Post-obduction carbonate system development in New Caledonia (Népoui, Lower Miocene)
2016
For the first time, depositional models of Lower Miocene carbonate systems from New Caledonia (Southwest
Pacific) are proposed, on the basis of a sedimentological and paleoenvironmental study of both cores and outcrops.
In the Nepoui area, two distinct stages of carbonate ramp development (Aquitanian Lower Nepoui and Burdigalian
Upper Nepoui carbonate systems), separated by a phase of siliciclastic deltaic deposition, are evidenced. The postobduction
marine transgression of the Western New Caledonian margin occurred at approximately 24 Ma and
is characterized by the development of an aggrading foraminiferal–coralline algal–scleractinian ramp system
(“Chapeau Chinois Limestone”) during the early Aquitanian (24–23 Ma). A retrogradational event is evidenced
at approximately 23 Ma followed by the development of a shallowing upward carbonate unit (Operculina
“Green Sands” and Xuudhen Limestone) during the late Aquitanian. This unit is topped by a major erosional unconformity
overlain by conglomeratic deposits (“Pindai conglomerates”), and interpreted to record a significant
uplift at around 21–19 Ma. During the Burdigalian, a marine transgression occurred at around 19 Ma, followed
by the development of a low-angle carbonate ramp or open platform (“Nepu Limestone”) up to the late
Burdigalian (19–17 Ma). In both Aquitanian and Burdigalian carbonate ramps, extensive sea-grass meadows are
shown to have colonized the proximal ramp environments within the euphotic zone. In the Aquitanian carbonate
ramp (Lower Nepoui Formation), carbonate production within sea-grass meadows is dominated by large benthic
foraminifera, together with red algae and sparse scleractinians. Mesophotic environments are characterized by
large and flat lepidocyclinids, rhodoliths and platy corals whereas in deeper oligophotic settings significant carbonate
producers consist mainly of large and flat benthic foraminifera. In the Burdigalian carbonate ramp (Upper Nepoui Formation),
porcellaneous foraminifera thriving in sea-grass meadows together with red algae and scattered coral colonies
characterize the carbonate production in the euphotic zone. Antecedent topography is regarded as a major factor controlling
the extension of carbonate systems at regional and local scale. The thickness and development pattern of Lower
Miocene deposits from Nepoui are dominantly controlled by tectonic subsidence. Finally, extensive sea-grass development
promoted the dominance of foralgal carbonate production within the euphotic zone.
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