Characteristics and Origin of a Precambrian Pediment

Abstract

The unconformity beneath the Torridonian (late Precambrian) sediments of northwest Scotland represents one of the most ancient of fossil landform assemblages. Cut across crystalline Lewisian rocks, it is highly irregular in most places, the relief locally reaching 1,000 m. This rugged surface is little weathered. In the extreme north, however, the unconformity generally parallels the major stratification of the sediments above. Before its burial, this planar surface was weathered to depths of several metres then partly stripped of its altered mantle. A solitary hill of gneiss represents the only major relief in the unconformity within this northern district. A piedmont alluvial fan environment is suggested by the overlying arkosic sediments. The planar, weathered surface and the solitary hill thus appear analogous to a pediment and knoll buried beneath alluvial fan deposits. Vertical variations in the sediments are consistent with the planar surface having formed contemporaneously with sedimentation as a pediment at the foot of a retreating elevated source area.