RELATIONSHIP BETWEEN SEDIMENTS AND THEIR IGNEOUS SOURCE ROCKS USING CLAY MINERAL MULTIELEMENT CHEMISTRY - THE CENOZOIC LACUSTRINE ANLOUA BASIN (ADAMAOUA, CAMEROON)

The Cenozoic lacustrine basin of Anloua (Adamaoua, Cameroon), is filled with the weathering products of the surrounding rocks. In order to determine the origin of these sediments, multi-element, especially trace element, analysis of their < 2-mu-m fraction was undertaken. The chemical compositions of these fractions were compared with those of the rocks surrounding the basin by multivariate statistical analysis and also graphically. A principal component analysis allowed the most discriminant elements to be selected, and a hierarchical ascending classification (HAC) made with these elements divided the samples into groups. Some of these groups contain < 2-mu-m fractions of both the lake sediments and surrounding rocks. These sediments are determined as deriving by weathering from the surrounding rocks. Some of the other groups contain no samples from the surrounding rocks and their petrographic origin cannot be given. A graphic comparison of the relative amounts of the elements (determined by HAC) from the < 2-mu-m fractions of the various groups and the rocks surrounding the basin allowed attribution of a petrographic origin for the various sediments analysed by using the fit between the curves. The clay mineralogy, the amounts of Ti, Zr, and heavy minerals in the < 2-mu-m fractions and their distribution in the area studied, as well as the presence of angular quartzite pebbles in some sediments gives an indication of the paleogeography of the Anloua basin. There was deltaic input towards the north and a high energy input, possibly torrential, to the west.