Parent material controlled subsoil weathering in a chronosequence, the Allier terraces, Limagne rift valley, France

The chronosequence in the Allier terraces is subject to erratic changes in parent material composition with time. Approximately 65% of the total variance in the basaltic elements content in the Allier terrace sands can be explained by the combined effect of parent material and time related neoformation of clay in the subsoil. It is shown that parent material composition, as a function of transport distance, strongly influences weathering rate in the Allier terrace chronosequence. In order to evaluate parent material controlled weathering effects a process model simulation was made based on an exponential decrease of MgO and neoformation of clay. Long term simulations with this model suggest that parent material controlled subsoil weathering is significant for sediments with easy weatherable fragments. After prolonged weathering the role of parent material decreases and becomes untraceable in strongly weathered materials.