The characteristics, classification and origin of late Pleistocene fan deposits in the Cass Basin, Canterbury, New Zealand

Abstract

The deposits comprising ‘valley‐side fans’ in a small intermontane basin of the Southern Alps are classified as debris flow, water‐laid, intermediate and mixed deposits on the basis of particle size and clast orientation characteristics. Five varieties of debris flow deposit are identified including unimodal and bimodal ‘mudflow gravels’. The fans comprise mainly unimodal mudflow gravels which although apparently similar to the mudflow gravels described from montane and periglacial environments are coarser, have less silt and clay and are better sorted than the mudflow deposits described from semiarid alluvial fans. Additions of airborne silt and fine sand to the fan catchments during later stages of fan building gave rise to bimodal debris flow deposits which appear similar to gravels described from cold‐climate fans in Tasmania and classed as water‐laid deposits. Braided stream deposits were added to the depositional sequence towards the closing stages of fan building indicating that the fan had become ‘wetter’. In many places, however, the youngest Pleistocene fan deposits are silt‐rich mixtons reflecting a peak in loess deposition.