Apport de la gravimétrie à l'étude des structures profondes du Sahel de Tunisie (cas de la région de Kairouan–Sousse–Monastir)

The monotony of the surface and of the deep structure of the Sahel domain in eastern Tunisia (low topographic area covered by a Quaternary series) induces the possible existence of an important subsiding collapsed block and associated faulted zones. Gravity data analyses have permitted the reconnaissance of the crustal and gravimetric setting of the northern part of the Sahel domain and the discussion of main outlines of subsurface structures. The deep structure of a particular zone (Kairouan–Sousse–Monastir area) demonstrates the existence of an east-west en-doigt-de-gant crustal thinning confirmed by the gravity data. This deep structuring is perfectly showed by the high-resolution second-order enhanced analytic signal technique developed to image geologic boundaries such as contacts and faults. This technique, correlated with the distribution of all seismic events in the last century, has permitted to define an important east–west Kairouan–Sousse–Monastir tectonic corridor (CKSM). This corridor corresponds to major faults oriented east-west, were some folded structures can be developed. To cite this article: H. Gabtni, C. R. Geoscience 337 (2005).     

Calpionellid zonation of the Jurassic–Cretaceous transition in North-Atlasic Tunisia. Updated Upper Jurassic stratigraphy of the ‘Tunisian trough’ and regional correlations

The analysis of calpionellid associations from jebels Amar and Jédidi sections in North-Atlasic Tunisia provides, for the first time, a precise biozonation of the Jurassic–Cretaceous transition succession. In the light of the new data obtained and considering recently published results, the age of Upper Jurassic formations is clarified, allowing correlations with the Tunisian ‘Dorsale’ and the North–South Axis successions. Within the Maghrebides' range, sections from the external zones correlated to the Tunisian successions are quite distinctive from their equivalent in the internal zones. Both have evolved in different palaeogeographic domains related to the early structuration of the northwestern and southwestern Tethys margins. To cite this article: M. Boughdiri et al., C. R. Geoscience 338 (2006).     

HP–LT Variscan metamorphism in the Cubito-Moura schists (Ossa-Morena Zone, southern Iberia)

Multi-equilibrium thermobarometry shows that low-grade metapelites (Cubito-Moura schists) from the Ossa–Morena Zone underwent HP–LT metamorphism from 340–370 °C at 1.0–0.9 GPa to 400–450 °C at 0.8–0.7 GPa. These HP–LT equilibriums were reached by parageneses including white K mica, chlorite and chloritoid, which define the earliest schistosity (S₁) in these rocks. The main foliation in the schists is a crenulation cleavage (S₂), which developed during decompression from 0.8–0.7 to 0.4–0.3 GPa at increasing temperatures from 400–450 °C to 440–465 °C. Fe³⁺ in chlorite decreased greatly during prograde metamorphism from molar fractions of 0.4 determined in syn-S₁ chlorites down to 0.1 in syn-S₂ chlorites. These new data add to previous findings of eclogites in the Moura schists indicating that a pile of allochtonous rocks situated next to the Beja-Acebuches oceanic amphibolites underwent HP–LT metamorphism during the Variscan orogeny. To cite this article: G. Booth-Rea et al., C. R. Geoscience 338 (2006).     

SEM–EDS analysis of western Mediterranean obsidians: a new tool for Neolithic provenance studies

The contents of Na, Al, Si, K, Ca and Fe of 99 obsidians from the western Mediterranean islands of Lipari, Palmarola, Pantelleria and Sardinia were determined with the energy-dispersive spectrometer of a scanning electron microscope (SEM–EDS). The Na and in a lesser way other elements contents characterize any obsidian source-island. In Sardinia (80 samples), the four Monte Arci obsidian geochemical types can be discriminated from binary diagrams of element contents or by a discriminant analysis based on the six elements measured. It is concluded that SEM–EDS offers a new option for Neolithic obsidian provenance studies in this region. To cite this article: F.-X. Le Bourdonnec et al., C. R. Geoscience 338 (2006).     

Discrimination between primary magmatic biotites, reequilibrated biotites and neoformed biotites

The ternary diagram TiO₂–FeO*–MgO (FeO* = FeO + MnO) is proposed as a quantitative objective tool for distinguishing between primary magmatic biotites and those that are more or less reequilibrated, or possibly neoformed, by or within a hydrothermal fluid. The limit of the domains of the primary magmatic biotites, the reequilibrated biotites and the neoformed biotites were determined on the basis of optical, paragenetic and chemical criteria. To cite this article: H. Nachit et al., C. R. Geoscience 337 (2005).     

Enhancement of regional variations in salinity and temperature in a coral reef lagoon, New Caledonia

The variability in salinity and temperature in the southwest lagoon of New Caledonia (2100 km²) under non-storm conditions is analysed using a 4-year dataset (1997–2001). Seasonal and interannual variations in salinity are amplified nearshore. Temperature is larger in bays than in the open lagoon during summer, the reverse is true in winter. El Niño Southern Oscillation (ENSO) related nearshore temperature variations parallel those at regional scale, but with seasonal variations amplified. Haline stratification, if any, is enhanced in coastal areas. To cite this article: S. Ouillon et al., C. R. Geoscience 337 (2005).     

The crystalline complexes of Hamadan (Sanandaj–Sirjan zone, western Iran): metasedimentary Mesozoic sequences affected by Late Cretaceous tectono-metamorphic and plutonic events

The Hamadan area is characterised by various metamorphic rocks where the slates yielded Jurassic fossils. The entire column, representing the Mesozoic from at least the Jurassic to the Mid-Cretaceous, has been affected by tectono-metamorphic events and the emplacement of Late Cretaceous granitic rocks. A timing of these events is based on the ⁴⁰K–⁴⁰Ar ages carried mainly on separated amphiboles, biotites and muscovites, and interpreted as the ages of their isotopic closure. Results are ranging between 91 and 70 Ma. To cite this article: A. Baharifar et al., C. R. Geoscience 336 (2004).

Résumé

La région de Hamadan expose des roches métamorphiques dont les termes les moins transformés contiennent des fossiles jurassiques. Au cours du Crétacé supérieur, elle a été affectée par un événement tectono-métamorphique régional et elle a été le siège d'une activité plutonique. Les résultats des datations ⁴⁰K–⁴⁰Ar des amphiboles et des micas séparés des roches métamorphiques et plutoniques qui s'étagent entre 91 et 70 Ma montrent l'importance de ces événements et leur étalement au cours du Crétacé supérieur. Pour citer cet article : A. Baharifar et al., C. R. Geoscience 336 (2004).     

Les argiles bleues du Cambrien inférieur de Saint-Pétersbourg et leur fissuration. Implications pour des stockages souterrains

The Lower Cambrian Saint Petersburg blue clays are composed of predominant illite and chlorite, sometimes accompanied by kaolinite. The <0.1 μm fraction has a high content of illite–smectite mixed layers. Particle-size distribution is more than 50% of clay particles and about 30% of silts. These blue clays correspond to plastic (and soft) clays; they may be compared to the Callovian clays of Bure (France), where storage of natural waste is envisaged. To cite this article: M. Arnould et al., C. R. Geoscience 334 (2002) 1135–1140.

Résumé

Les « argiles bleues » du Cambrien Inférieur de Saint-Pétersbourg sont constituées d'illite dominante et de chlorite, avec parfois présence de kaolinite. Des interstratifiés illite/montmorillonite sont très abondants dans la phase <0,1 μm. Du point de vue granulométrique, outre plus de 50% de particules argileuses, il existe une phase silteuse de l'ordre de 30%. Du point de vue pétrophysique, ce sont des argiles plastiques, de consistance molle. Elles sont subhorizontales et ont jusqu'à 116 m d'épaisseur. Outre l'absence de métamorphisme, malgré leur âge, leur caractère le plus remarquable est leur réseau de fracturation, bien observable en carrière. Les joints sont nets, sans remplissage ni cimentation. Les plans verticaux sont particulièrement développés. Le volume unitaire des blocs de matrice ne dépasse guère 1 m³. Des traces d'oxydation témoignent de circulations d'eau. Ces argiles silteuses anciennes peuvent aider à mieux connaı̂tre et comprendre les argilites silteuses épigénétiques calloviennes du site de Bure (Haute-Marne, France), où un stockage souterrain de déchets nucléaires est envisagé. Pour citer cet article : M. Arnould et al., C. R. Geoscience 334 (2002) 1135–1140.     

Enregistrement de phénomènes de resédimentation au cours du Maastrichtien supérieur et du Paléocène supérieur (J. Serj, Tunisie atlasique) : apport micropaléontologique et signature tectono-eustatique

Sedimentological and biostratigraphic analysis of Campanian–Maastrichtian reduced series of the J. Serj permitted to characterize two episodes of submarine erosion. The first is pre-Upper Maastrichtian (Rosita contusa zone), the second is pre-Upper Palaeocene (Morozovella velascoensis zone). Thus, two type-1 boundary surfaces are distinguished. These surfaces limit three stratigraphic intervals in apparent continuity. Syn-sedimentary tectonic activity is attested by gravity deposits or processes (conglomerates, slumps). To cite this article: A. Amri et al., C. R. Geoscience 337 (2005).     

Le gisement AgHg de Zgounder (Jebel Siroua, Anti-Atlas, Maroc) : un épithermal néoprotérozoïque de type Imiter

The Zgounder ore deposit (Anti-Atlas, Morocco), is hosted in a PII–PIII Proterozoic volcanosedimentary series. Disseminated mineralization is dominated by mercuriferous native silver (2 to 30 wt.% Hg), with few silver sulfosalts (acanthite, pearceite), arsenopyrite and base-metal sulfides. Arsenic grade of arsenopyrite and homogenisation temperatures of fluid inclusions indicate initial conditions of high temperature (above 400 °C). Lead isotope compositions comfort a Late-Proterozoic age and a crustal origin for metals. Similarities are obvious with the neighbouring silver ore deposit of Imiter and lead to consider Zgounder as another example of Neoproterozoic epithermal deposit in the Anti-Atlas of Morocco, a region that appears more and more as a silver metallogenic province. To cite this article: É. Marcoux, A. Wadjinny, C. R. Geoscience 337 (2005).     

L'érosion des chaînes de montagnes : influence de la sédimentation de piedmont

The applanation of mountain belts that results in peneplain is generally considered to be caused by the long-term activity of erosion. Peneplanation has been previously defined as the lowering of an elevated topography and the concomitant subduing of its relief. We propose a model following which piedmont sedimentation induces the base level rise, allowing applanation to develop at high elevation and resulting in an elevated ‘peneplain’. This model is illustrated by the morphological evolution of the southern flank of the Pyrenees during the Cainozoic. To cite this article: J. Babault, J. Van Den Driessche, C. R. Geoscience 337 (2005).     

Tracing and dating recent chemical transfers in weathering profiles by trace-element geochemistry and U---U---Th disequilibria: the example of the Kaya lateritic toposequence (Burkina-Faso)

The paper presents a synthesis of the approach recently developed to constrain the nature and the age of recent element mobility in weathering profiles. The approach relies on a comparison of variations of trace elements and U-series disequilibria in weathering profiles. It is exemplified in the case of old lateritic profiles from the Kaya toposequence, Burkina Faso. Trace elements and ²³⁸U---²³⁴U---²³⁰Th disequilibria have been analysed in whole rock samples from two pits located in very contrasted topographical positions. Trace element data show that the whole toposequence is marked by an intense chemical remobilization, including uranium, from the cap to the lower part of the profiles. ²³⁸U---²³⁴U---²³⁰Th disequilibrium data outline that all the levels of the profiles are affected by recent U---Th fractionations, and that each level of the toposequence is marked by U gains and losses. The chronological approach developed in this paper leads to an age of about 400 kyr for the dismantling of the iron cap, and ages ranging from 0 to 400 kyr for U accumulation in the pink clay horizon of the profiles. The depth repartition of U accumulation in the profile implies that the remobilisation processes in this toposequence varied through time, on time constants similar to those of climatic variations. To cite this article: F. Chabaux et al., C. R. Geoscience 335 (2003).     

A Late Hauterivian short-lived anoxic event in the Mediterranean Tethys: the ‘Faraoni Event’

A Late Hauterivian interval (127.5 Ma), called the ‘Faraoni Event’, which is characterised by the deposition of deep-marine black shales in the Mediterranean Tethys, is demonstrably of sufficient geological brevity to be qualified as an anoxic event. This event lies within the Pseudothurmannia catulloi ammonite subzone, coincides with the extinction of the calcareous nannofossil species Lithraphidites bollii, and records an increase in a globular planktonic foraminifer. High quantities of marine organic matter were preserved in pelagic successions from northern and central Italy, Switzerland, southeastern France, southern Spain and probably elsewhere in the Mediterranean Tethys and Atlantic Ocean. Carbon-isotope stratigraphy from Tethyan and Atlantic sections shows a minor positive excursion in the uppermost part of the Hauterivian and Lowermost Barremian, suggesting accelerated extraction of organic carbon from the ocean reservoir just after the ‘Faraoni Event’. The duration of this short event is less than 100 ka according to cyclostratigraphy and coincides with a third-order sea-level rise. It is likely that similar forcing mechanisms responsible for global OAEs operated during this short time interval. To cite this article: F. Baudin, C. R. Geoscience 337 (2005).     

Paléoclimat des Petites Antilles depuis 4000 ans BP : l'enregistrement de la lagune de Grand-Case à Saint-Martin

A 2.5-m-thick sequence of lake sediments at the Étang de Grand-Case, Saint Martin (French West Indies) is studied here. Significant hydrological budget fluctuations allows distinction of three main climatic periods: (1) an overall dry period (4200 BP–2300 BP), characterised by carbonated mud, gypsum and storm sand layers; (2) a wet phase (2300 BP–1150 BP) dominated by organic mud; and (3) a more complex phase (1150 BP to present), with detrital inputs due to human activities. Comparison with other regional high-resolution records shows that similar climate modifications typify the whole Mesoamerican and Caribbean area. The climatic phenomena that are implicated in the variations of both precipitation and hurricane frequency over the Lesser Antilles are due to the latitudinal displacement of the inter-tropical convergence zone. These data give new support to the hypothesis of the existence of a correlation between peopling phases and climate variations in the Caribbean as previously proposed by some archaeologists. To cite this article: P. Bertran et al., C. R. Geoscience 336 (2004).     

A High-Resolution Sea-Surface Temperature Record from the Tropical South China Sea (16,500–3000 yr B.P.)

The timing and magnitude of sea-surface temperature (SST) changes in the tropical southern South China Sea (SCS) during the last 16,500 years have been reconstructed on a high-resolution, ¹⁴C-dated sediment core using three different foraminiferal transfer functions (SIMMAX28, RAM, FP-12E) and geochemical (U^k′₃₇) SST estimates. In agreement with CLIMAP reconstructions, both the FP-12E and the U^k′₃₇ SST estimates show an average late glacial–interglacial SST difference of 2.0°C, whereas the RAM and SIMMAX28 foraminiferal transfer functions show only a minor (0.6°C) or no consistent late glacial–interglacial SST change, respectively. Both the U^k′₃₇ and the FP-12E SST estimates, as well as the planktonic foraminiferal δ¹⁸O values, indicate an abrupt warming (ca. 1°C in <200 yr) at the end of the last glaciation, synchronous (within dating uncertainties) with the Bølling transition as recorded in the Greenland Ice Sheet Project 2 (GISP2) ice core, whereas the RAM-derived deglacial SST increase appears to lag during this event by ca. 500 yr. The similarity in abruptness and timing of the warming associated with the Bølling transition in Greenland and the southern SCS suggest a true synchrony of the Northern Hemisphere warming at the end of the last glaciation. In contrast to the foraminiferal transfer function estimates that do not indicate any consistent cooling associated with the Younger Dryas (YD) climate event in the tropical SCS, the U^k′₃₇ SST estimates show a cooling of ca. 0.2–0.6°C compared to the Bølling–Allerød period. These U^k′₃₇ SST estimates from the southern SCS argue in favor of a Northern Hemisphere-wide, synchronous cooling during the YD period.     

Vp/Vs ratio along the Vøring Margin, NE Atlantic, derived from OBS data: implications on lithology and stress field

A total of 13 regional Ocean Bottom Seismograph (OBS) profiles with an accumulated length of 2207 km acquired on the Vøring Margin, NE Atlantic have been travel time modelled with regards to S-waves. The V_p/V_s ratios are found to decrease with depth through the Tertiary layers, which is attributed to increased compaction and consolidation of the rocks. The V_p/V_s ratio in the intra-Campanian to mid-Campanian layer (1.75–1.8) in the central Vøring Basin is significantly lower than for the layers above and beneath, suggesting higher sand/shale ratio. This layer was confirmed by drilling to represent a layer of sandstone. This mid-Cretaceous ‘anomaly’ is also present in the northern Vøring Basin, as well as on the southern Lofoten Margin further north. The V_p/V_s ratio in the extrusive rocks on the Vøring Plateau is estimated to be 1.85, conformable with mafic (basaltic) rocks. Landward of the continent/ocean transition (COT), the V_p/V_s ratio in the layer beneath the volcanics is estimated to be 1.67–1.75. These low values suggest that this layer represents sedimentary rocks, and that the sand/shale ratio might be relatively high here. The V_p/V_s ratio in the crystalline basement is estimated to be 1.67–1.75 in the basin and on the landward part of the Vøring Plateau, indicating the presence of granitic/granodioritic continental crust. In the lower crust, the V_p/V_s ratio in the basin decreases uniformly from southwest to northeast, from 1.85–1.9 to 1.68–1.73, suggesting a gradual change from mafic (gabbroic) to felsic (granodioritic) lower crust. Significant (3–5%) azimuthal S-wave anisotropy is observed for several sedimentary layers, as well as in the lower crust. All these observations can be explained by invoking the presence of liquid-filled microcracks aligned vertically along the direction of the present day maximum compressive stress (NW–SE).     

Dependence of Near-Surface Magnetic Susceptibility on Dust Accumulation Rate and Precipitation on the Chinese Loess Plateau

Magnetic susceptibility (MS) of surface sediment varies systematically across the Loess Plateau in central China, decreasing exponentially from >200×10⁻⁸ m³/kg at the northern margin of the Qinling Shan to ≤30×10⁻⁸ m³/kg near the southern margin of the Mu Us Desert. MS correlates highly with loess median grain size (r²=0.79), which decreases south-southeastward across the plateau. It also correlates with mean annual temperature (MAT) and mean annual precipitation (MAP) (r²=0.58 and 0.60, respectively), and with their product MAT×MAP (r²=0.83), which is considered a measure of potential pedogenic activity. Because regional isopleths depicting grain size and the primary meteorological parameters are nearly parallel, it is difficult to determine their relative influence on MS. A simple MS model, based on the observed spatial variation in loess thickness, permits quantitative assessment of the effect of the dust accumulation rate on the MS signal of surface sediment and isolates the likely role of climate in the production of magnetic minerals. The model suggests that 84% of the loess MS variance is dictated by the diluting effect of dust and 10–11% is associated with meteorological factors, primarily precipitation. The observed and modeled relationships support hypotheses that attribute variations in MS in the loess-paleosol succession to varying rates of dust deposition and in situ production of magnetic minerals in the accretionary soils, both of which are controlled by monsoon climate.     

Eclogite and pyroxenite xenoliths from off-craton kimberlites near the Kaapvaal Craton, South Africa

Mantle xenoliths brought to the surface by kimberlite magmas along the south-western margin of the Kaapvaal craton in South Africa can be subdivided into eclogites sensu stricto, kyanite eclogites and orthopyroxene eclogites, all containing omphacite, and garnet clinopyroxenites and garnet websterites characterised by diopside. Texturally, chemically (major elements) and thermally, we observe an evolution from garnet websterites (T_EG = 742–781 °C) towards garnet clinopyroxenites (T_EG = 715–830 °C) and to eclogites (T_EG = 707–1056 °C, mean value of 913 °C). Pressures calculated for orthopyroxene-bearing samples suggest upper mantle conditions of equilibration (P = 16–33 kb for the garnet websterites, 18 kb for a garnet clinopyroxenite and 23 kb for an opx-bearing eclogite). The overall geochemical similarity between the two groups of xenoliths (omphacite-bearing and diopside-bearing) as well as the similar trace element patterns of clinopyroxenes and garnet suggest a common origin for these rocks. Recently acquired oxygen isotope data on garnet (δ¹⁸O_gnt = 5.25–6.78 ‰ for eclogites, δ¹⁸O_gnt = 5.24–7.03 ‰ for garnet clinopyroxenites) yield values ranging from typical mantle values to other interpreted as resulting from low-temperature alteration or precursors sea-floor basalts and associated rocks. These rocks could then represent former magmatic oceanic rocks that crystallised from a same parental magma as plagioclase free diopside-bearing and plagioclase-bearing crustal rocks. During subduction, these oceanic rock protoliths equilibrated at mantle depth, with the plagioclase-bearing rocks converting to omphacite and garnet-bearing lithologies (eclogites sensu largo), whereas the plagioclase-free diopside-bearing rocks converted to diopside and garnet-bearing lithologies (garnet websterites and garnet clinopyroxenites).     

Two Tephra Layers Bracketing Late Holocene Paleoecological Changes in Northern Germany

Paleoecological records from two Holocene peat bogs in northern Germany are linked by two microscopic volcanic ash layers, correlated by petrology and geochemistry to explosive volcanism on Iceland. The younger “Microlite tephra” cannot be correlated to any known eruption, while the older tephra layer is identified as a deposit of the Hekla 3 eruption. The tephra layers are dated by an age–depth regression of accelerator mass spectrometry ¹⁴C ages that have been calibrated and combined in probability distributions. This procedure gives an age of 730–664 cal yr B.C. for the “Microlite tephra” event and 1087–1006 cal yr B.C. for the Hekla 3 event. Accordingly, the tephra layers were deposited during the late Bronze Age. At this time, human settlement slowly increased pressure on the environment, as indicated by changes in woodland pollen composition at the two bogs. The tephra-marker horizons further show that the palynologically defined transition from the Subboreal to the Subatlantic Period is synchronous in the investigated area. However, the macroscopic visible marker in peat, the change from fibrous to sapric peat, the “Schwarztorf-Weißtorf-Kontakt,” is asynchronous. Bog vegetation did not immediately react in unison to a climatic change at this pollen zone boundary; instead, the timing of vegetation change depended on the location within the bog.     

Interpretation of gamma-ray logs: The distribution of uranium in carbonate platform

High gamma-radioactivity in carbonates is usually ascribed to uranium of detrital minerals and organic matter, and to thorium and potassium of clays. The present study based on Urgonian marls and marly limestones (France) shows that some of the most radioactive values correspond instead to some ‘pure’ limestones. These peaks are generally associated with a sequence boundary or a maximum flooding surface. Low-level γ-spectrometry and ICP–MS analyses show that although high radioactivities are mostly associated with uranium, there is no obvious correlation between uranium enrichment and lithology. Also, correlation between high radioactivity and argillaceous beds might not be systematic. To cite this article: M.C. Raddadi et al., C. R. Geoscience 337 (2005).