Micro-ripples on silt-dominated beds: observations at the glacier Austerdalsisen, Norway

Abstract

Micro-ripples (wavelength 0.5-2.0 cm, amplitude less than 0.2 cm) form on silt-dominated beds at the margins of ephemeral stream channels in reworked glacial lake sediments as a result of unidirectional currents, oscillatory flow or both. Pulsation of flow and the presence of bed irregularities much larger than the dominant particle size result in local stress concentration, and micro-ripples may form even though the mean stress is below the theoretical threshold for the grain sizes involved. Grain shape and composition may be partly responsible for differentiation of crest and trough material. X-ray diffraction studies have confirmed that samples of sediments from micro-ripple troughs and crests at one site differ in mineralogical composition, and differences of grain size and shape are confirmed by scanning electron microscope investigations.