3-D architecture, depositional patterns and climate triggered sediment fluxes of an alpine alluvial fan (Samedan, Switzerland)

Abstract

The sedimentary architecture of an alpine alluvial fan with a surface of 300,000 m 2 near Samedan, Switzerland, was three-dimensionally investigated using 9 km of ground penetrating radar sections with a penetration depth of 10 m. Radar facies patterns could be very well calibrated to sedimentology and dated horizons due to a 300 m long outcrop section at the foot of the fan. Six major reflectors have been identified and represent first order, fan-wide palaeosurfaces. They are made up of up to 20 cm thick fine-grained deposits partly with initial pedogenetic structures and wood remains which yielded ages between 5670 ± 60 and 7515 ± 65 a 14C BP. Between these surfaces different depositional lobes and specific architectural elements like channels, levees or snouts of debris flows could be identified. All these data were geo-referenced to establish a complete quantitative 3-D time-stratigraphic framework. This allowed us to calculate deposited sediment volumes and sediment fluxes for different time slices between the dated palaeosurfaces. Sediment fluxes show an overall decline during the Holocene which we interpret as the decling sediment production in the catchment area—a function of the paraglacial cycle. Since the middle Atlantic period, the aggradation of the fan almost ceased and climate perturbations are no longer reflected in the sedimentary record since then. Within the aggradation period, distinct peaks of high sediment fluxes could be correlated with known periods of glacier retreats in the Swiss Alps, which points to a high sensitivity of the system to climate changes. The results of our study give valuable new insights into thresholds of activity and quiescence of an alpine catchment-fan system and its stage in the course of a paraglacial cycle. Although very important for the understanding of such dynamic systems under global warming scenarios and geo-risk assessment, such data sets are rarely available.