Sedimentary record of tectonic and climatic erosional perturbations in an experimental coupled catchment-fan system
Résumé
This article deals with the stratigraphic record of a climatic or tectonic perturbation of an experimental coupled catchment-fan system. Following Bonnet & Crave's results (2003), which suggest that it is possible to differentiate between climatic or tectonic causes of surface uplift of an erosional topography from the record of sediment flux output, we design a new experimental device to test this proposition in the sedimentary signal. This device allows the study of a coupled erosion- sedimentation system at the laboratory scale for given and changing uplift and rainfall rates. On the basis of experimental results, we propose a methodology to study alluvial fan architecture from large-scale geometries to stacking pattern and sequence analysis. In these experiments, the erosional perturbation resulting from climate or tectonic forcing induces a typical dynamic in terms of both sediment supply and the ratio between the sediment and water supply, which controls the transport capacity. The four possible forcings (rainfall rate and uplift rate increase or decrease, respectively) then result in unique dynamics of the combined parameters such as the fan slope, apex aggradation, mean sedimentation rate, grain size distribution, bed thickness and frequency and facies stacking. We first analyse large-scale geometries (onlap, toplap, downlap or truncation) and then fine-scale sedimentological features (fining, thinning, coarsening, thickening) in order to discriminate the nature of the forcing. This conceptual model could be adapted to real world alluvial fans in order to recognize and separate the driving mechanisms from each other.