Post-glacial colluvium in western Norway: depositional processes, facies and palaeoclimatic record

Abstract

The postglacial Quaternary colluvial systems in western Norway are arrays of steep fans, often coalescing into aprons, developed along the slopes of valley sides and fjord margins. The coarse debris, derived from weathered gneissic bedrock and its glacial-till mantle, varies from highly immature to mature. The depositional processes are mainly avalanches, ranging from rockfalls and debrisflows to snowflows, but include also waterflow and debris creep. The mechanics and sedimentary products of these processes are discussed, with special emphasis on snow avalanches, whose role as an agent of debris transport is little-known to sedimentologists. The subsequent analysis of sedimentary successions is focused on colluvial-fan deltas, which are very specific, yet little-studied, coastal depositional systems. The stratigraphic variation and depositional architecture of the colluvial facies assemblages, constrained by abundant radiometric dates, are used to decipher the signal of regional climatic changes from the sedimentary record. The stratigraphic data from two dozen local colluvial successions are compiled and further compared with other types of regional palaeoclimatic proxy record. The analysis suggests that the colluvial systems, although dependent upon local geomorphic conditions, have acted as highly sensitive recorders of regional climatic changes. The study as a whole demonstrates that colluvial depositional systems are an interesting and important frontier of clastic sedimentology.