Duricrust Influence on the Geological Record: Insights from Numerical Modelling

Duricrusts are hard mineral layers that develop in regions with contrasting climatic conditions, ranging from tropical to arid environments. These formations are distributed worldwide and can be found, e.g. in Europe, Africa, and South America. Typically found capping hills, inverting landscapes, and shielding underlying softer material, duricrusts play a crucial role in preserving landscapes and altering sedimentary archives. They act as both sources and sinks within geomorphic and sedimentary systems, depending on the spatial and temporal scale of analysis. This research focuses on the influence of duricrusts on landscapes and how they impact the sedimentary record.

Duricrust formation is explained by two main hypotheses: the hydrological hypothesis and the laterisation hypothesis. The hydrological hypothesis suggests that duricrust-forming elements are transported from distant sources and accumulate through processes associated with water table fluctuations. In contrast, the laterisation hypothesis attributes their formation to in-situ processes, where the underlying material undergoes leaching of soluble elements and compaction and cementation of less soluble ones.

Recently, we introduced two new numerical models (EGU abstracts: Fenske et al., 2022, 2023, 2024). These models incorporate a dimensionless hardening factor, κ, to account for reduced surface erodibility, i.e. a distinctive feature of duricrusts. Using independently constrained parameters derived from field data, hydrology, climate, and geochronology, our models successfully reproduce observed conditions for duricrust formation. Additionally, we improved the computation of regolith and duricrust ages to better align modelled results with empirical data.

Simulations demonstrate that, according to the hydrological model, duricrust thickness depends on the water table fluctuation range, λ. Duricrust formation is highlighted when two dimensionless numbers, W and R_t, exceed 0.1 and 1, respectively, indicating that duricrusts form preferentially under stable tectonic conditions. Conversely, according to the laterisation model, duricrust thickness is driven by vertical material supply, such as uplift or base-level drop, and duricrust formation occurs when Ω > Ω_min. This suggests that duricrusts evolve continuously in tectonically active cratonic environments. Tracing these dimensionless parameters and the computed ages through time provides tectonic and climatic constraints on duricrust formation across the geological timescale.

To illustrate these findings, we present a case study of Kaw Mountain in the Guiana Shield. The geological record preserved in duricrust ages enables the simulation of different stages of uplift since the Cretaceous, including a quiescent, dry 20-million-year period during the Oligo-Miocene, followed by a wetter and more active period after the Mid-Miocene Climatic Optimum. Additionally, the presence of duricrusts increases slope steepness, which accelerates erosion. This explains the typical topography observed at Kaw Mountain, with limited extensive duricrust covers in a mountainous region while accounting for the persistence of flat surfaces over time. In areas suitable for duricrust formation, achieving topographic steady-state is unlikely.

These results confirm the ability of our models to simulate duricrust formation under real-world conditions. The established tectonic and environmental parameters for duricrust formation serve as valuable tracers to reconstruct past conditions. Furthermore, these models have significant potential for future applications in understanding how duricrusts influence topographies and the geochronological record.