Claude Censier

Origin and function of a closed depression in equatorial humid zones: The Lake Télé in North Congo

Lake Tele has a nearly perfect ellipsoidal shape and is surrounded by swamps and barely penetrable flooded forests in the heart of the Congo—Zaire watershed basin, and has intrigued the international scientific community for decades. In June 1992, a first Franco—Congolese multidisciplinary scientific expedition was finally able to reach the lake which is vast (23 km2) and shallow (3 m). Its volume is estimated at 71 × 106m3 and is 40% filled with an organic silt layer a metre thick. Its hydrological exchanges are almost exclusively vertical with very little lateral contribution from the surrounding swamp. This leads to the observation that the waters are very slightly mineralized (< 3 mg 1−1), but are very rich in organic carbon (44% of suspended matter and of total dissolved matter) and are very acidic (pH < 4). In addition, a magnetic anomaly of some hundred nanoTesla from a magnetic body at shallow depth has been detected in the lakes northern half, although its origin remains unknown. Botanical and palynological observations suggest the persistence of a strongly hydromorphic forest environment for at least the last 6600 years. Identified pollen taxa, whether ancient or recent, come mainly from pioneer and colonizing taxa such as Macaranga, which tends to indicate that the forest is continuing to gradually fill in the lake.

Characteristics of Mesozoic fluvio-lacustrine formations of the western Central African Republic (Carnot Sandstones) by means of mineralogical and exoscopic analyses of detrital material

Abstract The so-called Carnot Sandstones, Mesozoic fluvio-lacustrine detrital formation, which stretch over an area of more than 40 000 km 2 in the western part of the Central African Republic, are made of a succession of conglomerates, sandstones and argilites which can reach 300 m in thickness. Heavy mineral analyses and quartz exoscopic studies of this detrital material allow to understand the geology of these formations. The detrital material origin: heavy mineral distribution in the lower levels indicates mostly a meridional origin with a quantitatively poor peripheral supply. The importance of the transport: quartz exoscopic observations show that the main part of the detrital material is highly evolved. Given the continental character of this formation and the geological context of the southwest of Central African Republic, it is possible that a reworking of one or several former detrital formations occurred; thus, the long evolution of this material is the result of the juxtaposition of several transports in an aquatic environment during geologic times. The extension boundaries of the Carnot Sandstones: using mineralogical and exoscopic analyses, it is possible to identify the origin of the material of certain alluvial deposits; this method of analysis allows us to demonstrate that the western boundaries of the mapped Carnot Sandstones are erosion boundaries and not extension boundaries; the latter are situated near the frontier between the Central African Republic and Cameroon.

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.