Sediment transport dynamics and grain size trends recorded by Oligo-Miocene megafans in the Swiss Molasse basin

Stratigraphic successions can record sediment transport dynamics and grain size trends within a temporal and spatial framework. Information on sediment- and water-fluxes are thus preserved by the arrangement and distribution of grains and by proxies of water depth conditions found in conglomerates and their associated stacking patterns.

Here we quantify long-term and instantaneous bedload sediment fluxes for three Oligo-Miocene depositional systems consisting of km-thick conglomerate sequences situated in the western, central, and eastern Swiss Molasse basin. We analyse these paleo-megafans for their stratal patterns, preserved channel depths and for temporal and spatial grain size trends. Our three target sections expose km-long spatial relationships from proximal (closer to the paleofan-apex) to distal positions as confirmed by their stacking patterns and clast morphometries, as well as supported by published palinspastic restorations. We apply a self-similarity approach to estimate threshold sediment fluxes based on grain size fining models for each section and we use paleo-hydrological techniques to estimate instantaneous bedload transport capacities associated with bankfull flow conditions.

Our analyses reveal that all three sections consist of amalgamated massive to cross-bedded, several m-thick conglomerate-beds, with a higher occurrence of sandstone- and mudstone-interbeds at distal positions. From this we infer that the clastic material has been deposited on alluvial fans consisting of a network of multiple braided streams. In contrast to the similar stacking pattern, grain size and channel depth measurements disclose a unique trend for each section. The Western section shows a ~60% decrease for both the D₈₄ and D₅₀ grain size percentiles down-system (along ~8.5 km). The Central succession reveals a decrease of ~30% (D₈₄) and ~40% (D₅₀), respectively (along ~29 km). The Eastern section reveals a decrease of ~30% (D₈₄) and ~7% (D₅₀) down-system (along ~9 km). Bankfull channel depths for all sections increase towards distal positions (threefold increase for the Western, increase of ~1/3 for the Central and twofold increase for the Eastern section, respectively).

All three sections show self-similar grain size distributions both down-system and between sites. From this, we conclude that the mechanisms that led to selective deposition down-system behaved similarly for all sections. However, calculations of threshold sediment fluxes reveal significant differences between these fans. The Western section discloses unit sediment fluxes of 18.8 ± 1.45 km³*Myr^-1*km^-1, while the Central suite reveals sediment fluxes twice as high of 39.8 ± 3.74 km³*Myr^-1*km^-1 and the Eastern section of 6.6 ± 1.6 km³*Myr^-1*km^-1. We argue that these differences are mostly controlled by variations in erosion rates within the source area in the Central Alps and tectonically-controlled variations in sediment deposition rates between the fans. Comparison of our threshold sediment fluxes with own estimates of instantaneous bedload transport capacities indicate that the fans were only active c. 1-5% of the time to balance the estimated long-term sediment budget. Our data capture an exceptional record of how these Oligo-Miocene megafans in the Swiss Molasse reflect long-term landscape processes within the foreland basin and the adjacent source areas in the Central Alps.