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81.
RésuméDans le Sud de la France et les Pyrénées espagnoles, les calcaires sous-jacents aux roches détritiques du Culm carbonifère ont fait l’objet d’une datation systématique par conodontes. Il en ressort que le dépôt de ces niveaux carbonates s’est poursuivi de plus en plus tardivement de l’Est vers l’Ouest : du Viséen inférieur (V3b) en Montagne Noire, au Namurien supérieur (G2) en Pays basque.Les roches détritiques à caractère flyschoïde du Culm remanient, localement, des éléments de calcaire de plate-forme. La datation de ces éléments par foraminifères, algues, pseudo-algues et végétaux supérieurs montre qu’ils sont de plus en plus récents d’Est en Ouest : leur âge s’échelonne du Viséen 3bp-Namurien A en Montagne Noire, au Westphalien C (Kachirien) au Pays basque (Massif des Cinco-Villas).Ces divers résultats montrent que les manifestations tectoniques, responsables des premiers épandages détritiques du Culm puis de la resédimentation des calcaires de plate-forme dans le bassin, ont commencé de plus en plus tardivement de la Montagne Noire au Pays basque.Dans sa branche Sud, l’orogenèse varisque, si l’on s’en tient à la position actuelle de ses segments observables, a migré progressivement du Nord vers le Sud, en Montagne Noire et dans le Massif de Mouthoumet, puis vers l’Ouest, des Pyrénées centrales aux Pyrénées orientales. 相似文献
82.
AbstractWe compared the disintegration processes and mineralogic and chemical evolution pathways of two Quaternary basalts at Bakrit and Ifrane, weathered in the same physiographic and hydrologic conditions, but differing in texture according to the quantity of glass present. At Bakrit, quite abundant glass favoured the formation of a microfissure network throughout the rock and its disintegration without any distinct weathering front. As a result, basaltic sand with polymineral grains and a clayey-silty matrix were produced. At Ifrane, weathering of a glass-poor basalt produced only a clayey-silty saprolite. In glass-rich basalts, secondary minerals formed in microfissures and were 2/1 clay-mineral rich. In glass-poor basalts, secondary minerals formed mainly within primary minerals and were 1/1 clay-mineral rich. Because glass could be easily dissolved, it protected the minerals of close chemical composition, especially the plagioclases. The order of basalt-mineral weathering (olivine, labrador, augite, Fe-Ti oxides) was modified when glass was abundant (glass, olivine, augite, labrador, Fe-Ti oxides). © Elsevier, Paris 相似文献
83.
Olivier Rabeau Jean-Jacques Royer Michel Jébrak Alain Cheilletz 《Mineralium Deposita》2013,48(7):817-824
The location of potentially unrecognized gold deposits in the close vicinity of the Cadillac–Larder Lake Fault Zone in the Archean Abitibi Subprovince (Canada) is predicted by applying a stochastic approach to the distribution of known gold deposits. The methodology uses the distances between neighboring orogenic gold deposits along the fault trace. The cumulative distribution of the curvilinear inter-distances along the fault zone, are adequately represented by a log-uniform model. The average inter-distance is 1.95 km, and an upper inter-distance of 5.6 km is observed. The same distribution pattern appears along the Destor–Porcupine Fault Zone (Abitibi). This log-uniform distribution shows that the spatial distribution of gold deposits is regionally controlled by the major crustal shear zone. Lithologies and structures only seem to exert a local influence at the deposit scale. The log-uniform spacing could be interpreted as the result of the crustal failure locations induced by hydraulic overpressure along mechanically independent segments on the main fault. 相似文献
84.
Mathieu Duval Christophe Falguères Jean-Jacques Bahain Rainer Grün Qingfeng Shao Maxime Aubert Jean-Michel Dolo Jordi Agustí Bienvenido Martínez-Navarro Paul Palmqvist Isidro Toro-Moyano 《Quaternary Research》2012,77(3):482-491
The combined U-series/electron spin resonance (ESR) dating method was applied to nine teeth from two Early Pleistocene archaeological sites located in the Orce area (Guadix-Baza Basin, Southern Spain): Fuente Nueva-3 (FN-3) and Barranco León (BL). The combination of biostratigraphy and magnetostratigraphy places both sites between the Olduvai and Jaramillo subchrons (1.78–1.07 Ma).Our results highlight the difficulty of dating such old sites and point out the limits of the combined U-series/ESR dating method based on the US model. We identified several sources of uncertainties that may lead to inaccurate age estimates. Seven samples could not be dated because the dental tissues had (230Th/234U) activity ratios higher than equilibrium, indicating that uranium had probably leached from these tissues. It was however possible to calculate numerical estimates for two of the teeth, both from FN-3. One yielded a Middle Pleistocene age that seems to be strongly underestimated; the other provided an age of 1.19 ± 0.21 Ma, in agreement with data obtained from independent methods. The latter result gives encouragement that there are samples that can be used for routine dating of old sites. 相似文献
85.
Jackie Despriée Pierre Voinchet Hélène Tissoux Jean-Jacques Bahain Christophe Falguères Gilles Courcimault Jean Dépont Marie-Hélène Moncel Sophie Robin Marta Arzarello Robert Sala Laurent Marquer Erwan Messager Simon Puaud Salah Abdessadok 《Quaternary Science Reviews》2011,30(11-12):1474-1485
This paper records the findings from c. 80 prehistoric sites that have been discovered in the alluvial deposits of the rivers Creuse, Cher, and Loir, tributaries of the middle Loire River, over the period since 1981. These deposits comprise river terrace aggradations formed during successive glacial–interglacial cycles which have recorded climate and environment during Quaternary time. The systematic dating of these river deposits by Electron Spin Resonance (ESR) applied to optically bleached sedimentary quartz has resulted in the establishment of a chronological framework for the evolution of these rivers during Lower and Middle Pleistocene (between 1.7 Ma and 130 ka). Evidence for Early Palaeolithic (Mode 1) industries with an in situ context (workshops, soils) in the highest aggradations indicates that Hominins were present in the study area, near the geographical centre of France (47°N), around 1.1 Ma. Examination of the sites indicates that Human occupations were located along valley bottom sites during temperate episodes. Subsequently, after a gap of several hundred thousand years industries with handaxes appear in the Middle Loire Basin in the interval between 700 and 600 ka, and then continuously from 400 ka. These two phases of settlement produced industrial assemblages with clear differences in their responses to the supplies of raw materials and in the modes of making flakes. 相似文献
86.
Regolith mass balance inferred from combined mineralogical, geochemical and geophysical studies: Mule Hole gneissic watershed, South India 总被引:3,自引:0,他引:3
Jean-Jacques Braun Marc Descloitres Jean Riotte Simon Fleury Jean-Loup Boeglin Eva Lacarce M. Sekhar M.S. Mohan Kumar S. Subramanian Bernard Dupré 《Geochimica et cosmochimica acta》2009,73(4):935-24
The aim of this study is to propose a method to assess the long-term chemical weathering mass balance for a regolith developed on a heterogeneous silicate substratum at the small experimental watershed scale by adopting a combined approach of geophysics, geochemistry and mineralogy. We initiated in 2003 a study of the steep climatic gradient and associated geomorphologic features of the edge of the rifted continental passive margin of the Karnataka Plateau, Peninsular India. In the transition sub-humid zone of this climatic gradient we have studied the pristine forested small watershed of Mule Hole (4.3 km2) mainly developed on gneissic substratum. Mineralogical, geochemical and geophysical investigations were carried out (i) in characteristic red soil profiles and (ii) in boreholes up to 60 m deep in order to take into account the effect of the weathering mantle roots. In addition, 12 Electrical Resistivity Tomography profiles (ERT), with an investigation depth of 30 m, were generated at the watershed scale to spatially characterize the information gathered in boreholes and soil profiles. The location of the ERT profiles is based on a previous electromagnetic survey, with an investigation depth of about 6 m. The soil cover thickness was inferred from the electromagnetic survey combined with a geological/pedological survey.Taking into account the parent rock heterogeneity, the degree of weathering of each of the regolith samples has been defined using both the mineralogical composition and the geochemical indices (Loss on Ignition, Weathering Index of Parker, Chemical Index of Alteration). Comparing these indices with electrical resistivity logs, it has been found that a value of 400 Ohm m delineates clearly the parent rocks and the weathered materials. Then the 12 inverted ERT profiles were constrained with this value after verifying the uncertainty due to the inversion procedure. Synthetic models based on the field data were used for this purpose. The estimated average regolith thickness at the watershed scale is 17.2 m, including 15.2 m of saprolite and 2 m of soil cover.Finally, using these estimations of the thicknesses, the long-term mass balance is calculated for the average gneiss-derived saprolite and red soil. In the saprolite, the open-system mass-transport function τ indicates that all the major elements except Ca are depleted. The chlorite and biotite crystals, the chief sources for Mg (95%), Fe (84%), Mn (86%) and K (57%, biotite only), are the first to undergo weathering and the oligoclase crystals are relatively intact within the saprolite with a loss of only 18%. The Ca accumulation can be attributed to the precipitation of CaCO3 from the percolating solution due to the current and/or the paleoclimatic conditions. Overall, the most important losses occur for Si, Mg and Na with −286 × 106 mol/ha (62% of the total mass loss), −67 × 106 mol/ha (15% of the total mass loss) and −39 × 106 mol/ha (9% of the total mass loss), respectively. Al, Fe and K account for 7%, 4% and 3% of the total mass loss, respectively. In the red soil profiles, the open-system mass-transport functions point out that all major elements except Mn are depleted. Most of the oligoclase crystals have broken down with a loss of 90%. The most important losses occur for Si, Na and Mg with −55 × 106 mol/ha (47% of the total mass loss), −22 × 106 mol/ha (19% of the total mass loss) and −16 × 106 mol/ha (14% of the total mass loss), respectively. Ca, Al, K and Fe account for 8%, 6%, 4% and 2% of the total mass loss, respectively.Overall these findings confirm the immaturity of the saprolite at the watershed scale. The soil profiles are more evolved than saprolite but still contain primary minerals that can further undergo weathering and hence consume atmospheric CO2. 相似文献
87.
Jerome Viers Bernard Dupre Jean-Jacques Braun Remi Freydier Sacha Greenberg Jules Ndam Ngoupayou Lus Sigha Nkamdjou 《Aquatic Geochemistry》2001,7(2):127-154
The knowledge of the biogeochemical cycle of chlorine (Cl)is important since this element is used as a tracer of geochemical and hydrological processes in oceanic or continental environments. More specifically, Cl can be used to correct surface water composition from atmospheric contribution in order to calculate precise chemical weathering rates in watersheds. Beyond the problem of potential Cl sources in a given watershed, which is directly related to the lithology, vegetation, and industrial activities, the Cl normalization is based on the assumption that this element behaves conservatively during surface processes (e.g., chemical weathering, adsorption/desorption processes).The purpose of the present study is to forecast the geochemical behavior of Cl in a forested ecosystem located under humid tropical environment.For this reason, we have analyzed the Cl (and also Ca and Na) concentrations ofsurface waters (rainwater, groundwater, river water) over a two-year period in the Nsimi–Zoetele watershed (Cameroon).The Cl mass balance for the watershed appears to be equilibrated over the studied period (1995–1996) but Cl behavior in Mengong River draining the watershed suggests a non-conservative behavior. Indeed, Cl concentrationsin the Mengong River are low during dry seasons and high during wet seasons, which is the reverse tendency to what is usually observed taking into account dilution and evaporation processes. As Cl concentrations in the Mengong River are lower than those measured in all the feeding reservoirs, Cl should be adsorbed onto the soils of the watershed. However, as the Cl mass balance is equilibrated over the whole-year, Cl should be adsorbed and releasedat a seasonal scale. The results we obtained for this small watershed were not generalized for a larger studied basin (i.e., Nyong River basin). Even if these results should be followed by further investigations, this study suggests that Cl normalization should be used with caution to avoid under- or over-estimation of chemical weathering rates. 相似文献
88.
Grard Bossire Issa Bonkoungou Jean-Jacques Peucat Jean-Pierre Pupin 《Precambrian Research》1996,80(3-4)
The Tarkwaian rocks of Burkina Faso overlie the Birimian Supergroup which is considered to be part of a juvenile crust formed during the Eburnean orogen. They comprise fluviatile pebbles (quartz, rhyolite, chert and schist), embedded in an acid volcaniclastic matrix. During a sinistral transpressive shearing they underwent a low-grade metamorphism characterized by a paragonitic muscovite assemblage formed during isoclinal folding. Fold axes are nearly horizontal, parallel to a stretching lineation and oriented 035–215°.A typologic study of zircons in the volcaniclastics, coupled with radiometric datings, enables the volcanism to be characterized and its age to be determined.The typologic study has shown that: (1) zircons from the rhyolitic pebbles, the matrix of the conglomerates and from the sandstone are basically the same; (2) the zircons' morphology characterizes a calc-alkaline to alkaline evolutionary trend implying three main episodes of volcanism; (3) the morphology of this zircon population is very similar to those usually observed in post-orogenic metasediments or sediments. The tectonic setting is one of intracontinental extension associated with rifting and crustal thinning in the course of a major period of crustal growth around 2.1 Ga. The continental volcaniclastics accumulated in the rift at the same time as the sediments.The radiometric study has shown that there are no reworked Archean rocks. The ages cover the period 2170 to 2124 Ma. Three main volcanic episodes may be defined 2170, 2150 and 2124 Ma; the last one was probably contemporaneous with the sedimentation. Deformation of Tarkwaian rocks took place between 2124 ± 9 Ma (the age of the youngest inherited zircon) and 1991 ± 12 Ma (the age of a post-Tarkwaian granite). This period, 2170 to 2124 Ma, fits well with the main Birimian crustal growth and the ages reported for the Tarkwaian rocks of Ghana. The same period of accretion is also recognized in South America.The Tarkwaian conglomeratic rocks of Burkina Faso do not have gold concentrations similar to those in Ghana and Guyana. With respect to the Tarkwaian rocks of Ghana, where gold is concentrated in the quartz pebbles, they also differ because: (1) there is a volcaniclastic contribution; (2) the zircons from the Tarkwaian rocks in Burkina Faso are euhedral, indicating that transport distances were short; and (3) they have been deformed by shearing.With respect to Guyana, the rocks are very similar and probably have the same origin; the short transport distance may not allow a secondary concentration of gold. These differences, at least, may help explain the lack of economic gold concentration in the Tarkwaian metasediments. 相似文献
89.
Aline Mign Dominique Davoult Jean-Jacques Bourrand Guy Boucher 《Journal of Sea Research》2005,53(4):223
In situ measurements of ammonium and carbon dioxide fluxes were performed using benthic chambers at the end of spring and the end of summer in two soft-bottom Abra alba communities of the western English Channel (North Brittany): the muddy sand community (5 m, about 10% of surface irradiance) and the fine-sand community (19 m, about 1% of surface irradiance). High rates of ammonium regeneration were measured in the two communities at the end of summer (296.03±40.07 and 201.7±62.74 μmolN m−2 h−1, respectively) as well as high respiration rates (2.60±0.94 and 2.23±0.59 mmolC m−2 h−1, respectively). Significant benthic gross primary production (up to 6.11 mmolC m−2 h−1) was measured in the muddy sand community but no benthic primary production was measured in the fine-sand community. It suggests that microphytobenthic production values used in simulations previously published for these two communities were overestimated while values of community respiration were underestimated. The study confirms that this benthic system is heterotrophic and strengthens the idea that an important pelagic-benthic coupling is required for the functioning in such coastal ecosystems. 相似文献
90.
Eric Gloaguen Yannick Branquet Philippe Boulvais Yves Moëlo Jean-Jacques Chauvel Pierre-Jacques Chiappero Eric Marcoux 《Mineralium Deposita》2007,42(4):399-422
In the Saint-Aubin-des-Chateaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects
an oolitic ironstone layer intercalated within the Lower Ordovician Grès armoricain Formation. The hydrothermal overprint
produced pervasive and massive sulphidation with stratoid pyritised lenticular bodies within the oolitic ironstone layer.
These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. After the massive sulphidation
stage (Fe–As, stage 1), subsequent fracturing allowed the deposition of base metals (stage 2) and Pb–Sb–Au (stage 3) parageneses
in veins. The dominant brittle structures are vertical extension veins, conjugate shear veins and strike-slip faults of various
orders. All these structures are filled with the same paragenetic sequence. Deformation analysis allows the identification
of structures that developed incrementally via right-lateral simple shear compatible with bulk strain affecting the Central
Armorican Domain. Each increment corresponds to a fracture set filled with specific parageneses. Successive hydrothermal pulses
reflect clockwise rotation of the horizontal shortening direction. Geothermometry on chlorite and arsenopyrite shows an input
of hot hydrothermal fluids (maximum of 390–350°C) during the main sulphide stage 1. The subsequent stages present a marked
temperature drop (300–275°C). Lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds
to the underlying Neo-Proterozoic basement. Lead isotope data, relative ages of deformation and comparison with neighbouring
deposits suggest that large-scale fluid pulses occurred during the whole Hercynian orogeny rather than pulses restricted to
the late Hercynian period. The vicinity of the Hercynian internal domain appears as a key control for deformation and fluid
flow in the oolitic ironstones, which acted as a chemical and structural trap for the hydrothermal fluids. The epigenetic
mineralisation of Saint-Aubin-des-Chateaux appears to be very similar to epigenetic sulphidation described in banded iron
formation-hosted gold deposits. 相似文献