Debris flow-dominated and rockfall-dominated talus slopes: Genetic models derived from GPR measurements

Abstract

Stratified talus deposits are reported from many different mountain environments. Numerous possible explanations are discussed in the literature; however, the sediment stores are rarely accessible as exposures are sparse. We applied ground-penetrating radar (25, 50 and 100 MHz antennae) to gain insight into the internal sediment structures of 23 alpine scree slopes; ten examples are presented in this paper. The study areas are spread over the Eastern European Alps at altitudes ranging from 1500 to 2900 m. The bedrock type is primarily limestone and dolostone; one area is composed of gneiss and mica–schist. GPR turned out to be highly suitable for investigating sediment structures of dry talus debris. The results showed that almost all of the deposits investigated are characterized by pronounced stratification. Several different types of layering were identified. Discordant layers which are restricted to confined parts of the talus are probably related to sediment redistribution processes like surficial debris flows or dry grain flows. These features frequently occur at the uppermost part of the slope caused by overland flow from the adjacent rock face, but may also develop in the downhill part of a talus. One talus in the Reintal area showed surface-parallel, persistent layers of different grain sizes which cannot be explained by any known models. We suggest a novel model of talus development which is driven by climatic fluctuations. In periods of enhanced freeze–thaw activity like the Little Ice Age, the delivery of coarse debris prevails. In warmer climate with a higher frequency of rainstorms, the depletion of finer-grained intermediate stores in less inclined rockwall positions leads to delivery of clasts smaller than 2 cm. The type of layering found within a talus is determined by rockwall parameters like height, steepness, topography and dissection of the rock face. The “storage depletion” model applies to high rockwalls with a considerable volume of intermediate storage.