Abstract

The Swiss Molasse Basin, situated north of the Central European Alps, documents the tectono-geomorphological evolution of the foreland basin and the adjacent mountain belt. Tectonic perturbations in the Alpine source area, shifts in climatic conditions, or a combined effect thereof were reported to have been reflected by changes in the volumes of material supplied to the foreland basin. A frequently used signal to infer such geodynamic changes is the size of grains deposited on alluvial fans, preserved as conglomerate beds forming several hundreds of meters thick sections. Accordingly, our contribution is tailored to explore how grain sizes reflect changes in environmental conditions at the scale of the entire Swiss Molasse Basin. For this, we analyzed the sizes of grains > 2 mm along 15 stratigraphic sections, which record the evolution of the Molasse basin during Oligo-Miocene times between 31 and 13 Ma ago. The analyzed conglomerate beds were deposited either at or close to the fan apex or in the basin axis > 10 km away from the Alpine thrust front. From this data, with a total of c. 50,000 measured grains, we modeled the transfer probability of the supplied material in paleostreams at the scale of the Swiss Molasse Basin. In general, we find that the values of the D₅₀ and D₈₄ grain-size percentiles scatter around an average of 40 and 80 mm, respectively, and that they were nearly constant over the course of 18 million years. This is particularly the case for the sediments deposited in the basin axis > 10 km away from the thrust front. In contrast, conglomerates that formed close to the apex and thus at the Alpine thrust front reveal a coarsening of the material through time. Modeling of the relative mobility revealed that the rivers transporting the sediment on the fans preferentially entrained particles smaller than 11 mm on average, whereas the coarser-grained material was preferentially stored on the fan. The calculations also revealed that the D₅₀ and D₈₄ have a higher probability of c. 75% and 88%, respectively, of being stored in the substrate rather than being entrained. This implies that large and intermittent floods accomplished most of the transport work, consistent with observations from modern fluvial systems. Similar to the grain-size percentiles, the relative mobility of the grains, and thus the transport capability of the dispersal systems, was nearly constant through time. This is particularly the case for the conglomerates deposited in the basin axis. Accordingly, the sedimentary processes were likely similar for these systems and seemed to be controlled largely by autogenic processes on the fans themselves. However, for conglomerate beds recording deposition near the fan apex where the dispersal systems entered the foreland basin, the coarsening of the material occurred during the same time interval when sediment supply to the basin increased. We thus conclude that supply signals were recorded by grain size only at the most proximal sites in the Molasse basin. Yet in the basin axis, patterns of grain size in fluvial conglomerate beds do not record shifts in tectonic processes and paleoclimatic changes in the hinterland, nor do they record shifts of sediment supply rates to the basin. The cause for this might be that autogenic processes on the depositional fan likely shred related signals between the apex and the basin axis, or that these signals were not recorded at all.