestone surfaces being probably the most suitable substrate for coral-larval and attachment recruitment.
Core analysis also shows that framework, generally, dominates over skeletal debris in the higher energy reefs, while
skeletal debris are the main components in the lower
energy reefs (Cabioch, 2003). Moreover, the branching
and tabular coral assemblages (acroporids) are mainly
developed during the early stages of reef growth, whereas
the domal (favids, poritids) and/or robust-branching
(acroporids) assemblages appeared later (Figure 2).
Unfortunately, the reef slope and front edge zones were
not investigated by coring so that two interpretations were
proposed to explain this observation (Cabioch, 2003):
a progressive succession from low-energy to high-energy
water environments interpreted as a progressive increase
of water energy as the reef approaches the sea surface, typical of a catch-up reef growth mode (Growth classification
and sea level); or alternatively, a transition from lagoonal
deposits typified by coral and skeletal debris to reef
coralgal biofacies interpreted as a reef initiation,
retrograded over its back-reef. Examination of the distribution of reef facies from reef to reef also provides additional information and shows a strong relationship with
the antecedent topography (Antecedent platforms) and
the local water-energy conditions. The high water energy
reefs characterized by steeply dipping antecedent platforms are dominated by frameworks while medium water
energy reefs by abundant reef detritus.
Barrier reefs
The history and the development of barrier reefs in New
Caledonia are less known because it is more difficult to
drill in the barrier reef tracts strongly exposed to the oceanic conditions. One of the first studies was performed
by Coudray in 1976 at Ténia, on the west coast
(Figure 1). Analysis of the 226 m-long core revealed the
occurrence of several episodes of reef development during
several interglacial quaternary stages. More recently, cores
recovered at Amédée in the southwest and Kendec in the
northwest (Figure 1) (Cabioch et al., 2008) showed that
the New Caledonian barrier reefs are composed of
a succession of depositional units resulting from
a combination of glacio-eustatic sea level variations and
subsidence (Antecedent platforms). In both areas, 11 carbonate units deposited during the high sea stands are
observed, separated by unconformities resulting from
emersion during the glacial periods (low sea stands). It
appears that the reef growth would have begun around
400 ka in marine isotope stage 11, considered as
a particularly favorable period for coral development.
Prior to this period, the carbonate units are not characterized by biofacies typical of coral reefs sensu stricto
although scarce isolated corals can be found.
Lagoon
Lagoons are the transition between the two morphological
features of the fringing and barrier reefs, but their history
is still little known. Nevertheless, a recent high resolution
seismic and multibeam surveys of the South-West lagoon
(Figure 1) has revealed several depositional sequences
bounded by erosional unconformities, indicating two
periods of deposition and erosion in the past 200 ka (Leroy
et al., 2008). Two generations of incised deep fluvial valleys, connected by passes to the continental slope, were
identified. These data would indicate that the lagoonal history does not extend beyond 200 ka. In addition, the analysis
of morphology of this network of drainage suggests a progressive south-westward tilt of the outer lagoon expressing
the role of tectonics in the formation of the lagoons.
Climatic and/or oceanographic factors affecting
the reef growth and the coral development
The distribution and the growth of modern fringing and
barrier reefs are dependent not only on the tectonic context
but also on the climatic variations. The climatic variations
at the scale of the last 100,000 years (astronomical theory),
marked by sea-level variations of the glacial and interglacial periods, influenced the internal structure and reef succession in the barrier reef tracts (see above the history of
Barrier reefs). At the scale of the Holocene, the oceanographic factors and climatic variations influenced on the
initiation and growth of both barrier and fringing reefs.
NEW CALEDONIA
719