Neogene to Quaternary evolution of carbonate and mixed carbonate-siliciclastic systems along New Caledonia's eastern margin (SW Pacific)

Neogene and Quaternary shallow-water carbonate records surrounding New Caledonia main island, Grande Terre, provide a good example for understanding the stratigraphic architecture of tropical mixed carbonate-siliciclastic systems. Due to a southeastern tilt of the eastern margin, the eastern shelf of Grande Terre has been better preserved from erosion than the western part, favouring the development and preservation of shallow-water carbonates. Based on the integration of bathymetric and seismic data, along with paleoenvironmental and biostratigraphic constraints derived from dredged carbonate rocks, a comprehensive geomorphological and architectural characterization of the offshore eastern margin of Grande Terre has been made. During the Mio-Pliocene, a wide, up to 750 m-thick carbonate build-up developed and extended over at least 350 km from north to south. This Mio-Pliocene build-up, currently lying at 300 to 600 m water depths, is overlain by a Pleistocene-Holocene barrier reef-lagoon complex and associated slope deposits. The switch from aggrading Neogene carbonate banks to backstepping Quaternary platforms likely reflects an increase in accommodation due to a high subsidence rate or to relative sea-level rise, and/or results from a switch in carbonate producers associated with global environmental changes. The internal architecture of the Quaternary barrier reef-lagoon complex is highlighted, especially the development of lowstand siliciclastic prisms alternating with transgressive shallow-water carbonate sequences. This pattern agrees with the reciprocal sedimentation model typically invoked for mixed sedimentary systems. This stratigraphic pattern is well developed in front of the Cap Bayes inlet in the north of our study area, yet it is not observed southward along the eastern margin. This difference suggests that other factors than relative sea-level variations directed the architecture of the margin, such as low terrigenous inputs, lagoon paleo-drainage networks or sediment by-pass towards deep basins.