ABSTRACT

Contorted beds are prevalent in sandstones and shales of Stage 5b (Upper Ordovician) in the Oslo area but are absent in coeval nearby limestone shale sequences and are also rare in the rest of the local Ordovician succession. Most of the contorted beds have been produced by the collapse of sand layers into underlying mud, by deformation within sand layers, or by contortion of several layers of interbedded sands and muds; a few resulted from translational slumping. The contorted structures are related to high sedimentation rates, possibly associated with storm activity, operating during the migration of a sandy shoreface across the area. However high sedimentation rates alone are thought to be inadequate to trigger off all the deformation, particularly that within the sandstone beds an seismic activity related to fault movements of the underlying basement was probably important.