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

Abstract

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 VaSlAn_alti 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 QuantGrav_situ index (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 (δ¹³C, δ¹⁸O) 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” of LFS C1. The coastal area LFS C1 is markedly different from the afore-mentioned coastal areas forming a prominent seaward bulge with a volcanic delta. Volcano-physical processes are held accountable after hot lava got in touch with water was quenched and solidified. The “headland- and bay coast” of LFS D1 is a degraded coastal system that has been derived from LFS C1, as the bird foot structures underwent strong longshore erosion leaving behind some headlands, stumps and stacks surrounding a bay with a very low cliff and gentle slopes passing into a flat sandy beach.

Away from the coastline, in the hinterland a quadripartite horizontal subdivision of the landscape overlaps with a binary vertical morpho-dynamic subdivision. The Older Hotspot Island (=OHI) spans the period 10.5–4.6 Ma and the Younger Hotspot Island (YHI) the interval <4.5 Ma. The OHI resides on the central axis. Strong vertical morphotectonic activity on its SW limb accounts for the evolution of the volcanogenic badlands (LFS B) and gave rise to their subsequent dissection (LFS A). The YHI is affected by strong lateral morphotectonic activity leading to the volcano-tectonic wedge of LFS C. The dissected volcanic subaerial shield area of LFS D hallmarks the preexisting shield morphology with a consequent drainage system the (sub) parallel volcanic ridges of which extend down to the present-day shoreline of LFS D where they end up in coastal marine headlands.

The numerical terrain analysis can be run fully satellite-based. It allows for a more objective handling of landform during a genetic and applied geosciences. This numerical terrain analysis may directly translate into the search of niobium (Nb), tantalum (Ta), lithium (Li), beryllium (Be), and rare-earth elements-bearing (REE) mineral deposits, which are coupled with certain volcanogenic metalliferous events. The metallogenic comparison of the Canary Islands and the Cape Verde Archipelago is based on terrain analysis and yields a contrasting picture of both alkali-magmatite-carbonatite hotspot provinces regarding their potential for REE-Nb-Ta-Be-Li metal deposits. The northern Canaries and their submarine terrain are rather promising targets for rare metal exploration whereas the potential of the Cape Verdes is rather bleak in this respect.

Limitations on this terrain analysis may be imposed by the climate (tropical humid zones) where true color satellite imagery should be supplemented by infrared-based - remote sensing techniques.