Jacques Lang

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’Europe ou encore avec les USA (Western Interior + Nouveau Mexique et Texas). La presence d’ammonites comme les Cibolaites (?) africaensis nov. sp. ou les Coilopoceras inflatum Cobban & Hook permettent egalement de bonnes correlations avec l’ouest et le sud des USA. Toutefois les faunes d’ammonites restent essentiellement d’affinite tethysienne comme l’ont deja souligne de nombreux auteurs avec en particulier Nigericeras gadeni ( Chudeau ) et Thomasites nigeriensis ( Woods ) qui montrent un endemisme saharien assez marque (Algerie, Niger, nord-est Nigeria). Bien que l’existence d’une liaison entre la Tethys et l’Atlantique sud, via le fosse de la Benoue pour le Cenomanien — Turonien, soit contestee dans des travaux recents, la repartition des ammonites nous incite, a l’instar des autres ammonitologues, a y voir l’existence d’un bras de mer entre ces deux domaines paleogeographiques. Cette connexion semblerait debuter vers la partie superieure de la zone a Gadeni (Cenomanien superieur) et se prolongerait aux environs de la fin de la zone a Nigeriensis (fin du Turonien inferieur). L’aspect ontogenetique et evolutif est egalement aborde dans cette etude, en particulier pour les Nigericeras gadeni ( Chudeau ) — N. jacqueti Schneegans — N. jacqueti involutus nov. subsp. qui tendent au cours de leur histoire a perdre leur ornementation juvenile par peramorphose alors que, chez le groupe Vascoceras (Paravascoceras) cauvini ( Chudeau ) — V. (P.) proprium ( Reyment ) ou le polymorphisme est exacerbe, la forte costulation des formes adultes tend a s’effacer par une tendance inverse: la paedomorphose. Il en resulte une tendance generalisee, chez les ammonites nigeriennes vivant dans des mers peu profondes, a acquerir des morphologies lisses et des sutures simplifiees en reponse probablement aux contraintes externes (synergie des contraintes d’ordre climatique, ecologique et physiographique) en developpant en fonction de leur contrainte interne (potentiel ontogenetique) des morphologies assez differentes et peut-etre plus performantes pour le groupe (canalisation de la derive morphologique).

Nouvelles données sur lesammonites du Niger oriental (Ténéré, Afrique occidentale) dans le cadre de la transgression du Cénomanien-Turonien

The new data on the ammonites faunas allow to specify the Cenomanian-Turonian biostratigraphy of the Nigerduring the transsaharian marine transgression with the description of two new horizons: Glabrum horizon and Wallsi horizon. If the endemism tendancy is corroborated mainly for the Damergou and the Iullemmeden Basin, the new results make conspicuous a strong North-South connection between the Tenere and the NE Nigeria with the presence of Vascoceras aff. glabrum (Barber), Neoptychites gr. cephalotus (Courtiller), Thomasites nigeriensis (Woods), Thomasites wallsi (Reyment) et Choffaticeras sp. Moreover a new species Thomelites (?) tenereensis nov. sp. is here describeb.

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–Early Silurian storm and tidal sediments of the Kandi Formation, made up of hummocky cross-stratified sandstones and siltstones. Computer-aided analysis of the populations of synsedimentary to synlithification microfaults observed in the Late Ordovician to Early Silurian sediments shows evidence of extensional paleostress tensors with a N90°E to N100°E horizontal σ 3 stress responsible for normal displacement along the Kandi Fault. The synsedimentary normal activation of this major fault, inherited from the Precambrian, controls the spatial arrangement of the glacial, braided-stream, storm to tidal, and offshore deposits as well as the deformation of the basin-fill into an asymmetric synsedimentary syncline associated with progressive unconformities. The characterization of glacial features and Late Ordovician deposits from the biostratigraphic distribution of traces of trilobites strongly supports the idea that the deposits of the Kandi Basin are contemporaneous with the melting of the wide ice sheet which overlay the Afro-Arabian Shield during Late Ordovician times. The successive deposits of the Were and Kandi Formations reflect a gradual change from tillites, through glaciofluvial outwash conglomerates (Wa Member), braided-stream sediments (Wb Member), and shoreface barrier sands (Ka Member), to offshore clays and sands (Kb Member). They correspond to reworked, glaciofluvial to marine facies laid down by the Late Ordovician glacial retreat. The Kandi Basin is therefore defined as a staging-post between the Late Ordovician––Early Silurian basins of the Sahara and those of South Africa.

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 fluvial-lacustrine and glacial continental sedimentation, punctuated by many sedimentary gaps and episodes of erosion, and were affected by Late Precambrian extensional faulting indicated by NNE-SSW oriented tilted blocks. This Precambrian extensional faulting was reactivated during subsequent tectonic episodes, and most notably during the Early Cretaceous rifting phase.

Unusual faunal associations during Upper Cenomanian-Lower Turonian floodings on the Niger ramp (central West Africa)

Abstract The Upper Cenomanian-Lower Turonian marine deposits of the Iullemmeden Basin and of the northern Chad Basin (Niger) consist of interbedded mudshales and fossil-rich limestones that formed on a ramp as a result of a Tethyan transgression. The faunal content of the limestone beds is characterized by (1) the absence of forms typically found on the Tethyan platforms: rudistids, corals and commonly associated forms (large benthic foraminifers, bryozoans, calcareous sponges, red and green algae); (2) the absence of pycnodonts and of brachiopods, and (3) the abundance of Gryphaeidae and Ostreidae, as well as the relative abundance of ammonites. This unusual faunal composition is ascribed to several factors. Rudistids, corals and associated forms may have failed to occur because of the general decrease in the diversity of these organisms in the Upper Cenomanian-Lower Turonian and because of the lack of suitable shoals on the Niger ramp for reefs to develop. Pycnodonts may have failed to gain a foothold in the face of severe competition from pioneer and opportunistic bivalves (Gryphaeidae, Ostreidae) that quickly colonized the ramp. The absence of brachiopods has not yet been explained satisfactorily; it could be due to several factors, including severe competition with pioneer bivalves and/or excessive turbidity of bottom waters.

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 under SEM and found to correspond to what under the light microscope appear to be ‘honeycomb’ structures. The diameter of alveoli can be the same as that of a ‘false branching’; their walls, formed by the outer sheath, have a dense network of microfibrils. These observations confirm the formation of calcitic lamellae at the false branching points and in contact with the microfibrils of the outer sheath. Confirmation of earlier hypotheses was sought through observation in thin section of the stromatolites collected over seven years. Micritic dark laminae are thought to form in three stages: (i) formation of a dark lamina by formation of a false branching zone in the wet season, (ii) initial calcification in this zone as microsparitic and sparitic lamellae during even a short dry spell, and (iii) subsequent bacterial micritization of the lamellae during an extended warm, dry season. The sparitic light laminae form around the hair-bearing regions. Lamination was first considered with regard to the annual radial growth rate of R. haematites. The thin section may display either a single dark lamina for two years’ growth, or a dark lamina thicker than the annual growth rate. Examination of local meteorological data accounts for the first phenomenon by a wet spell of more than 13 months and the second by a warm, dry period of one month. It may therefore take several years for a couplet to form as growth of the cyanobacterial colony is influenced by climatic variations. The different calcification phenomena were also correlated with dry spells. These interpretations are consistent with results obtained by experiments.

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 south of the Carnot Formation. These results provide useful indications as to the provenance of diamonds mined in the southwest Central African Republic.

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

The Interior Basin of Gabon, created during the break-up between South America and Africa, displays thick Neoproterozoic to Aptian p.p. fluvio-lacustrine deposits overlain by Aptian to Albian marine facies. Rock–Eval analyses from outcrop and drillhole samples show high content in organic matter (up to 25%) related to types I and II. These intervals are encountered within Permian, Neocomian–Barremian as well as Aptian siliciclastic succession. They constitute fairly good to excellent

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.

Discontinuities in the recent and Quaternary margino-littoral sedimentation of Benin (humid tropical zone)

Abstract In the first part of this paper, factors involved in the present margino-littoral sedimentary dynamics are reviewed, with the ⪡normal⪢ biosedimentary phenomena or “background noise” on a daily and seasonal scale. The breaks in sedimentary and environmental equilibria which generate sedimentary discontinuities and major facies variations are then analyzed. Within the Quaternary sequences, beside the breaks in sedimentary and environmental equilibria which act on a daily or seasonal time-scale (e.g. hurricanes, floods), it is possible to observe slower phenomena lasting over a hundred or a thousand years (sea-level oscillations, epirogenic and neotectonical movements, palaeoclimatic changes, streams wanderings). The recent margino-littoral domain of Benin, with alternating wet and dry seasons, adding their effects to those of saline intrusions, is a very favourable environment where biosedimentary phenomena, ⪡normal⪢ as well as ⪡exceptional⪢, on various time scales, can be recorded. The aim of this paper is to recover the information “recorded” from these “recordable” biosedimentary phenomena , more or less magnified or dampened by the “recording” environment , in the Upper Quaternary sequences of Benin (peat layers, mollusc thanatocoenoses, coprolites, facies variations, clays and heavy-mineral assemblages). These phenomena may appear either as short-lived events ( “crises” ) or as critical events such as palaeogeographical events (40 000 to 39 000 years B.P., 6000 to 5000 years B.P.) or palaeoclimatic events (18 000 years B.P.).