Modelling Solutions for Ferricrete Formation and their Impacts on Topography

Ferricretes, or iron duricrusts, are hard iron layers, which predominantly develop in tropical and subtropical environments. They commonly cap elevated topographical features, potentially safeguarding old landscapes. The genesis of duricrusts is intricately tied to climatic conditions, particularly relying on intense seasonal precipitation cycles.

Two hypotheses for iron duricrust formation exist: the hydrological or horizontal hypothesis and the laterisation or vertical hypothesis. In the first case, elements forming the duricrust are transported from distant areas and concentrated by hydrological processes. In the second case, the protolith is the underlying basement, and ferricretes form through leaching of soluble elements and compaction of less soluble ones.

As no numerical model has been proposed for ferricrete formation until recently, we incorporated both formation hypotheses in a previously described numerical model for regolith formation (Braun et al., 2016).  The hydrological model was profusely described last year (Fenske et al. at EGU23), thus, we will concentrate on the laterisation model. In accordance with the second hypothesis, ferricrete formation follows laterisation of the regolith. During laterisation, the most soluble elements gradually dissolve and leach, resulting in the enrichment of non-soluble elements like iron and compaction. The model is characterized by two parameters: the time scales for iron enrichment τ_l and compaction τ_c, respectively. Various numerical scenarios were performed under diverse tectonic and climatic. The threshold Ω_min was determined to state formation or not of ferricretes.

To calibrate the model, a case study has been defined in the southeastern part of Brazil, the Quadrilátero Ferrífero (QF) region. Ranging almost 7 000 km², this region is known for its abundance in iron ores and distinct topography with escarpments and high plateaus commonly topped by cangas. Cangas are a type of ferricrete which form from the weathering of BIFs (Banded Iron Formations). The oldest registered formations are up to 70 Ma old and seem to protect some of the mountain peaks for extended periods of time. Multiple scenarios are proposed to describe today’s landscape, with different climatic and tectonic parameters in play. With the help of the laterisation model, it is possible to model different scenarios to attempt to depict the formation of the QF.