A Numerical Laterisation Formation Model for Ferricretes

Ferricretes are hard iron layers forming in semi-arid to subtropical environments. We can observe them in i.e. Africa, Australia or Brazil. They are an important part of regional geomorphology, capping hills and protecting landscapes. Climatic dependance is very high. Ferricrete formation occurs under strongly seasonally contrasting climates with precipitation and dry cycles. During wet seasons, transport and accumulation of elements happens while during dry seasons, precipitation and hardening dominate. It is also known that ferricretes form in tectonically “quiet” environments, and approximately need 1 Myr to form meter thick layers.

There are two ferricrete formation hypotheses, the hydrological hypothesis and the laterite hypothesis.

Recently, we developed a numerical model for ferricrete formation based on the laterisation hypothesis. 33% of land surfaces are covered by laterites today. The thickest lateritic profiles evolved for millions of years and are found in the centre of tectonically inactive areas. Weathering is the main process responsible for laterisation, transforming bedrock into regolith. A typical lateritic profile is divided into multiple stages from the weathering front to the surface, starting with a coarse grained and then fine grained saprolite, a mottled zone and at the top, a ferricrete.

In our model, we assume that as the regolith ages, it undergoes a process of transformation that leads to hardening and compaction. Material is constantly eroded away from the regolith, thus making the model dependant on a constant material input for example through uplift. This is necessary to reach sufficient laterisation levels for ferricrete formation.