Abstract :
[en] In Southern High Katanga, the ferruginous formations such as iron duricrusts and stone lines with lateritic nodules
show a diversity as broad as in Western Africa. Bauxites are nevertheless almost absent proving that, here, aluminium
does not contribute to the genesis of duricrust, leaving iron as the only responsible.
Among that wide diversity, the indurated sands form a special case because of their dual origin. Because of a large
porosity favouring a fast iron penetration and fixation, some of the sandy concretions or duricrusts may have a short
history and some of them may even be very recent. Nevertheless the others have a genesis that fits in with the general
longer scheme.
The most frequent type of laterite crust is also the most complex one. Its facies is both nodular and vermicular.
Several agents and climatic phases are involved in its formation :
• A first planation surface with wide-open valleys where the water table lies near the surface induces favourable
circumstances for the next step;
• In such circumstances, nodules are shaped in the mottled soil horizon. One exceptionally well-preserved site
evidences the proximity between mottles (the nodules to be) and water table level at the end of the dry season;
• A revival of erosion enhances river down cutting which in turn lowers the water table so that termites can reach the
mottled horizon;
• Nodules are mechanically concentrated by elimination of the non-indurated zones (relative accumulation): within
the soil, by termites (on shaly rocks), at the surface by run off (often, on sandy rocks, unfavourable to termites
activity);
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• Finally the ran off extends to both types of rock and denudes the indurated zone;
• A ferruginous coating forms on different substrates : upper laterite surface, internal joints, termite galleries as well
as on any gravel at the soil surface. The coating lies unbroken from one object to the other but the outer surface is
frequently eroded. The superficial coating could be a rock varnish formed by bacteria during a desertic period.
The coating of galleries partly inhibits bioturbation because they collapse no more. The network of the
strengthened galleries constitutes an internal skeleton reinforcing the laterite mantle;
• With the return of a less severe climate, termites from below the laterite mantle again bring up material. The
resulting upper fine deposit often exceeds 2 meter.
All those processes imply important climatic variations :
• A steppe vegetation with a scanty grass cover favouring run off and planation;
• A relatively arid climate for the coatings formation;
• A contrasted seasonal regime, not so different from the present day climate (six months dry season) for vertical
erosion and intense termite activity.
The weathering of rocky materials except for the iron impregnated ones (nodules, ferricretes and so on) requires a
humid climate. This is happening nowadays but the more humid Quaternary phases were more effective in that respect.
The characteristics of the outer coatings (thickness, colour, conservation state of the included quartz grains) allow
us to identify four levels of ferricrete. Their succession is confirmed by the relative position of the coatings between
them and by the correlated continental deposits. Because of their coatings, all those ferricretes are dated back to the
Tertiary (because of the desertic climate needed for the formation of the rock varnish). Their levels overlook the
Quaternary river terraces.
The oldest ferricrete is quite different from the others. It shows neither nodules nor termite galleries. Its facies is
thus very different from the nodular/vermiform of the three younger ones. It shows three facies, sometimes
superimposed : pisolithic, massive or breccia-like. Moreover, the ferricrete of the corresponding level in West Africa
contains bauxite. Its genesis is the result of a very different sequence of processes, some aggressive and linked to a
humid climate with a great supply of iron and others resulting from more arid conditions producing the thick pisolith
coatings.
Ferruginous stone-lines present a wider variety of concretions than duricrast. Their upper part is the result of a
concentration mainly by run off that occurred during several periods of the Quaternary. Cover loam is each time built
up again by the termites from weathered rocks below the stone-line. The same phenomenon is seen on ferricretes.
The soil profile where ferruginous formations can be seen consists of horizons of different ages. Weathering of
saprolite and soil lifting are still active nowadays. Whereas some stone-line profiles are renewed at each Quaternary
dry phase, the ferricrete profiles are build up during a long period of time extending from the Tertiary till now.
A new definition of iron duricrast is proposed that is not based on weathering in depth or on the presence of
aluminium
