16 research outputs found
Rock magnetic investigation of possible sources of the Bangui magnetic anomaly
International audienceThe Bangui magnetic anomaly (BMA) is the largest lithospheric magnetic field anomaly on Earth at low latitudes. Previous studies investigated its geological source using constraints from satellite and ground magnetic field measurements, as well as from surface magnetic susceptibility measurements on rocks from the Panafrican Mobile Belt Zone (PMBZ). Here we combine magnetic field data modelling and rock magnetic property measurements (susceptibility and natural remanent magnetization, NRM) on many samples from this PMBZ and the surrounding formations. It reveals that NRM is a significant component of the total magnetization (Mt) of the BMA source, which reaches 4.3 A/m with maximum thicknesses of 38 and 54 km beneath the western and eastern parts of the BMA. Only the isolated and relatively thin banded iron formations and some migmatites show such Mt values. Thus we suggest that the thick BMA source may be composed either by overlapped slices of such metamorphic rocks, or by an iron-rich mafic source, or by a combination of these two geological structures
GenÚse des indices polymétallifÚres des formations néoprotérozoïques (1000 543 Ma) dans la région du Béli (Nord du Burkina Faso) : contrÎle structural, nature des fluides, rÎle de la silice
A l instar des formations sĂ©dimentaires des bassins de Taoudeni et des Volta, celles de la rĂ©gion du BĂ©li comportent une triade (tillite-calcaire-silexite) et une formation molassique. Ces marqueurs stratigraphiques ont permis la subdivision verticale des ensembles sĂ©dimentaires des bassins ouest-africains en trois surpergroupes. La triade repose sur des grĂšs-quartzites et est surmontĂ©e par des shales et des dolomies. Comme les formations carbonatĂ©es, certaines roches du complexe silexitique renferment des stromatolites, de mĂȘme que des micro-organismes. Dans la rĂ©gion du BĂ©li, exceptĂ© le deuxiĂšme, on reconnaĂźt les quatre Ă©pisodes tectoniques de l orogenĂšse panafricaine dont les caractĂ©ristiques permettent de distinguer deux unitĂ©s structurales. L unitĂ© structurale du Nord-BĂ©li (allochtone), caractĂ©risĂ©e par une dĂ©formation de type ductile et un anchimĂ©tamorphisme, est charriĂ©e sur l unitĂ© structurale du Sud-BĂ©li (para-autochtone). Celle-ci est marquĂ©e par une dĂ©formation Ă dominante cassante, n a subi aucun mĂ©tamorphisme et est chevauchant sur le socle birimien. Le rĂ©seau de fractures intĂ©ressant la rĂ©gion du BĂ©li a favorisĂ© la circulation de fluides de tempĂ©ratures variĂ©es et d origines diverses (mĂ©tĂ©oriques, marines, saumĂątres, Ă©pithermales). Ces fluides sont responsables de la prĂ©sence des abondantes expressions siliceuses, mises en place en au moins deux phases. Les fluides hydrothermaux, par lessivage des formations silexitiques et schisteuses, et probablement du socle birimien, ont favorisĂ© le dĂ©pĂŽt, sous forme de sulfures, de nombreux mĂ©taux, prĂ©fĂ©rentiellement dans les zones des brĂšches silexitiques. Celles-ci comportent l essentiel des affleurements ferrugineux dont la plupart ont Ă©tĂ© reconnus comme Ă©tant des chapeaux de fer.AIX-MARSEILLE3-BU Sc.St JĂ©rĂŽ (130552102) / SudocSudocFranceF
Impact biostratigraphique et géographique dans le secteur nord-est du bassin cÎtier togolais
LâĂ©tude lithologique et micropalĂ©ontologique des carottes de cinq sondages exĂ©cutĂ©s dans le secteur nord-est du bassin cĂŽtier togolais a permis de prĂ©ciser la stratigraphie et la palĂ©ogĂ©ographie de ce secteur au cours du Maastrichtien et du PalĂ©ogĂšne. Lâanalyse lithologique met en Ă©vidence une sĂ©rie marine comportant un ensemble infĂ©rieur maastrichtien, un ensemble moyen palĂ©ocĂšne Ă Ă©ocĂšne et un ensemble supĂ©rieur attribuĂ© au Continental terminal sensu lato. LâĂ©tude biostratigraphique, basĂ©e sur des foraminifĂšres planctoniques permet dây caractĂ©riser les biozones allant de la biozone Ă Globotruncana aegyptiaca Ă la zone Abathomphalus mayaroensis et les biozones P5 Ă P11 dĂ©finissant ainsi un Maastrichtien moyen Ă supĂ©rieur, un PalĂ©ocĂšne supĂ©rieur et un EocĂšne infĂ©rieur Ă moyen. LâĂ©volution palĂ©ogĂ©ographique montre que cette sĂ©rie comprend deux cycles sĂ©dimentaires : le premier se termine Ă la fin du CrĂ©tacĂ© et le second au PalĂ©ogĂšne.Lithological and micropaleontological studies of core-samples from five boreholes drilled in the northeastern part of the coastal basin of Togo allow for a specification of the stratigraphy and the paleogeography of this area during the Maastrichtian and Paleogene. A lithological analysis reveals a marine series consisting of a Lower Maastrichtian unit, a Middle Paleocene to Eocene unit, and an upper unit attributed to the Continental terminal sensu lato . The biostratigraphical study, based on planktonic foraminifera, has led to a characterization of the basin in terms of biozones ranging from the Globotruncana aegyptiaca to the Abathomphalus mayaroensis biozones and biozones P5 to P11, thus specifying a Middle to Upper Maastrichtian, an Upper Paleocene and a Lower to Middle Eocene units. The paleogeographical evolution of the area shows that the series recorded two sedimentary cycles: the first one stops at the end of the Cretaceous and the second one in the Paleogene.</p
Pan-African Paleostresses and Reactivation of the Eburnean Basement Complex in Southeast Ghana (West Africa)
This faulting tectonics analysis concerns the southernmost segment of the Dahomeyide Orogen and the West-African craton eastern margin in southeast Ghana. The analysis of strike-slip faults in the frontal units of the Dahomeyide Belt indicates that four distinct compressive events (NE-SW, ENE-WSW to E-W, ESE-WNW to SE-NW and SE-NW to SSE-NNW) originated the juxtaposition of the Pan-African Mobile Zone and the West-African craton. These paleostress systems define a clockwise rotation of the compressional axis during the structuring of the Dahomeyide Orogen (650â550 Ma). The SE-NW and SSE-NNW to N-S compressional axes in the cratonic domain and its cover (Volta Basin) suggest that the reactivation of the eastern edge of the West African craton is coeval with the last stages of the Pan-African tectogenesis in southeast Ghana. An extensional episode expressed as late normal faulting is also recorded in this study. This E-W to SE-NW extension, which is particular to the southernmost part of the Dahomeyide Belt, appears to be post-Pan-African. This extension probably contributed to the formation of a major Jurassic rifting zone that originated the Central Atlantic and the Benue Trough
New Computerized Method for the Geochemical Classification of Precambrian Carbonate Rocks: Case of a Set of African Cap Carbonates
International audiencePost-sedimentary transformations have masked or completely obliterated the structures and textures of Precambrian carbonate rocks. Therefore, methods of classification of the carbonate rocks founded on the observation of primary structures or textural characteristics are ill-adapted. Consequently, only certain geochemical classification methods al- low us to distinguish the various rock-types in the case of Neoproterozoic carbonates. After presenting the most suit- able geochemical classifications, we propose a new classification into 14 groups based on a regular ternary diagram with computerized data input. For each sample of carbonate rock, analysis of calcium and magnesium contents allows us to calculate the input data for our diagram i.e. the percentages of Calcite, Dolomite and Insoluble Residue. To auto- mate the application of this diagram, input parameters are created in a descriptive file "Roches.ternaires.txt" using an option called "Ternaires" in the "Diagrammes" software developed by Roland Simler. Thirty cap carbonates of Africa are used to validate this new method
Comparaison des résultats analytiques obtenus aprÚs fusion alcaline au métaborate avec ceux obtenus aprÚs fusion alcaline au tétraborate dans les niveaux d'altération des grÚs de l'Inkisi (secteur de Brazzaville, République du Congo) : applications dans l'étude des profils d'altération
We have compared the analytical results obtained by ICP-AES after alkali fusion with lithium metaborate and with lithium tetraborate in the analysis of samples from an arkosic sandstone from «Inkisi» (Congo, Central of Africa) along a weathering profile. The observation of some differences of behaviour of the two fluxes at the upper levels has conduce us to assume that this differences was in relationship with the degree of weathering process on potassic feldspars and sodic feldspars (albite) contained in those sandstones. In order to locate easily the weathering front on the weathering profile we have defined a coefficient of variation Ki:Ki (%) = 100 * ((oxidei (tf) - oxidei (mf)) / oxidei (mf))(with tf : result obtained after tetraborate alkaline fusion and mf : result obtained after metaborate alkaline fusion for the i oxide analyzed).The study of the Ki curves obtained for three chemical elements (K, Na, Si) typical of alteration of sandstones in function of the depth permits to show that the variations of the Ki curves was able to locate precisely the alteration levels of the different minerals. So, sodium from albite is the first released at a quite deep level of the profile. It is followed by the potassium from potassic feldspars. We have also showed that, according to the positive or negative sign positive of Ki variations, the chemical element released will integrate more or less refractory mineral structures. All of those results have been confirmed by the study of the X-ray diagrams of the same weathering profile. So, in case of arkosic sandstone our method would permit to locate with precision the weathering front on the profile. This method might be used for the study of other kind of rocks.Les rĂ©sultats analytiques obtenus en ICP-AES aprĂšs fusion alcaline au mĂ©taborate de lithium sont comparĂ©s Ă ceux obtenus aprĂšs fusion alcaline au tĂ©traborate de lithium sur un profil dâaltĂ©ration des grĂšs de lâInkisi (Sud de Brazzaville, Congo). Les teneurs obtenues montrent des diffĂ©rences parfois importantes, notamment dans les niveaux supĂ©rieurs de lâaltĂ©ration. Ces diffĂ©rences sont, soit positives, soit nĂ©gatives. Elles varient selon l'Ă©lĂ©ment chimique analysĂ©. Certains Ă©lĂ©ments chimiques, comme l'aluminium, prĂ©sentent des diffĂ©rences trĂšs faibles attribuables Ă la prĂ©cision des analyses; pour d'autres, par contre, comme le silicium, elles sont importantes et fluctuantes. Nous avons conçu un coefficient Ki de la maniĂšre suivante: Ki (%) = 100 * ((oxydei (tf) - oxydei (mf)) / oxydei (mf))(tf : rĂ©sultat obtenu aprĂšs fusion alcaline au tĂ©traborate et mf : rĂ©sultat obtenu aprĂšs fusion alcaline au mĂ©taborate, pour un oxyde i analysĂ©). Les courbes de variation du Ki en fonction de la profondeur, Ă©tablies de la base au sommet du profil dâaltĂ©ration permettent de localiser les niveaux plus fusibles au tĂ©traborate (Ki positif) et les niveaux plus fusibles au mĂ©taborate (Ki nĂ©gatif). En comparant ces courbes avec lâĂ©volution minĂ©ralogique au cours de lâaltĂ©ration, montrĂ©e par les diagrammes rayons X, nous avons obtenu les rĂ©sultats suivants: -les variations des courbes du Ki du sodium et du potassium ont pu ĂȘtre corrĂ©lĂ©es, respectivement, avec les niveaux d'altĂ©ration de lâalbite et ceux d'altĂ©ration des feldspaths potassiques.-les variations des courbes du Ki du silicium ont pu ĂȘtre corrĂ©lĂ©es avec la nature des minĂ©raux secondaires apparaissant successivement le long du profil d'altĂ©ration. Ainsi lâutilisation du coefficient Ki, tel que nous lâavons dĂ©terminĂ©, peut contribuer, en appui avec d'autres mĂ©thodes comme la diffraction RX, Ă lâĂ©tude des processus dâaltĂ©ratio