Subsidence and sea-level controls on the evolution of a mixed siliciclastic-carbonate platform: the eastern margin of New Caledonia (SW Pacific)

Abstract

The island of Grande-Terre in New Caledonia (NC) hosts one of the world's largest lagoon complexes, and its sediment infill makes it an ideal case study to understand the evolution of mixed siliciclastic-carbonate platforms. The under-researched eastern margin of Grande-Terre provides an opportunity to examine the major changes in lagoon sedimentation since the end of the Mio-Pliocene in response to climate variation, vertical movements and anthropogenic activities. Using geomorphological and seismic interpretations constrained with dated sediment cores, this study identifies three seismic sequences separated by major channelized and erosional unconformities. The upper and last unconformity is supposed to be linked to the sea-level lowstand during the Last Glacial Maximum. The overlying sedimentary record, dated to the Holocene, contains both clastic and carbonate deposits, resulting from the reflooding of the shelf. This mixed sediment infill displays significant spatial variation. The inner lagoon concentrates terrigenous inputs whereas the median lagoon is dominated by a series of large carbonate reef flats. The outer shelf preserves a remarkable succession of falling-stages deposits that reflect 100 kyr depositional sequence cycles resting atop the Mio-Pliocene carbonate platform. Stratigraphic analysis of these Pleistocene sequences dates their deposition from 630 kyr (MIS 16) and allows us to estimate a subsidence rate of approximately 0.06 m/kyr on the outer shelf, enabling the barrier reef to keep up and the successive lowstand sediment wedges to stack.