Despite recent works on the Schisto-calcaire Group of the West-Congolian Supergroup in Central Africa, Neoproterozoic Cap Carbonate have not been well documented in the Niari-Nyanga basin that ...extends from Gabon to Angola. The couplet “Cap Carbonate (SCIa) and Upper Diamictite”, that is a marker of the “Snowball Earth event” was studied in 5 sections from the Niari-Nyanga basin and in one section from the external zone of the Mayombe folded belt (MFB). Our paper presents a high-resolution data set based on results from petrography, geochemistry, mineralogy and stable isotope studies performed on Cap Carbonate samples. The estimation of the illite crystallinity indexes indicates different degrees of post-sedimentary transformations evolving from deep diagenesis in the syncline basin to an epimetamorphism in the external zone of the MFB. Thin-sections of rocks sampled in this latter section display metamorphic features materialized by recrystallization of minerals in a S1 cleavage plane. Despite such an observation, all our samples clearly show preservation of primary petrographic structures such as biolaminations, fenestrae and peloids. Geochemical data leads us to highlight that the Cap Carbonate is composed by a variety of carbonate types. These results support the fact that the Cap Carbonate was derived from a combination of biological and chemical processes: organomineralization and chemical precipitation from supersaturated solutions, with a negligible impact of post-depositional transformations. For all the studied sections, stable isotope profiles display a drop in δ13C values, from about −2.6‰ to −4‰, associated with δ18O values ranging from −6‰ to −10‰. Moreover, the similarity of the isotopic profiles of the studied Cap Carbonate (from the Niari-Nyanga basin and the MFB) and the close fit to the expected Neoproterozoic δ13C and δ18O seawater values constitute a strong indication of a weak impact of a diagenetic to epimetamorphic evolution on the carbon isotopic signature of the SCIa Cap Carbonate. In the light of this data set, our isotopic records must be considered as a reliable tracer of the Marinoan Glaciation within the West-Congolian Supergroup.
A major issue in understanding weathering processes is to determine to what extent fabrics, structures and volumes from the parent rock are preserved through the weathering profile. Iso-element mass ...balance methods (IMB) are often used to estimate volume changes during progressive weathering, based on the assumption that a given element (either Ti, Zr or Th) is not mobile. The petrofabric of the weathered material is often characterized by a mimetic replacement of primary minerals that fully preserve the primary fabric. This suggests an isovolume weathering as volume change must be associated with shape change due to boundary conditions not allowing horizontal strains. So collapse or dilation should induce vertical compaction or constriction, respectively. The weak petrofabric of weathered materials may be precisely quantified using the anisotropy of low-field magnetic susceptibility technique (AMS). This paper reports, for the first time, a combined IMB and AMS study of two different environments: a complex lateritic sequence on Precambrian metamorphic rocks in Cameroon and a weathering profile on a Pleistocene basaltic flow in Morocco. The lateritic profile, divided into a lower saprolite zone (>12 m) and an upper nodular iron-rich unit (4 m), is characterized by neoformed magnetic minerals (goethite/hematite/spinels) and a weak but rather consistent magnetic fabric. Saprolitization, which induces low susceptibility values (50×10
−9 m
3/kg), preserves the linear tectonic fabric of the parent gneiss. AMS evidence for isovolumetric weathering agrees with Th based IMB, while Ti and Zr partly indicate apparent collapse and dilation (up to 50%), respectively. Ferralitization in the nodular iron crust, which enhances susceptibilities (500 to 700×10
−9 m
3/kg, due to spinel phases), induces drastic reductions in anisotropy due to multiple generation of neoformed minerals destroying primary fabric. However, a weak horizontal planar magnetic fabric suggests compaction, in agreement with Th and Ti based IMB while Zr would indicate isovolume weathering. Conversely, in the one meter thick weathering profile on basalt, the strong susceptibility (1.5 to 3×10
−6 m
3/kg) is dominated by inherited titanomaghemite grains which totally preserve the weak magmatic fabric of the fresh basalt. This again suggests that volume change is minor, while Ti and Zr based IMB indicate 20 to 50% of dilation. In both examples, large volume changes inferred from the IMB in units showing preserved primary AMS fabric appear unrealistic. Our AMS study, together with the large discrepancies between the IMB results produced by various elements, indicate that the assumption of immobility of a given element is probably not fulfilled all along these profiles. Isovolumetric weathering in the saprolitic zone may be more widespread than suggested by IMB results.
A palaeomagnetic study was carried out on Neoproterozoic samples from seven sites of the sub-basins of Gourma and Bobo Dioulasso, which include a Marinoan glaciogenic deposit. Magnetic mineralogy is ...represented essentially by magnetite and hematite. The mean directions of the sites are calculated on the high temperature component (500–670
°C). Two locations provide data constrained by statistical reversal and fold tests and determining Neoproterozoic virtual geomagnetic poles. The palaeolatitudes display very low values which place the West-African craton in the sub-equatorial position during the Marinoan glaciation. This result enhances the Snowball Earth hypothesis, which places most of the continental landmasses, and notably Africa, at low latitudes during the Neoproterozoic.
Une étude paléomagnétique a été réalisée sur des échantillons du Néoprotérozoïque provenant de 7 sites des sous-bassins du Gourma et de Bobo Dioulasso, au Sud-Est du bassin de Taoudéni et renfermant un dépôt glaciogénique marinoen. La minéralogie magnétique est représentée essentiellement par la magnétite et l’hématite. Les directions moyennes des sites ont été calculées sur la composante de haute température (500–670
°C). Deux sites présentent des directions contraintes par des tests statistiques d’inversion et de pli et déterminant des pôles géomagnétiques virtuels du Néoprotérozoïque. Les paléolatitudes montrent des valeurs très basses qui placent le craton Ouest-Africain en position subéquatoriale pendant la période de mise en place de la glaciation marinoenne. Ces résultats rehaussent l’hypothèse du «
Snowball Earth » qui place l’essentiel des masses continentales, et notamment l’Afrique, à de basses latitudes pendant le Néoprotérozoïque.
The Khushaym Matruk site in central Jordan may represent a natural analogue depicting the interaction of alkaline solutions with a clayey sedimentary formation or with clay-rich confining barriers at ...the interface with concrete structures in waste disposal sites. In this locality, past spontaneous combustion of organic matter in a clayey biomicritic formation produced a
ca. 60
m-thick layer of cement-marble containing some of the high-temperature phases usually found in industrial cements (e.g., spurrite, brucite, and Ca-aluminate). A vertical cross-section of the underlying sediments was used in order to study the interaction between cement-marbles and neighbouring clayey limestones under weathering conditions. A thermodynamic approach of the alteration parageneses (calcite–jennite–afwillite–brucite and CSH phases) in the cement-marbles constrains the interacting solutions to have had pH-values between 10.5 and 12. Over 3
m, the sediments located beneath the metamorphic unit were compacted and underwent carbonation. They display large C and O isotopic variations with respect to “pristine” sediments from the bottom of the section. Low
δ
13C-values down to −31.4‰/PDB show the contribution of CO
2 derived from the oxidization of organic matter and from the atmosphere to the intense carbonation process affecting that particular sedimentary level. The size of the C isotopic anomalies, their geometrical extent and their coincidence with the variations of other markers like the Zn content, the structure of organic matter, the mineralogical composition, all argue that the carbonation process was induced by the percolation of high pH solutions which derived from the alteration of cement-marbles. The temperature of the carbonation process remains conjectural and some post-formation O isotopic reequilibration likely affected the newly-formed carbonate. Carbonation induced a considerable porosity reduction, both in fractures and matrixes. The Khushaym Matruk site may have some bearing to the early life of a repository site, when water saturation of the geological formations hosting the concrete structures is incomplete, enabling simultaneous diffusion of alkaline waters and gaseous CO
2 in the near field.
Spontaneous combustion, less than 1
Ma ago, affected a 60-m thick sediment pile of biomicrite at the Khushaym Matruck site (Jordan). The present study shows that three retrograde alteration stages ...occurred: weathering, thermal stress and oxidative alkaline perturbation. μ-FT-i.r. spectra of isolated kerogens and oxygen index of whole rocks indicate that oxidation of organic matter occurred down to ∼10
m beneath the metamorphosed zone at Khushaym Matruck. The occurrence of the oxidative weathering bacterially mediated, as suggested by the mass chromatograms of saturated hydrocarbons, can explain high Rock-Eval
T
max values and low petroliferous potential measured along the sedimentary pile. On the other hand, the thermal extent of combustion events was limited to the first 2
m from the contact. The mean reflectance of 0.20–0.24% and porosity of ca. 50% of the grey clayey biomicrites indicate that organic matter was very immature and sediments were unconsolidated at the time of the combustion event. Using mineralogy, microscopic analyses of vegetable debris and magnetic susceptibility, a suite of characteristic points corresponding to the thermal imprint can be assessed: (i)
x
=
0
m,
T
∼
1000
°C, (ii)
x
=
1
m,
T
∼
350
°C, (iii)
x
=
2
m,
T
∼
150
°C and (iv)
x
>
∼
8
m,
T
∼
30
°C. Paleocirculation of meteoric groundwater in the ‘cement-marbles’ generated high-pH fluids that have circulated via fractures and through the matrix porosity of the underlying biomicrites but have also induced alkaline hydrolysis and oxidative attack of the organic matter. The polysaccharide/lignin ratio derived from μ-FT-i.r. analyses shows that the delignification of vegetable debris and degradation of polysaccharides progressively decline in the indurated zone, which indicates a decrease in the pH of migrating solutions. The latter also severely oxidized organic matter at 2.10 and 3.05
m as revealed by the oxygen index and induced the generation of bitumen. The spatial correlation between the oxidation levels of organic matter and the metal contents (Fe, Ti and Cr) suggests that redox reactions were responsible for the immobilization of metals in the indurated biomicrites. The intensity of these reactions is attributed to changes in the fluid flow regime within the sedimentary column.
•A depositional model for post-Marinoan cap carbonates is proposed.•Cap carbonates are interpreted to have deposited in peritidal environments.•A south-westward migration of the carbonate factory ...though time is evidenced.•The top of cap carbonates is a diachronous surface.
In the Niari–Nyanga Basin, the Neoproterozoic SCI Formation of the Schisto-Calcaire Group includes a lower member (SCIa), made up of carbonates and corresponding to the cap carbonate that unconformably overlies the Marinoan glacial diamictite. Petrographic analysis of carbonate and terrigenous rocks from seven stratigraphic sections of the Niari–Nyanga Basin led to define six microfacies (MF0 to MF5). The vertical and lateral variations of the cap carbonate microfacies suggest a shallow-water carbonate system with a dominating microbial carbonate factory. The area of carbonate production migrated through time to the South-West prior to the final demise of the carbonate system in the basin. Such a transition is marked by a sharp negative excursion in bulk carbon and oxygen isotope ratios, that is widely correlatable across the basin, at the top of the SCIa dolostones. The presence of depositional features, such as oolites, peloidal stromatolites with fenestral structures, together with evidences of early leaching of carbonate grains within the SCIa member and at the base of the SCIb member strongly suggest a peritidal marine carbonate production in the Niari–Nyanga Basin, during the deposition of the cap carbonate.
The Neoproterozoic-age Mali Group of the southwestern Taoudeni Basin, NW Africa, represents, in the Walidiala Valley, a glaciogenic and post-glacial succession that brackets the Cryogenian-Ediacaran ...period boundary. At its base, debris flows and turbidite-like, sandy units of the Pelel Member pass upward into siltstone and shale of the Diagoma Member. These two units represent the progressive evolution from some portion of a fan delta fed by a nearby ice shelf to a more distal environment disturbed only by the occasional fallout from passing icebergs. The appearance of coarse-grained, cross-bedded sandstone beds and gravels of the overlying Tanague Member heralds a return to a shallower, fluvially influenced environment before abrupt transgression caps the glaciogenic succession. The transgressive unit consists of a regionally extensive, 2-7 m-thick, silty dolostone, the Bowal Member, which is isotopically and petrographically indistinguishable from ca. 635-Ma cap dolostone units elsewhere in NW Africa and worldwide. The Bowal Member comprises microcrystalline dolomite in turbidite-like depositional sheets disrupted by internal brecciation, fracturing and cementation by first chert and then dolomite. The stratigraphic succession in the Walidiala Valley closely resembles facies models relating to glacial retreat in a proximal glaciomarine environment affected by glacioeustasy. A large volcaniclastic debris flow has caused slumping and soft-sediment deformation within the cap dolostone of the Bowal Member. The widespread association of pyroclastic deposits with cap dolostone throughout the Taoudeni Basin implies that volcanism and deglaciation were roughly contemporaneous across a huge area. We consider that the volcaniclastic debris flow and soft-sediment deformation within the underlying Tanague Member were possibly triggered by seismic activity during deglaciation, caused by isostatic relaxation of the lithosphere. However, fitted brecciation of cap dolostone beds here and elsewhere in the world is more consistent with pervasive dolomite cementation.Original Abstract: Dans la vallee de Walidiala, au sud-ouest du bassin de Taoudeni, le groupe du Mali comprend une succession glaciaire et postglaciaire neoproterozoique se rapportant a la limite Cryogenien-Edicarien. Cette succession debute par des coulees de debris et turbidites sableuses formant l'unite de Pelel, laquelle passe vers le haut a des siltstones et argilites a blocs laches de l'unite de Diagoma. Ces deux unites marquent le passage d'un environnement de type lobe deltaique, proche d'une marge glaciaire, a un environnement plus distal, alimente notamment par des icebergs. Au-dessus, la presence de gres grossiers a stratifications obliques de l'unite de Tanague marque le retour a un environnement moins profond et sous influence fluviatile, avant la grande transgression regionale representee par un banc de 2 a 7 m de dolomies silteuses formant l'unite de Bowal. La composition isotopique et petrographique de cette unite carbonatee est identique a celle des cap dolostones qui coiffent les depots glaciaires neoproterozoiques ( 635 Ma) en Afrique de l'Ouest et ailleurs dans le monde. Il s'agit de dolomie microcristalline formant des lits turbiditiques localement brechifies, fractures, avec un ciment siliceux puis dolomitique. La succession stratigraphique de la vallee de Walidiala est tres proche des modeles proposes pour illustrer un retrait glaciaire en milieu marin proximal affecte par la glacio-isostasie. Un important ecoulement de debris volcanoclastiques a provoque glissements et deformations souples dans les bancs dolomitiques de l'unite de Bowal. L'association generalisee de depots pyroclastiques avec le depot des cap dolostones sur l'ensemble du bassin de Taoudeni suggere que les deux phenomenes sont contemporains et en relation avec les evenements tectoniques panafricains. On considere que l'ecoulement de debris et la brechification de l'unite de Bowal, ainsi que les deformations souples dans les gres de Tanague sous-jacents, ont pour origine une activite sismique liee a la relaxation isostatique de la lithosphere lors de la deglaciation
The results of our new paleomagnetic investigations on 21 sites in the Cévennes and Lure regions as well as previous studies demonstrate that all Mesozoic marly limestones of SE France exhibit ...similar paleomagnetic behavior with remagnetization disputed in age. The studied areas have the particularity to have been folded before (Late Eocene), the Alpine folding (Oligo–Miocene). Samples (201 marly limestones) dated from Lower Jurassic to Lower Cretaceous have been demagnetized by thermal treatment. They all present a well-defined component with a normal polarity which was mostly obtained between 200 and 350 °C. Numerous arguments lead from pretectonic to syntectonic widespread remagnetization related to orogenic fluid circulation affecting the whole basin. An Eocene age (between 35 and 40 Ma) is obtained for this remagnetization thanks both to the comparison of the average inclination of all regional paleomagnetic studies (+54.9°/−1.5°) with the expected paleomagnetic inclination and the syntectonic character of remagnetization.
The Néel temperatures (TN) of synthetic goethites (α‐FeOOH), substituted with diamagnetic elements (Al, Ga) have been determined from the temperature dependence of their low‐field susceptibility ...using a Kappabrige AC bridge. These temperatures proved to be identical to those derived from more conventional measurements of high‐field or remanent magnetization versus temperature, and in good agreement with those inferred from Mössbauer models. The low‐field susceptibility based‐technique appears to offer a precise and rapid method of determining TN. While Néel temperature coincides with maximum unblocking temperature in Gagoethite s, supporting the assumption that antiferromagnetism and parasitic ferromagnetism have a similar origin in goethite, the lack of clear correlation between saturation remanent magnetization and increasing diamagnetic substitution suggests that the weak ferromagnetism of goethite is due to substitution independent defects induced during crystal growth.
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