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Analyzing grain-size distribution is of great interest for many questions related to fluvial architecture and sediment transport, or to characterize the evolution of a system in response to a perturbation (in particular to a tectonic or climatic change). Many methods have been developed to measure accurately the grain-size distribution of an outcrop, my work has so far been based on Wolman and volumetric measurements in natural and experimental settings.
Based on measurements performed on the Urumqi River (Chinese Tian Shan), we showed that the grain-size distributions of the bars and the anabranches are actually similar in surface and in sub-surface. The same general pattern is observed on the Sunwapta River (Canadian Rockies) and on its experimental equivalent at the London University, Ontario.
Therefore, the three morpho-sedimentary elements that built the river bed correspond to only two distinct grain-size distributions. This suggests that at the scale of active threads, such rivers can be characterized by only two grain-size distributions.
We also worked at larger scale and compared the grain-size distributions of 30 samples issued from a trench (volumetric sample) and of a vertical surfacec count performed along the walls of the trench. All the grain-size distributions are similar. This implies 1) that the granulometry of the Urumqi River is homogeneous at the scale of the active layer of the river and 2) that vertical surface counts are accurate to sample the granulometry of conglomerates (as far as we know, this is the first field confirmation).
It is thus possible to use the granulometry of conglomerates to reconstruct the paleo-environments that prevailed at the time of deposition. We used this method to quantify the increase in water discharge associated to the Paleocene-Eocene Thermal Maximum about 56 Myrs ago. This climatic perturbation is a quite short but intense global warming event associated with strong perturbations in precipitation rates. We worked in the southern pyrenean foreland (Tremp Basin, Spain) and documented the evolution of the grain sizes together with channel geometries across the PETM. We show that the water discharge of the rivers increased by a factor up to 14.
Related publications:
The grain-size patchiness of gravel-bed streams: the example of the Urumqi River, Chinese Tian Shan (pdf), L. Guerit, L. Barrier, C. Narteau, F. Métivier, Y. Liu, E. Lajeunesse, E. Gayer, L. Malverti, P. Meunier, B. Ye, Advances in Geosciences, 37, p-29-37, 2014.
Uniform grain-size distribution in the active layer of a shallow, gravel-bedded, braided river (the Urumqi River, China) and implications for paleo-hydrology (pdf), L. Guerit, L. Barrier, Y. Liu, C. Narteau, E. Lajeunesse, E. Gayer, F. Métivier, Earth Surface Dynamics, 6, p. 1011-1021, 2018.
Estimating regional flood discharge during Palaeocene-Eocene global warming (pdf), C. Chen, L. Guerit, B. Foreman, H. Hassenruck-Gudipati, T. Adatte, L. Honegger, M. Perret, A. Sluijs, S. Castelltort, 8, 13391, 2018.
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