Post-obduction tectono-sedimentary architecture of the Cornes Sud platform (New Caledonia): New insights from the POPCORN geophysical cruise (April 2026)

The lake region of the Massif Sud of New Caledonia, designated as a Ramsar site in 2014, represents the largest freshwater reservoir on the island and hosts fossil remnants of an ancient detrital system, the Fluvio-Lacustrine Formation (FLF). This formation is mainly composed of sediments eroded from the surrounding lateritic massifs and displays a high degree of internal structural complexity. Recent studies conducted at ISEA (University of New Caledonia) have shown that certain sedimentary levels are particularly enriched in metallic elements, raising questions about potential recurrent transfers between these sediments and present-day fluvial systems. To investigate the system as a whole using a “source-to-sink” approach, we carried out the POPCORN campaign (Post-obduction tectono-sedimentary characterization of the Cornes Sud platform of New Caledonia: What influence of ultrabasic massifs?), scheduled from 2 to 22 April 2026. The campaign aims to acquire very high-resolution (VHR) Sparker seismic profiles and multibeam bathymetry across the Cornes Sud platform, extending offshore from the FLF, combined with Kullenberg coring to provide geological and chronostratigraphic calibration of the geophysical data. The Cornes Sud platform, located between Grande Terre and Île des Pins, has received very limited investigation compared to the southwestern lagoon (Le Roy and Cabioch, 2004; Le Roy and Jorry, 2013; Le Roy et al., 2008, 2019) and the eastern margin (Chardon et al., 2008; Le Roy et al., 2022a, 2022b; Kerouédan et al., 2024a, 2024b). The western and eastern lagoons display contrasting morphologies associated with opposite vertical movements since the New Caledonian obduction (Lagabrielle et al., 2005 ; Tournadour et al., 2021). The Cornes Sud platform lies in direct continuation of the Massif Sud and forms the southern link between these two lagoons. It is characterized by several channelized systems (5-Miles, Prony, and Port Boisé), all affected by the Havannah Fault. The acquisition of Sparker seismic profiles and multibeam bathymetry, combined with sediment sampling, will allow identification and characterization of the tectono-sedimentary architecture of this poorly explored area and reconstruction of sedimentary and associated metallic element transfer pathways from continental fluvio-lacustrine systems (Fluvio-Lacustrine Formation, FLF) to carbonate platform deposits. This campaign forms part of a strategic research effort aimed at improving geological and environmental knowledge of a largely unexplored coastal zone. It is integrated within a broader research program focused on the FLF conducted by the same scientific team (TelluS-SYSTER PONCES and METALFLAP projects) and represents a key component of the land–sea continuum. The diversity of methodologies employed, together with the multidisciplinary nature of the research team, will enable reconstruction of the post-obduction evolution of the Massif Sud of New Caledonia based on sedimentary records, from the onshore Fluvio-Lacustrine Formation to the offshore Cornes Sud platform. Preliminary results from the POPCORN campaign will be presented here, focusing on seafloor morphology, sedimentary architecture, and styles of tectonic deformation within the study area. Particular emphasis is placed on identifying offshore expressions of the Fluvio-Lacustrine Formation previously studied on land.