Quantification of sediment fluxes and intermittencies from Oligo–Miocene megafan deposits in the Swiss Molasse basin

Abstract

The conglomerates preserved in alluvial fans in the Swiss Molasse basin provide crucial insights into the sedimentary dynamics of these routing systems. In particular, the architectural trends and grain sizes of such deposits reveal information on the sediment fluxes and record variations in the intermittency — a proxy for the fan's activity — that indicate on the relative importance of tectonic or climatic controls on sediment production and transport. Here, we calculated intermittencies from sediment transport dynamics using the ratio between the long‐term average and the short‐term instantaneous unit sediment fluxes during bankfull discharge conditions. For this, we collected grain size data from three palaeo‐fan systems that were active through Oligo–Miocene times and that reveal preserved proximal–distal relationships. The three fan systems, which we term the western, central and eastern fans, show significant differences in their long‐term sediment budget but equivalent magnitudes of the sediment transport dynamics expressed through the intermittency factor. The eastern fan records a low long‐term sediment flux (5.7 km2 Myr−1), which needed the fan to be active during ca. 8 h yr−1 (intermittency factor of 0.89 × 10−3). The western fan reveals a higher long‐term sediment flux (16.2 km2 Myr−1), which could have accumulated during ca. 16 h yr−1 (intermittency factor of 1.83 × 10−3), thereby reflecting a more active system. The central fan records the largest long‐term sediment flux (40.3 km2 Myr−1), where ca. 57 h yr−1 of sediment transport would be required to deposit the supplied material (intermittency factor of 6.53 × 10−3), thus representing the most active system. By relating these characteristics to the regional exhumation history, we consider that the central fan mainly recorded the transient response of the Alpine surface to the break‐off of the European mantle lithosphere slab. Contrarily, the western and eastern fans were formed during the Alpine evolution when steady‐state conditions between uplift and erosion were reached and when sediment fluxes to the basin were lower. Despite differences in the tecto‐geomorphic and climatic boundary conditions, our data suggest that these Oligo–Miocene megafans could accomplish their mean annual sediment transport work within a few hours or days per year.