Terrestrial Sedimentation

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Aus dem Unter-Ems der Klerf-Formation in der West-Eifel werden erstmals fossile Regentropfen-Marken vorgestellt und kurz diskutiert. Zusammen mit Trockenrissen-Verfüllungen belegen sie die zeitweise subaerische Exposition von Schichtflächen während der Sedimentation. Der Ablagerungsraum im untersuchten Abschnitt wird als möglicherweise tidal beeinflusste Deltaebene gedeutet. Im Profil dominieren tonig-schluffige Sedimente, die in flachem bis sehr flachem, wenig bewegtem Wasser zum Absatz gelangt sind.Temporär stärkere hydrodynamische Verhältnisse zeigen kleine, sandige Rinnenkörper an.
Abstract
Raindrop imprints as evidence of subaerial exposure from the
Klerf Formation (Lower Devonian, Emsian) of the Köppen Quarry at Waxweiler, Eifel
Fossil raindrop imprints from the lower Emsian Klerf Formation of the western Eifel are presented and briefly discussed for the first time. Together with mud-crack fillings they prove the temporary subaerial
exposure of bedding planes during sedimentation. The depositional environment of the studied section is interpreted as a possibly tidally influenced deltaic plain. The profile is dominated by clayey-silty sediments that were deposited in shallow to very shallow, less agitated water. Small, sandy channel bodies point to temporarily stronger hydrodynamic
conditions.

The Zapalco-Amacuzac area in the Morelos and Guerrero states, southern Mexico, constitutes one of the classic sectors where Paleogene continental beds of the Balsas Group were originally described. The exposed succession has an apparent thickness of ~2,500 m and it is constituted by two distinct sedimentary units separated by a slightly angular unconformity and by two intercalated ignimbrites (35.1 ± 0.4 and 38.5 ± 0.7 Ma, sanidine,. The basal beds of the lower sedimentary unit (Tepetlapa Formation) are constituted by matrix-supported conglomerates that grade upwards to sandstone with a poorly defined planar parallel stratification and some intercalated andesitic and mafic lava flows. One of the andesites yielded a whole rock K-Ar date of 44.5 ± 0.7 Ma. Intercalations of conglomerate beds increase again toward the top of this unit. The upper sedimentary unit (Huajintlán Formation) is constituted by a ~300 m thick succession of matrix-supported conglomerate with some lenses of conglomeratic sandstone. This unit unconformably underlies the Tilzapotla ignimbrite whose best age estimate is 34.3 Ma (sanidine Ar-Ar). The beds of the two sedimentary formations dip east to northeast with higher dips for the lower unit (50-30º). Recognized facies in both units are indicative of an alluvial fan environment with more proximal facies for the Huajintlán Formation. The studied succession is located immediately northwest of a large elliptical (50×30 km) dome structure that resulted from the tumescence stage, in the early evolution of the Tilzapotla caldera, previous to the collapse. The northwestern termination of the dome merges and interferes with the NNE-trending Laramide fold and thrust front of the Iguala Cacahuamilpa highs, near Taxco. In the study area, the dome contours display a local inflection defined by a change in the dip direction of the Morelos and basal beds of the Tepetlapa Formation from north to east. The slight angular unconformity between the Tepetlapa Formation and the overlaying units indicates a first stage of eastward tilting related to an uplift episode in the Taxco area, probably originated by the emplacement of silicic magmas (ca. 38 Ma). The proximal facies of the overlaying Huajintlán Formation and the age (35.1 ± 0.4 Ma) of the Piedras Altas Tuff , indicate that this formation resulted from the rejuvenation of the source area triggered by the tumescence and the initiation of the volcanic activity in the nearby Tilzapotla caldera area. This is confirmed by the higher dips (~50º) of the Tepetlapa Formation near the dome structure and the decreasing dips toward the northeast. The age of one andesite lava flow (44.5 Ma) intercalated in the lower unit is indicative of an Eocene age for the beginning of the Balsas Group accumulation in this area.

The post-obduction formations of Grande Terre, New Caledonia, comprise igneous intrusions, regolith cover, and marine and terrestrial sedimentary rocks. Two restricted Late Oligocene granitoid bodies are intruded into the Peridotite Nappe and its substrate in the south of the island. Thick regolith cover developed over the Peridotite Nappe from the Late Oligocene or earlier. The Népoui Group comprises Late Oligocene–Early Miocene mixed marine carbonate and siliciclastic deposits. It mainly reworks the Peridotite Nappe and its regolith cover. Its development pattern is mainly controlled by tectonic uplift and subsidence. The Gwa N'Doro Formation on the eastern coast and the Fluvio-lacustrine Formation in the south are remnants of the Miocene–Present river network. Offshore, thick Oligocene to Neogene sedimentary successions are imaged by seismic surveys on the margins of Grande Terre, although these successions have not been drilled and remain undated. Several dredges have recove...

Terra Nova, 24, 77–85, 2012Comparing palaeolatitudes estimated from remanent magnetization data of ferricrete developed over laterite at two sites (Goro and Tiébaghi) against the apparent polar wandering path of Australia provides new time constraints for the peridotite weathering in New Caledonia. The Tiébaghi Massif of the West Coast klippes displays a single episode of ferricrete development during latest Oligocene times. By contrast, the Goro site in the southern part of the ‘Massif du Sud' reveals a more complex history with several episodes of ferricrete development from latest Oligocene to Pliocene–Quaternary times. These data have made it possible to constrain the post‐obduction geodynamics and associated morphogenesis of New Caledonia since related uplift probably prevented regolith development until latest Oligocene times. Moreover, the later uplift and deep erosion of the West Coast klippes during the Early Miocene probably prevented further regolith preservation.

The uplift movements of the New Caledonia and the Loyalty Islands, which are located on the Indian lithospheric plate that descends beneath the New Hebrides arc, have been studied in the geological context. This paper concentrates on the older (since Oligocene) uplift of the ultramafic peneplain in New Caledonia and the younger (since about 2 m.y.) regional uplift of the Loyalty Islands and southern New Caledonia. We computed the amount of uplift and deformation caused by three possible processes: erosional unloading, eustatism, and flexure of the lithosphere as it underthrusts beneath the New Hebrides island arc. The calculations indicate that the older uplift cannot be completely explained by erosional unloading of New Caledonia and may be partially caused by dynamic factors not yet understood. Eustatic uplift of New Caledonia in compensation for the rise in sealevel is also of insufficient amplitude to explain the uplift observed since about the last 2 m.y. Studies of the flexure of the lithosphere caused by subduction beneath the New Hebrides arc provide an explanation for the observed recent uplift of the emergent atolls of the Loyalty Islands. * New Caledonia is 400 km long, about 45 km wide, surface area 18,000 km2. It is surrounded by a barrier reef which extends 200 k m beyond the northern limit of the island. ¡Il3 FE\@-6. 3is s. T;. %Je @qv ?+i

We present new data on the lithospheric bulge in the vicinity of the Loyalty and New Hebrides Islands, and attempt to interpret the difference of the topographic profile of the outer wall of the New Hebrides trench from a theoretical profile. The free edge of the lithospheric plate appears to correspond to a line of maximum seismic strain release. The uplift rate and consequently the subduction rate have been calculated from studies of the dynamic aspect of the bulge and from 230Th/234U ages of raised coral terraces.

New Caledonia holds one of the largest supergene nickel ore deposits worldwide. Two main ore types are recognized: i) oxide and ii) hydrous Mg silicate types. Our study focuses on the latter type, usually located in elevated parts of the Peridotite Nappe, forming the so-called plateau and slope deposits. Understanding the controls of these ore deposits is important in regard to sustainability of nickel mines; however, to date there are only a few models to refer to, and structural aspects have been generally underestimated. From its formation at a spreading ridge to Eocene obduction, the Peridotite Nappe of New Caledonia underwent several episodes of brittle fracture that mostly preceded weathering. We suggest that inherited fractures coated with serpentine "polymorphs" play a major role in the Ni enrichment process. Fracture analysis shows that this early fracture network share the same orientations with lineaments of peridotite massifs and controlled erosion and plateau dissection during uplift events. The subsequent steep slopes together with circulating waters within serpentinized fractures parallel to underlying valleys lead to slope failure. The collapse of plateau edges provoked plateau decompression and hence multi-directional extension. Gravity-driven faulting caused preferential opening of joints within the hanging walls of faults. The lateral permeability contrast favored preferential weathering and eventually resulted in an increase of Ni content in the saprock. The controlling faults have a specific polyphase infill, which could be a metallotect and a potential guide for nickel exploration.

Dolines are closed geomorphological depressions which are surface manifestations of karstic systems. Usually developed on limestones, they also typify the morphology of the New Caledonian landscape, particularly on the southern massif of the main island (known as Massif du Sud). The specificity of dolines here lies in their development on ultramafic rocks. They are evidences of subsidence, suffosion and collapse phenomena resulting from dissolution weathering of peridotites. However, extensive underground drainage systems are still not yet recognized. Semi-automatic mapping of dolines is carried out on a 148 km 2 area of the Massif du Sud from a high accuracy LiDAR digital elevation model. In this area 8601 dolines ranging from 1 m 2 to 2 km 2 are identified and morphologically characterized with precision. Most are small, shallow and round-shaped, yet more complex shapes are locally observed. Size distribution analysis allows the setting of a threshold of 20 000 m 2 above which surface processes rather than chemical weathering control doline evolution. Doline density analysis reveals high concentrations on flat areas where ferricrete overlies the complete weathering profile, especially in the case of elevated rainy watersheds. Dolines are aligned and elongated along a north 135°± 5°major fracture direction, which is inherited from the obduction of the Pacific Plate upper mantle in the Late Eocene. Finally, we propose a pioneering morphometric typology of dolines that provides important clues as to pseudokarstic activity. We define collapse, bowl-shaped and flat bottom dolines. Collapse and bowl-shaped dolines are assumed to denote active pseudokarst. They may widen and deepen, or eventually be filled by sediments. They are distinguished from flat bottom dolines that are partially to completely filled, which suggests that they are associated with paleo-pseudokarsts. However the groundwater flow paths associated with the genesis and evolution of dolines must be clarified, thus collapse and bowl-shaped dolines should be hydrologically monitored.

A high-resolution cyclostratigraphic and magnetostratigraphic study was carried out on cyclically bedded successions of middle Miocene lacustrine to distal alluvial fan-floodplain deposits from the Calatayud basin, in northeast Spain. Eight (partially overlapping) subsections near the village of Orera are correlated in detail using distinct sedimentary cycle patterns or by following marker beds in the field. Together they form the Orera Composite Section (OCS). Sedimentary cycles are recognised on at least two different scales. The basic small-scale cycles in the OCS consist of an alternation of grey and, occasionally, red clays with white, dolomite-rich, carbonate beds. They are arranged in largerscaled, so-called large-scale cycles based on repetitive changes in the overall carbonate^clay lithology. Two other, but less distinct, types of intermediate scale cyclicity are also recognised. In terms of depositional environment, the cycle hierarchy is interpreted to represent periodic lake expansion over the palaeo-alluvial fan-floodplain area. The palaeomagnetic results yield a reliable magnetostratigraphic record, which confirms the cyclostratigraphic correlations between the subsections in detail. Rock magnetic experiments reveal that haematite is the main magnetic carrier of the primary component. The magnetostratigraphy of the OCS is correlated straightforwardly with the geomagnetic polarity time scale. This resulted in an age of 10.7^12.8 Ma for the entire succession, which is supported by fossil micromammal findings. In addition, it also reveals the presence of two, possibly three, short new polarity intervals. Based on the number of cycles in the OCS the average periodicity of the basic small-scale cycles is approximately 23 000 yr, while the large-scale cycle indicates a periodicity of 400 000 yr. This suggests that these sedimentary cycles are controlled by astronomically induced climate changes causing lake-level fluctuations and thus resulting in the deposition of carbonate-clay cycles. The continental sequences of the OCS provide a unique opportunity to extend the astronomical polarity time scale into the middle Miocene. The overlap of such continental sequences in the Mediterranean area with time-equivalent astronomically induced marine sequences is fundamental for establishing marine^continental, bed-tobed, correlations and for understanding regional climate change.

A high-resolution cyclostratigraphic and magnetostratigraphic study was carried out on cyclically bedded successions of middle Miocene lacustrine to distal alluvial fan-floodplain deposits from the Calatayud basin, in northeast Spain. Eight (partially overlapping) subsections near the village of Orera are correlated in detail using distinct sedimentary cycle patterns or by following marker beds in the field. Together they form the Orera Composite Section (OCS). Sedimentary cycles are recognised on at least two different scales. The basic small-scale cycles in the OCS consist of an alternation of grey and, occasionally, red clays with white, dolomite-rich, carbonate beds. They are arranged in largerscaled, so-called large-scale cycles based on repetitive changes in the overall carbonate^clay lithology. Two other, but less distinct, types of intermediate scale cyclicity are also recognised. In terms of depositional environment, the cycle hierarchy is interpreted to represent periodic lake expansion over the palaeo-alluvial fan-floodplain area. The palaeomagnetic results yield a reliable magnetostratigraphic record, which confirms the cyclostratigraphic correlations between the subsections in detail. Rock magnetic experiments reveal that haematite is the main magnetic carrier of the primary component. The magnetostratigraphy of the OCS is correlated straightforwardly with the geomagnetic polarity time scale. This resulted in an age of 10.7^12.8 Ma for the entire succession, which is supported by fossil micromammal findings. In addition, it also reveals the presence of two, possibly three, short new polarity intervals. Based on the number of cycles in the OCS the average periodicity of the basic small-scale cycles is approximately 23 000 yr, while the large-scale cycle indicates a periodicity of 400 000 yr. This suggests that these sedimentary cycles are controlled by astronomically induced climate changes causing lake-level fluctuations and thus resulting in the deposition of carbonate-clay cycles. The continental sequences of the OCS provide a unique opportunity to extend the astronomical polarity time scale into the middle Miocene. The overlap of such continental sequences in the Mediterranean area with time-equivalent astronomically induced marine sequences is fundamental for establishing marine^continental, bed-tobed, correlations and for understanding regional climate change.

Page 2 of 16 with characteristics indicating that they were emplaced during pervasive top-to-SW shear deformation. The oxygen isotope composition of magnesite is homogeneous (27.4 ‰ < δ O < 29.7 ‰) while its carbon isotope composition varies widely (−16.7 ‰ < δ 13 C < −8.5 ‰). These new data document an origin of magnesite from low temperature meteoric fluids. Laterization on top of the Peridotite Nappe and carbonation along the sole appear to represent complementary records of meteoric water infiltration. Based on the syn-kinematic character of magnesite veins, we propose that syn-laterization tectonic activity has enhanced water infiltration, favoring the exportation of leached elements like Mg, which led to widespread carbonation along the serpentine sole. This calls for renewed examination of other magnesite-bearing ophiolites worldwide in order to establish whether active tectonics is commonly a major agent for carbonation.

Eluvial concentration of platinum-group minerals (PGM) has developed in the south of New Caledonia, from the weathering of a mafic and ultramafic cumulate. The platinum/palladium (Pt/Pd) ratio evolution from the bottom to the top of the weathering profile indicates a chemical mobilization of Pd in supergene environment. The examination by scanning electron microscopy of the residual PGM collected in the weathering profile and their chemical characterization by electronic microprobe show a preservation of the structure and chemical signature (in the core of the grains) of the fresh PGM. This clearly demonstrates that the PGM studied here are residual and are affected by dissolution process.

This work aims to elucidate the factors governing the mobility of metals in soils of a lateritic nickel ore deposit in New Caledonia. The transfer of nickel and associated metals is determined along a topographic sequence ranging from a plateau to a thalweg. Mining exploration borehole data and soil pit data enabled us to define the general geochemical trends of the lateritic weathering. The homogeneous topsoil, which consists mainly of iron oxy-hydroxides, exhibits various amounts of plant available metals. Two interrelated factors control the differences in metal availability: (1) the presence of nickel-rich soil horizons underlying the topsoil and formed of silicate minerals, and (2) the biological cycling of metals from these horizons to the surface. These results have major implications for topsoil management during the mining activity where it is important to minimise metal inputs to water systems and to restore endemic vegetation after mining.

Extensive mapping of relict lateritic landsurfaces over the 1600 m of relief of mainland New Caledonia is used to evaluate longterm morphogenesis of the island since its emergence in the Oligocene. The results support the interpretation of the lateritic levels as abandoned, stepped remnant landsurfaces that may be grouped into eight island-scale erosion levels recording episodic lowering of base level. The separation of the erosion levels results from the combination of positive epeirogenic movements of the New Caledonia continental ridge and long-term Neogene climate variability, which is recorded by the contrasted erosion and weathering patterns of the three youngest levels and tentatively correlated with known global and regional paleoclimatic events.

The New Caledonia Ophiolite (Peridotite Nappe), represents about one third of the island’s surface (i.e. 5 500 km). The ophiolite is composed of harzburgites, dunites, lherzolites, minor mafic-ultramafic cumulates, and various dykes and sills. The mantle section underwent a polyphase evolution, which involved prominent depletion and re-fertilization. The oldest events are probably recorded by abyssal-type lherzolites of the northern massifs, which bear traces of moderate partial melting. Plagioclase lherzolites were formed by shallow entrapment of highly depleted MORB melt in residual spinel lherzolites. Nd isotope compositions are consistent with derivation from an asthenospheric mantle source that experienced a recent MORB-producing depletion. This evolution was most likely accomplished during the late Cretaceous breakup of the eastern Australian margin. The harzburgite-dunite association, which forms the bulk of Peridotite Nappe was probably formed through a multistage magma-prod...

Page 2 of 16 with characteristics indicating that they were emplaced during pervasive top-to-SW shear deformation. The oxygen isotope composition of magnesite is homogeneous (27.4 ‰ < δ O < 29.7 ‰) while its carbon isotope composition varies widely (−16.7 ‰ < δ 13 C < −8.5 ‰). These new data document an origin of magnesite from low temperature meteoric fluids. Laterization on top of the Peridotite Nappe and carbonation along the sole appear to represent complementary records of meteoric water infiltration. Based on the syn-kinematic character of magnesite veins, we propose that syn-laterization tectonic activity has enhanced water infiltration, favoring the exportation of leached elements like Mg, which led to widespread carbonation along the serpentine sole. This calls for renewed examination of other magnesite-bearing ophiolites worldwide in order to establish whether active tectonics is commonly a major agent for carbonation.

Chemical weathering and supergene dissolution processes of Pt-bearing chromite have been studied in a lateritic weathering profile developed on ultramafic rocks in New Caledonia (southwest Pacific). The chemical distributions of alkaline earth, transition metals, and precious metals (including Pt and Pd) were determined in a weathering profile varying from bedrock at the base upward through coarse and fine saprolites, and capped by a mottled zone and a lateritic colluvial nodular horizon. Chemical analyses and mass-balance calculations suggest that progressive weathering of the parent rock is characterized by an enrichment of Fe, Co, and Mn, a segregation of Ni at the boundary between the bedrock and the coarse sapolite and in the lower part of the fine saprolite, and a depletion of Mg, Ca, Si, Al, and Cr. The higher concentration of transition metals at the interface between the coarse and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile. In such a lateritic environment, the Pt-bearing chromite grains are progressively dissolved and the Pt-group minerals (PGM) are released in the weathering mantle with a preferential depletion of Pd with regard to Pt.

Page 2 of 16 with characteristics indicating that they were emplaced during pervasive top-to-SW shear deformation. The oxygen isotope composition of magnesite is homogeneous (27.4 ‰ < δ O < 29.7 ‰) while its carbon isotope composition varies widely (−16.7 ‰ < δ 13 C < −8.5 ‰). These new data document an origin of magnesite from low temperature meteoric fluids. Laterization on top of the Peridotite Nappe and carbonation along the sole appear to represent complementary records of meteoric water infiltration. Based on the syn-kinematic character of magnesite veins, we propose that syn-laterization tectonic activity has enhanced water infiltration, favoring the exportation of leached elements like Mg, which led to widespread carbonation along the serpentine sole. This calls for renewed examination of other magnesite-bearing ophiolites worldwide in order to establish whether active tectonics is commonly a major agent for carbonation.

Chemical weathering and supergene dissolution processes of Pt-bearing chromite have been studied in a lateritic weathering profile developed on ultramafic rocks in New Caledonia (southwest Pacific). The chemical distributions of alkaline earth, transition metals, and precious metals (including Pt and Pd) were determined in a weathering profile varying from bedrock at the base upward through coarse and fine saprolites, and capped by a mottled zone and a lateritic colluvial nodular horizon. Chemical analyses and mass-balance calculations suggest that progressive weathering of the parent rock is characterized by an enrichment of Fe, Co, and Mn, a segregation of Ni at the boundary between the bedrock and the coarse sapolite and in the lower part of the fine saprolite, and a depletion of Mg, Ca, Si, Al, and Cr. The higher concentration of transition metals at the interface between the coarse and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile. In such a lateritic environment, the Pt-bearing chromite grains are progressively dissolved and the Pt-group minerals (PGM) are released in the weathering mantle with a preferential depletion of Pd with regard to Pt.

A lower Miocene coarse conglomerate that crops out in the Népoui Peninsula does not represent the base of the marine transgression that followed obduction in New Caledonia. Instead, the conglomeratic alluvial fan that contains peridotite cobbles and reworked weathering products records a short-lived episode of terrestrial erosion intercalated between two intervals of subsidence marked by marine carbonate deposition. Considering the Miocene sea level evolution reported in the literature, it is concluded that neither lower Miocene transgression nor erosion were driven by sea level variation. In contrast, a southeastward propagating slab tear that initiated at the latitude of the high pressure/low temperature metamorphic complex of northern New Caledonia likely generated east to west tilting of New Caledonia, subsidence along the West Coast and hence fringing reef development together with moderate erosion of older regolith. Coincidence between conglomerate deposition and hence prominent erosion that closely followed emplacement of postobduction granitoids influenced by a slab window suggests a genetic link. Therefore, it is concluded that short-lived lower Miocene erosion was due to slab breakoff and subsequent uplift that occurred at~22 Ma. Lower Miocene erosion profoundly dissected the Peridotite Nappe and in the northern half of New Caledonia only left isolated klippes along the West Coast.

Page 2 of 16 with characteristics indicating that they were emplaced during pervasive top-to-SW shear deformation. The oxygen isotope composition of magnesite is homogeneous (27.4 ‰ < δ O < 29.7 ‰) while its carbon isotope composition varies widely (−16.7 ‰ < δ 13 C < −8.5 ‰). These new data document an origin of magnesite from low temperature meteoric fluids. Laterization on top of the Peridotite Nappe and carbonation along the sole appear to represent complementary records of meteoric water infiltration. Based on the syn-kinematic character of magnesite veins, we propose that syn-laterization tectonic activity has enhanced water infiltration, favoring the exportation of leached elements like Mg, which led to widespread carbonation along the serpentine sole. This calls for renewed examination of other magnesite-bearing ophiolites worldwide in order to establish whether active tectonics is commonly a major agent for carbonation.

2) CNRS-UMR 7619 Sisyphe &quot;Structure et fonctionnement des systèmes hydriques continentaux&quot;, France, (3) IPGP Laboratoire de Paléomagnétisme et de Géomagnétisme, 4 Place Jussieu 75252 Paris Cedex 05, France, (4) BRGM – Guyane, Domaine de Suzini, route de Montabo, B.P.

Eluvial concentration of platinum-group minerals (PGM) has developed in the south of New Caledonia, from the weathering of a mafic and ultramafic cumulate. The platinum/palladium (Pt/Pd) ratio evolution from the bottom to the top of the weathering profile indicates a chemical mobilization of Pd in supergene environment. The examination by scanning electron microscopy of the residual PGM collected in the weathering profile and their chemical characterization by electronic microprobe show a preservation of the structure and chemical signature (in the core of the grains) of the fresh PGM. This clearly demonstrates that the PGM studied here are residual and are affected by dissolution process. D

Although the description of the emplacement and the weathering of the New Caledonia peridotites are well documented in the literature (Trescases, 1975), the processes and the age of the planation surfaces formed upon the ultramafic massifs are poorly documented. Ambiguities remain on: 1) the timing and modality of weathering, 2) formation of supergene nickel ore and 3) the role of the parameters controlling its evolution: •The overall geometry of the ophiolitic nappes and structures possibly inherited from the obduction history, •The lithology of the bedrock (dunite, wehrlite, harzburgite, gabbro, amount of serpentinisation ...), •The post-obduction uplift history, and the paleogeographic and paleoclimatic evolution of New Caledonia up to Present. •The geomorphologic evolution related to recent tectonics and sea level changes, Several planation surfaces have been recognised along the island and their correlation in the southern part of the Grande Terre has been done (Chardon and Che...

Extensive mapping of relict lateritic landsurfaces over the 1600 m of relief of mainland New Caledonia is used to evaluate longterm morphogenesis of the island since its emergence in the Oligocene. The results support the interpretation of the lateritic levels as abandoned, stepped remnant landsurfaces that may be grouped into eight island-scale erosion levels recording episodic lowering of base level. The separation of the erosion levels results from the combination of positive epeirogenic movements of the New Caledonia continental ridge and long-term Neogene climate variability, which is recorded by the contrasted erosion and weathering patterns of the three youngest levels and tentatively correlated with known global and regional paleoclimatic events.

Eluvial concentration of platinum-group minerals (PGM) has developed in the south of New Caledonia, from the weathering of a mafic and ultramafic cumulate. The platinum/palladium (Pt/Pd) ratio evolution from the bottom to the top of the weathering profile indicates a chemical mobilization of Pd in supergene environment. The examination by scanning electron microscopy of the residual PGM collected in the weathering profile and their chemical characterization by electronic microprobe show a preservation of the structure and chemical signature (in the core of the grains) of the fresh PGM. This clearly demonstrates that the PGM studied here are residual and are affected by dissolution process. D

This work aims to elucidate the factors governing the mobility of metals in soils of a lateritic nickel ore deposit in New Caledonia. The transfer of nickel and associated metals is determined along a topographic sequence ranging from a plateau to a thalweg. Mining exploration borehole data and soil pit data enabled us to define the general geochemical trends of the lateritic weathering. The homogeneous topsoil, which consists mainly of iron oxy-hydroxides, exhibits various amounts of plant available metals. Two interrelated factors control the differences in metal availability: (1) the presence of nickel-rich soil horizons underlying the topsoil and formed of silicate minerals, and (2) the biological cycling of metals from these horizons to the surface. These results have major implications for topsoil management during the mining activity where it is important to minimise metal inputs to water systems and to restore endemic vegetation after mining. D

Chemical weathering and supergene dissolution processes of Pt-bearing chromite have been studied in a lateritic weathering profile developed on ultramafic rocks in New Caledonia (southwest Pacific). The chemical distributions of alkaline earth, transition metals, and precious metals (including Pt and Pd) were determined in a weathering profile varying from bedrock at the base upward through coarse and fine saprolites, and capped by a mottled zone and a lateritic colluvial nodular horizon. Chemical analyses and mass-balance calculations suggest that progressive weathering of the parent rock is characterized by an enrichment of Fe, Co, and Mn, a segregation of Ni at the boundary between the bedrock and the coarse sapolite and in the lower part of the fine saprolite, and a depletion of Mg, Ca, Si, Al, and Cr. The higher concentration of transition metals at the interface between the coarse and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile. In such a lateritic environment, the Pt-bearing chromite grains are progressively dissolved and the Pt-group minerals (PGM) are released in the weathering mantle with a preferential depletion of Pd with regard to Pt.

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 could document the last 25 Ma of the geological history of the island. The age of this Formation, which mainly comes 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. Afterwards, sediments filled-up 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 due to the involvement of eastern Australian Plate in the New Hebrides (Vanuatu) subduction zone locally changed the drainage pattern and deeply eroded the sediments.

Extensive mapping of relict lateritic landsurfaces over the 1600 m of relief of mainland New Caledonia is used to evaluate longterm morphogenesis of the island since its emergence in the Oligocene. The results support the interpretation of the lateritic levels as abandoned, stepped remnant landsurfaces that may be grouped into eight island-scale erosion levels recording episodic lowering of base level. The separation of the erosion levels results from the combination of positive epeirogenic movements of the New Caledonia continental ridge and long-term Neogene climate variability, which is recorded by the contrasted erosion and weathering patterns of the three youngest levels and tentatively correlated with known global and regional paleoclimatic events.