Clast fabric of diamictons associated with some roches moutonnées

Abstract

Clast fabric patterns in thin, dense, over‐consolidated diamictons overlying and abutting the stoss sides of 23 Canadian and Swedish roches moutonnées were investigated. These diamictons display sedimentary characteristics that are commonly regarded as typical of basally‐deposited tills formed under actively flowing ice. The macrofabrics are marked by strong unimodal orientations in the direction of ice flow, parallel to the long axes of the roches moutonnées. Many have low down‐ice plunge angles. in contrast to the up‐ice plunges normally associated with ortho‐till deposited under actively flowing ice. The degree of divergence between the macrofabric mode and the long axis, whereas the plunge angle is highest in the central part of the stoss slope. The down‐ice plunge of the large clasts appears to be a consequence of deposition in areas where glacial flow and shear stress surfaces are oriented at a shallow angle towards an ice/rigid substratc interface. The angle of plunge depends upon the relative importance of the vertical and lateral stress components. pervasive deformation during deposition, and subsequent reworking. The presence of diamictons over rigid substrates on the stoss flanks of roches moutonnées is an uncommon situation. suggesting that this macrofabric pattern may be relatively rate in general for ortho‐tills deposited under actively flowing ice.