Synthesis of ages obtained by paleomagnetism on ferricretes and iron-rich sediments of New Caledonia. Implications on the morphogenesis of Grande-Terre
Résumé
Although the description of the emplacement and the weathering of the New Caledonia peridotites are well documented, the reconstruction of the planation surfaces formed upon the ultrabasic massifs are poorly documented. Ambiguities remain on the timing and modality of weathering and related supergene nickel ore. The data and interpretations presented here are based on paleomagnetic analysis of lateritic ferricretes and fluvial sediments. In Tiébaghi an age of 25 Ma has been obtained along a section crossing the plateau. Well constrained ages of 0-5 Ma to 25 Ma have been obtained in Goro, highlighting possible stepped paleosurfaces. The Tiébaghi Massif displays a single episode of ferricrete development during latest Oligocene times, whereas the Goro site reveals several episodes of ferricrete development from latest Oligocene to Pliocene-Quaternary times. Samples from the Kopeto-Boulinda Massif and lower Nepoui surfaces have been further studied; they could not be dated by paleomagnetism, except the silico-ferruginous matrix of the Nepoui conglomerate giving an age close to 10-25 Ma. This age is rather imprecise but 1) the weathering of the fluvial conglomerates fossilized by the silicification followed their deposition and 2) the conglomerates are locally interstratified in Lower Miocene marine limestones. Moreover the detailed petrographic study of the ferricretes confirms rearrangements of nodules and pisolites, more important in the upper parts of the toposequence, where they also follow the smooth slopes of the topography. They probably result from the dismantling of early lateritic ferricrete and laterites capping a former landscape. All the data can be interpreted in reconstructing a fossil paleolandscape in New Caledonia triggered by a Lower Miocene uplift, responsible for the drastic erosion of the oldest profiles, the creation of highly incised river systems and the deposition of coarse fluvial conglomerates. In the Fluvio-lacustrine Fm of the southern Grande Terre, new paleomagnetic data enable to reconstruct old (25 to 15 Ma) ferruginisation episodes interstratified within fluvial units, while younger (0-5 Ma) secondary concretioning phases are also recorded. They could be attributable to recent incision of the succession, until the creation of the current hydrographic network.