Landsat TM data processing for the characterization of the anorthositic complex of Cunene (south Angola)

Landsat TM data have been used for a lithological and structural characterisation of the anorthositic complex of Cunene that can be considered, with its 23.000 Km2 of extension, one of the greatest emerging anorthositic complexes on the surface of the earth. The work was carried out along two main lines. We first proceeded to a visual interpretation of the single bands and of the colour composite images, band-ratio composites and Principal Component Analysis for the identification of the contacts, the overburdens, the families of fractures and the zones of alteration and lithological differentiation of anorthosites. We then implemented digital analysis procedures in order to obtain a supervised classification of the lithology in a limited zone of the anorthositic complex. This area has an extension of about 1000 Km2 and is characterised by an elevated aridity and scarce vegetation. The multi-spectral image classification was carried out on a sample of many hundreds of pixels representing portions of territory known through the existing cartography and literature or through reconnaissance fieldwork. This classification has given satisfying results discriminating, with good approximation, the anorthositic outcrops from the alluvial and regolithic overburden. On the other hand, the rare granitical outcrops have given classification problems due to the elevated effects of weathering on the formation.