Neogene terrestrial sediments: a record of the post-obduction history of New Caledonia

The poorly studied iron-rich terrestrial sediments, referred to as the Fluvio-lacustrine Formation, that crop out in the southern part of the Grande Terre of New Caledonia document the last 25 Ma of the geological history of the island. The age of this formation, which is mainly derived from the erosion of an ultramafic regolith, is not tightly constrained yet; however, it has recorded several episodes of post-obduction erosion and sedimentary infill preceded and followed by weathering and reactivated erosion. A correlation with early Miocene slab break-off, which may have triggered a first stage of erosion marked by coarse conglomerate, is suggested. Thereafter, sediments filled topographic lows and were in turn weathered during an interval of tectonic quiescence. Finally, Holocene sea-level drop and southward tilt of southern New Caledonia, owing to the involvement of eastern Australian Plate in the New Hebrides (Vanuatu) subduction zone, locally changed the drainage pattern and deeply eroded the sediments.