La formation glaciaire de la Mambéré (République Centrafricaine) : reconstitution paléogéographique et implications à l'échelle du Paléozoïque africain

L'analyse stratigraphique et sedimentologique de la Formation de la Mambere, unite horizontale situee dans l'Ouest et le Sud-Ouest de la Republique Centrafricaine, a apporte des arguments en faveur de l'origine glaciaire des depots; une reconstitution paleogeographique a ete esquissee en vue de preciser l'âge de la formation. Deux grandes categories de depots glaciaires ont ete identifiees: L'origine meridionale du materiel detritique est demontree par l'analyse exoscopique des quartz et l'etude des mineraux lourds. Ce materiel provient essentiellement du demantelement du Complexe Schistoquartzitique precambrien, et accessoirement du Complexe Granito-gneissique. D'apres les trajectoires du …

Sedimentation and tropical vegetation changes in Southern Benin (West Africa), carbon-13 tracing of organic matter dynamics and fluctuations of the sea level during the last 7,000 years.

12 pages; International audience

Deep-rooted “thick skinned” model for the High Atlas Mountains (Morocco). Implications for the structural inheritance of the southern Tethys passive margin

A re-interpretation of the deep structure of the High Atlas is presented through integration of geophysical and geological data, highlighting the architectural significance of the southern Tethys palaeomargin. Previous crustal models suggest the occurrence of a flat intra-crustal detachment at a depth of � 20 km, a zone where surface thrusts merge and below which the lower High Atlas crust appears continuous. However, within this study seismic refraction data, electrical resistivity surveys and gravity modelling all appear to detect a jump in crustal thickness between the High Atlas and the northern plains. We interpret these data as penetration by thrusts within the ‘‘South-Atlasic fault’’…

Nautiles paléocènes du Niger

Resume De nombreux nautiles tertiaires ont ete recoltes au Niger tant dans le Nord-Ouest du pays (region de Mentes et Tillia) que dans le centre (region de Kao) et, dans le Sud, aux environs de Malbaza. Leur etude montre deux faunes assez differentes, la plus riche, dans la region centrale (Kao), caracterisee par de nombreux specimens de Deltoidonautilus molli, la seconde, plus pauvre, caracterisee par Deltoidonautilus sudanensis et D. chudeaui aussi bien au Nord qu'au Sud. Malgre des differences considerables, ces formes qui peuvent etre attribuees au Paleocene, ne semblent pas, d'apres les fossiles associes (notamment l'echinide Linthia sudanensis sudanensis), indiquer une difference d'âg…

Sedimentary dynamics and extensional structuring related to early Cretaceous rifting of Neocomian and Barremian deposits of the interior basin of Gabon.

18 pages; International audience; Recent field and subsurface data about the early Neocomian N'dombo series and the Neocomian to mid-Barremian Schistes series of the interior basin of Gabon further our understanding of the initial stages of early Cretaceous N40–60E extensional rifting. The syn-rift series comprise fluvial–lacustrine claystones–sandstones, rare conglomerates, and carbonates. The syn-rift fill begins with braided-stream feldspathic sandstones. These are overlain first by fluvial–lacustrine deposits and then by predominantly lacustrine–palustrine claystones, which are potential petroleum source rocks. The claystones are eroded in part and are capped by the pre-Aptian angular u…

Erratum to “Lamination of swamp-rivulets Rivularia haematites stromatolites in a temperate climate” [Sediment. Geol. 143 (2001) 125–147]

Le bassin sédimentaire a phosphates du Togo (Maastrichtien-Eocène): stratigraphie, environnements et évolution

Resume Les observations faites a l'affleurement de la carriere de Tabligbo (materiaux pour la fabrication de klinker) et a celles de Hahotoe et Kpogame (phosphates), ainsi que l'analyse de cinq forages, permettent d'affiner la stratigraphie du bassin cotier togolais: rajeunissement des Calcaires de Tabligbo (Paleocene superieur) et des Argilites feuilletees a palygorskite (Eocene inferieur). Un apercu de l'environnement du gisement de phosphates est propose a travers l'evolution sedimentaire et dynamique du bassin: transgression sur une marge continentale, approfondissement progressif du milieu avec depot et conservation de la matiere organique, phosphates en debut de regression, flechissem…

Biocoenosis and induration of freshwaterRivulariastromatolites in a temperate climate

Freshwater Rivularia haematites (D.C.) Agardh and Rivularia biasolettiana Menegh‐ini stromatolites were collected from the Plateau de Langres and Lac d'Annecy (France) and investigated experimentally. In addition to cyanobacteria, the community of organisms consists of Desmococcus (green algae) and many bacteria including Agrobacterium tumefaciens and Pseudomonas diminuta for Rivularia haematites, and Achromobacter group B and Pseudomonas acidovorans for Rivularia biasolettiana. Stromatolite induration is not confined to calcification and may locally exhibit apatite or gypsum crystals, with the latter appearing only in cultures exposed to light. The effect of certain microorganisms of the b…

Sedimentary processes in the Carnot Formation (Central African Republic) related to the palaeogeographic framework of Central Africa

Abstract The depositional environment, provenance and processes of emplacement of the detrital material of the Mesozoic Carnot Formation are defined, by bedding and sedimentological analysis of its main facies, and are reconstructed within the palaeogeographic framework of Central Africa. The clastic material was laid down between probably the Albian and the end of the Cretaceous, in a NNW-oriented braided stream fluvial system that drained into the Doba Trough (Chad) and probably also into the Touboro Basin (Cameroon). The material was derived from weathering of the underlying Devonian–Carboniferous Mambere Glacial Formation and of the Precambrian schist–quartzite complex located to the so…

Breaks in the continental environmental equilibrium and intensity changes in aridity over the past 20 000 years in the Central Sahel

Abstract The record of arid climatic events in continental sedimentation is subject to the intensity of the climatic mechanism which generated them and to the nature of the environment which recorded them. In them Sahel, where the environmental equilibrium is particularly precarious, all types of arid climatic events are likely to be recorded. But the consequences of events of small global intensity (exceptional climatic events and local climatic anomalies) merge with the effects of the ⪡average climate⪢. On the other hand, climatic events on a global scale (climatic crises and climatostratigraphic events) are clearly pointed out, due to the large breaks in environmental equilibrium that th…

First carboniferous conulariids from Niger (west Africa)

Abstract Two new conulariid species, Paraconularia feldmanni sp. nov. and P. sahara sp. nov., are described from the Lower Carboniferous (Visean) Talak Formation of northern Niger. These are the first example of the genus Paraconularia to be reported from Africa and the first Carboniferous conulariids from that continent to be assigned to species level. Previously reported Carboniferous conulariids from Africa were collected from Morocco, assigned to other genera, and left in open nomenclature. Paraconularia is interpreted to have been cosmopolitan by the Early Carboniferous, and is the dominant genus in most assemblages of Carboniferous conulariids.

Sedimentation in a tropical, microtidal, wave-dominated coastal-plain estuary

The Mono estuary is an infilled, microtidal estuary located on the wave-dominated Bight of Benin coast which is subject to very strong eastward longshore drift. The estuarine fill comprises a thick unit of lagoonal mud deposited in a ‘central basin’between upland fluvial deposits and estuary-mouth wave-tide deposits. This lagoonal fill is capped by organic-rich tidal flat mud. In addition to tidal flat mud, the superficial facies overlying the ‘central basin’fill include remnants of spits resting on transgressive/washover sand, an estuary-mouth association of beach, shoreface, flood-tidal delta and tidal inlet deposits, and a thin sheet of fluvial sediments deposited over tidal flat mud. Af…

Paléoenvironnements et caractérisation des roches mères pétrolières des séries pré-salifères du bassin intérieur du Gabon

Resume Le bassin interieur du Gabon, dont l'histoire est liee a la separation des continents Sud-Americain et Africain, comprend des depots continentaux fluvio-lacustres, localement glaciaires, dates du Precambrien superieur a l'Aptien p.p., surmontes par des facies marins aptiens et albiens. L'analyse par pyrolyse Rock–Eval d'echantillons d'affleurements et de forages preleves dans ce bassin met en evidence la presence de facies siliciclastiques riches en matiere organique (jusqu'a 25 % de carbone organique total) de types I et II. Ces niveaux, qui se rencontrent dans le Permien, le Neocomien-Barremien ainsi que dans l'Aptien, constituent d'excellentes roches meres et sont a l'origine des …

Données nouvelles biochronologiques et paléoécologiques sur le bassin sédimentaire côtier du bénin entre le crétacé supérieur et l'éocène basal (Afrique de l'ouest)

Resume Un sondage carotte (Ouinhi) et des couples levees dans la carriere d'Onigbolo, situee a une vingtine de kilometres du forage, sur la partie aujourd'hui on-shore du bassin sedimentaire cotier beninois, ont fait l'objet de ce travail. Les conditions paleoenvironnementales, fort inegales d'un niveau a l'autre, imposaient une etude pluridisciplinaire, aussi bien sedimentologique que paleontologique. Les resultats obtenus, a partir des foraminiferes, ostracodes et palynomorphes, fournissent des donnees biostratigraphiques et paleoecologiques nouvelles, apportant des complements a la connaissance de l'histoire de ce bassin entre le Cretace terminal et l'Eocene basal. La grande rarete de do…

Les ammonites du Sinémuriensupérieur du Jebel-Bou-Hamid (Haut-Atlas central, Rich, Maroc). Approches paléontologique et biostratigraphique

Resume L'etude des faunes d'ammonites - plus de 25 taxons - et de leur succession biostratigraphique dans unecoupe exceptionnellement complete et fossilifere du Haut-Atlas central de la region de Rich (Maroc) a permis la mise en evidence de 10 unites biochronologiques elementaires (=assemblages) pour une periode debutant au sein de la partie terminale du Sinemurien inferieur et s'achevant avec le Sinemurien superieur. Pour les zones a Oxynotum et Raricostatum, il s'agit des premieres donnees biostratigraphiques collectees avec une telle precision dans le Nord de l'Afrique. Pour le Sinemurien inferieur et la zone a Obtusum, les nombreuses donnees nouvellement acquises, completent, precisent …

Erratum to “Sedimentary dynamics and extensional structuring related to Early Cretaceous rifting of Neocomian and Barremian deposits of the interior basin of Gabon” [J. African Earth Sci. 51 (5) (2008) 239–256]

Modelo de cabalgamiento profundo para el Alto Atlas (Marruecos). Implicaciones sísmicas en la zona de colisión entre Eurasia y Africa

Previous crustal models of the High Atlas suppose the existence of a mid-crustal detachment where all the surface thrusts merged and below which the lower crust was continuous. However, both seismic refraction data and gravity modeling detected a jump in crustal thickness between the High Atlas and the northern plains. Here we show that this rapid and vertical jump in the depth of Moho discontinuity suggests that a thrust fault may penetrate the lower crust and offset the Moho (deep-rooted “thick skinned” model). The distribution of Neogene and Quaternary volcanisms along and at the northern part of the High Atlas lineament can be related to the beginning of a partial continental subduction…

Sedimentary dynamics and structural geology of pre-rift deposits of the interior basin of Gabon

Abstract Recent studies combining field data (facies and paleocurrent analyses, structural surveys), sub-surface data (core analyses, well logs, seismic sections) and palynological data provide insight into the sedimentary dynamics of pre-rift deposits of the interior basin of Gabon and their structural and paleoclimatic setting. These sedimentary deposits belong to the Noya, Agoula and M’vone series of Late Precambrian to Jurassic age. Although these deposits are potential sources of petroleum, they are partly known because of poor outcrop in a wet equatorial climate, low density and uneven distribution of exploration wells and poor seismic survey coverage. The pre-rift deposits reflect fl…

Les ammonites du Niger (Afrique occidentale) et la transgression transsaharienne au cours du Cénomanien-Turonien

Resume Notre travail s’inscrit dans le cadre d’une revision de la paleontologie des ammonites liees a la transgression transsaharienne du Cenomanien-Turonien et d’un affinement de la biostratigraphie dans le bassin sedimentaire des Iullemeden (Niger occidental et meridional: Mt Iguelela et Damergou). Plus de 700 ammonites ont ete recoltees avec precision dans 8 localites, Il a ete ainsi possible de mettre en evidence une serie de 9 horizons biostratigraphiques et de decrire egalement 3 taxons nouveaux. En particulier, la presence de Metoicoceras geslinianum (d’ Orbigny ), associe aux Nigericeras gadeni ( Chudeau ) permet de mieux correler les niveaux a N. gadeni avec le nord-ouest de l’Euro…

Dynamique sédimentaire des dépôts post-rift du bassin intérieur du Gabon.

14 pages; International audience

Sedimentation in the Kandi extensional basin (Benin and Niger): fluvial and marine deposits related to the Late Ordovician deglaciation in West Africa

Abstract The Lower Paleozoic detrital succession of the half-graben Kandi Basin in West Africa (Niger-Benin) is about 600 m thick and rests unconformably on the Pan-African basement. Along the western edge of the basin, the base of the succession locally features large glacial fault-bounded paleovalleys. These valleys are filled by the lowermost continental deposits of the Were Formation characterized by massive diamictites with dropstones, and coarse to conglomeratic sandstones associated with large-scale channel structures and internal erosional truncations. The uppermost braided-river deposits of the Were Formation deposited across the entire basin are overlain by the Late Ordovician–Ear…

Sedimentation in a fluvially infilling, barrier-bound estuary on a wave-dominated, microtidal coast: the Oueme River estuary, Benin, west Africa

The Oueme River estuary is located on the seasonally humid tropical coast of Benin, west Africa. A striking feature of this microtidal estuary is the presence of a large sand barrier bounding a 120 km2 circular central basin, Lake Nokoue, that is being infilled by heterogeneous fluvial deposits supplied by a relatively large catchment (50 000 km2). Borehole cores from the lower estuary show basal Pleistocene lowstand alluvial sediments overlain by Holocene transgressive–highstand lagoonal mud and by transgressive to probably early highstand tidal inlet and flood-tidal delta sand deposited in association with non-preserved transgressive sand barriers. The change in estuary-mouth sedimentatio…

Etude expérimentale de la lamination des stromatolithes à Rivularia haematites en climat tempéré: édification des lamines micritiques

The lamination of Rivularia haematites stromatolites (D.C.) Agardh was studied experimentally for 7 years. Micritic laminae are found to form in three stages: biological formation of dark laminae during the wet season, microsparitic calcification of these laminae in the form of clearly individualized polycrystalline aggregates and, finally, micritization of the latter by bacterial action. These three stages develop over 2 to 3 years. The occurrence of transverse, longitudinal and circular microfibrils in the outer sheath is thought to explain the nucleation and the three-dimensional structure of the microsparitic crystals of the dark laminae and of the polycrystalline aggregates.

Lamination of swampy-rivulets Rivularia haematites stromatolites in a temperate climate

Abstract Swampy-rivulets Rivularia haematites (D.C.) Agardh stromatolites were collected from the same site on the Plateau de Langres (Chaugey, France) over a seven year period. Specimens were studied in thin sections, in culture media under varying conditions of temperature and illumination, and by scanning electron microscope (SEM) in order to investigate the processes of calcification and lamination. New SEM observations confirm the polycrystalline structure of the ‘lamellae’ reported by the authors and show that the crystals composing these lamellae are elongate and aligned in the same direction as the filaments linked by them. The lamellae were also isolated in transverse sections unde…

Évolution tectono-sédimentaire de l'unité d'Akokan au cours du Viséen dans le bassin de Tim Mersoï (région d'Arlit, Niger)

Resume Dans le bassin de Tim Mersoi, les facies greseux fins, argilo-silteux, de l'Unite d'Akokan, typiques de milieux tidaux, se substituent progressivement aux depots argileux sous-jacents a caracteres marins du Talak. Ils sont separes de la formation surincombante du Guezouman par une discontinuite erosive, largement representee dans la region, ce qui est un argument determinant pour leur rattachement au meme cycle sedimentaire que les Argilites du Talak sous-jacentes. Au cours du depot de cette Unite d'Akokan, des accidents cassants N60 °E a 80 °E apparaissent en bordure immediate de l'Air et engendrent des sous-bassins ou la serie devient de plus en plus epaisse. Le jeu continu de ces …