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 km2 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.