Quaternary river terrace formation in the Allier basin, France : a reconstruction based on sand bulk geochemistry and 3-D modelling

<TT>The research presented in this thesis is focussed on a quantitative reconstruction of the effects of past environmental dynamics within a fluvial system. The study area is part of the Allier basin (Limagne) in the Auvergne, Massif Central, France.</TT><TT>The research was carried out in several stages. At first field work was carried out to determine the terrace stratigraphy and chronology in more detail. A new age estimate of the Fva (65 m above present river bed) is based on pumice clasts found in the terrace sediments. Younger terraces were dated with</TT><sup>14</SUP><TT>C and Th/U disequilibrium methods. Fx terrace sediments (15 and 10 m above present river bed) were mainly deposited during the Late Weichselian, while the Fwb terrace sediments (25 m above present river bed) have most probably a Late Saalian age. Due to these new age estimates a revision of the existing Allier terrace chronology is necessary. This new chronology shows a large time gap between the deposition of the Fv and Fw terrace sediments.</TT><TT>Next, sands of various terrace units were collected and bulk geochemically measured with XRF. This bulk geochemical research allowed a statistically significant discrimination of different terrace levels. The processes which shape and shaped the actual sand geochemistry were successfully quantified. It was found that grain size has only a very limited effect on bulk geochemical variability while longitudinal sorting processes and weathering have a stronger impact on actual sediment composition. Although the effects of parent material controlled weathering in the Allier sands were successfully modelled, the older terrace sediments are unsuitable for paleoenvironmental reconstructions. Such a reconstruction was done for the Weichselian and Holocene terrace deposits at the Allier/Dore confluence. The sediment mixing behaviour of these rivers is estimated by calculating sediment mixing ratios. This reconstruction started with an investigation of spatial mixing effects of Allier and Dore sediments in time by means of mapping and geostatistics. Results suggest an environmental</TT><TT>control over the spatial variability of sediment mixing at this confluence in time. The reconstructed relative sediment fluxes of the Allier in time show a good correspondence with known past environments. Relative Allier sediment fluxes seem mainly climate controlled whereby large fluvio-glacial fluxes at the end of a glacial played a dominant role in the Allier system. These large sediment fluxes in the Allier system caused a strong rise in the Allier riverbed level contributing to the development of lake basins (Marais) in Grande Limagne.</TT><TT>Further a large scale and long term model of terrace formation was constructed using finite state modelling. This methodology allows the construction of a general 3-D terrace formation model containing as well quantitative as qualitative knowledge on fluvial systems. Finally, an adapted version for the Allier (LIMTER) is made incorporating all present knowledge on this system. LIMTER allows the formulation and evaluation of long term terrace formation scenarios for the Allier system. Simulation results suggest that terrace stratigraphy in the study area is mainly the result of the internal Allier dynamics and climatic change. Local tectonism caused the development of unpaired terraces while the general regional uplift played a dominant role in terrace formation and preservation in general.</TT><TT>The terrace research as presented in this thesis shows that it is well possible for any fluvial system to simulate the interaction climate/tectonism and fluvial dynamics. The for the Allier simulated dynamics, net sedimentation during and at the end of a Glacial and net dissection during an Interglacial has no general validity.</TT>