Numerical terrain analysis of fluvial-marine watersheds on the Isle of Santiago, Cape Verde, based on satellite imagery, ground-truthing and landform indices - A preparatory study in search of Nb -Ta - REE deposits related to hotspot islands

Numerical terrain analysis constitutes the missing link between the classical descriptive geomorphology and geomorphometry providing landform indices by means of which issues in applied and genetic geosciences can be solved. This method is fully satellite-based and supplemented by ground-truthing dealing with the interior parts of the positive landforms and the inter-landform sediments. This approach can successfully be taken prevalently for immature modern landscapes, exemplified by Cenozoic volcanic hotspot islands such as Santiago, the main island of the Cape Verde Archipelago, where four lithofacies types could be delineated. Corresponding to the scale of observation, a tripartite subdivision is achieved in numerical terrain analysis (1st order regional, 2nd order local, 3rd order outcrop scale). The first order indices allow for a tripartite compartmentalization of the volcanic island into the presumed paleosurface of all volcanic summits within a certain altitude ("Gipfelflur"), the volcanic pediment and the coastal zone. Among the second order indices, the VaSlAnalti index (Variation of Slope Angle altitude) giving the variation of slope angle as function of altitude is an excellent environmental marker. Its correlation coefficients provide a measure for the homogeneity of volcanogenic, mass wasting, fluvial, and coastal landform series. Among the third order indices, the QuantGravsituindex (Quantification Gravel situmetry) lends much support to the afore-mentioned environmental markers. It is a meticulous measure of the modality, sharpness and fan width of the orientation of clasts with their data, illustrated in semicircle rose diagrams. It is used to fine-tune volcaniclastic deposits at outcrop scale. The compositional quantification encompasses mineralogy, biosedimentology and isotope geochemistry (delta 13C, delta 18O) as well as 14C-dating. The compositional study of the terrain analysis reveals on the biosedimentological part a low-relief coral accumulation with an impoverished fauna of Caribbean affinity. On the mineralogical part, the two strings heavy and light minerals unravel different processes. The heavy minerals accumulated in the fluvial-marine sediments of the coastal region point to magmatic host rocks from basaltic andesites to picrobasalt and basanite. Zeolites among the light minerals are indicative of a meteoric to low-temperature hydrothermal alteration confined to the lithofacies types C and D. The coastal zone shows a characteristic quadripartite subdivision. It is the reference terrain for any inter-island comparison regarding volcanogenic islands. And it is the starting level for a more detailed characterization of the volcanic island's upper slope. LFS A1 (Landform Series) is characteristic of a curvilinear-rectilinear cliff coast sculped by a strong marine wave action with subordinate subaerial- submarine point sources. The "ria coast" is typical of LFS B1 and demonstrates the fluvial-marine interaction. LFS C1 developed a "volcanic bird-foot delta"