Influence of bedrock on sedimentary processes on two adjacent alluvial fans (Julian Alps, NW Slovenia)

Alluvial fans are sedimentary bodies typical for mountain environments of all latitudes and climatic zones. Sediment, building alluvial fans, is transported from the catchments area by fluvial and/or sedimentary gravity flow processes resulting in a variety of distinct sedimentary facies. Several different factors control the prevailing sedimentary process on alluvial fans such as climate, catchment and fan area, catchment relief, aspect, vegetation types and density, tectonic setting, and bedrock lithology. In this study we present a detailed study of fluvial and sedimentary gravity flow facies on two adjacent Holocene alluvial fans. The fans are located in the Planica Valley in the Julian Alps (NW Slovenia). Both alluvial fans have very similar dimension, same climatic conditions, relief, and vegetation cover, however, differ in the geology setting of the catchment areas. While the catchment area of the first fan consists entirely of Julian aged peritidal to shallow subtidal carbonates Conzen dolomite, the adjacent second catchment area is partly composed of Julian-Tuvalian shallow-water claystone, marlstone, dolomite, and marly limestone. We made twelve sedimentary logs in a scale of 1: 10 and performed a granulometric analysis of several sediment layers to determine sedimentary processes forming the two alluvial fans. The first alluvial fan is entirely built of either clast-supported open-framework gravel or sandy gravel layers. The layers, up to 20 centimetres thick, contain less than 1% of mud fraction and the clasts are Conzen dolomite. Gravel is predominantly pebble sized, with occasional cobbles and boulders in the upper and middle part of the fan. Cobble sized clasts are occasionally imbricated parallel with slope inclination. We attribute these layers to fluvial and/or sheetflood depositional processes. The second alluvial fan is also predominantly composed of the same fluvial to sheetflood deposited layers, which are intercalated with crudely stratified and up to 50 cm thick layers of muddy sandy gravel. These layers contain up to 16% of mud fraction with gravel clasts being matrix-to clast supported. Based on sediment texture and the amount of mud fraction contained we attribute these layers to cohesive and highly viscous debris flow deposits. The debris flow deposits however do not represent the predominant sedimentary process in the second alluvial fan. This suggests that debris flows occur only sporadically indicating to their stochastic depositional occurrence. The study show that the catchment bedrock is the most important parameter determining the sedimentary process in a typical alpine alluvial fan when all other controlling factors are equal.