NEBKHAS - THEIR SPATIAL-DISTRIBUTION, MORPHOMETRY, COMPOSITION AND AGE - IN THE SIDI-BOUZID AREA, CENTRAL TUNISIA

Nebkhas, accumulations of dust and sand that collect in the lee of, or around, a clump of vegetation, were studied in an extensive area in c entral Tunisia. Their spatial distribution, as well as their morphomet ry, composition and age, were analysed. The results indicated that neb khas that have accumulated around the Ziziphus lotus thornscrub have p rimarily developed within alluvial fans of wadis that drain mountainou s rocky areas. These areas supplied the material that formed the nebkh as. An increase in the extent of nebkhas can thus be caused by an enha nced erosion rate in mountainous areas generated by, for example, defo restation. Secondly, nebkhas have developed along the margins of the m ajor wadi beds that dissect the plains. Statistical tests showed that the nebkhas morphometry varied geographically in the area. It was conc luded that the variation reflected differences in sediment supply. The relatively poor sorting of the nebkhas sediments can be explained by the fact that nebkhas were situated close to a sediment source. The mo st important sediment sources for the nebkhas were found to be (a) all uvial fans, (b) broad wadi beds and (c) cultivated ground on weakly de veloped, sandy soils. The ages of four nebkhas were determined, by mea ns of tree-ring analyses, as being between 50 and 200 years.