ABSTRACT

Sets of trough cross-bedding in the glacio-fluvial sands and gravels of the Pleistocene Floral Formation near Saskatoon, Saskatchewan, are deformed into recumbent synclines by, the overturning of individual cross-strata. The direction of overturning of these folds is to the south and east, and is the same as the palaeocurrent direction indicated by the dips of cross-strata and the orientations of trough axes. These folds, which are exposed at several levels in the sequence have apical angles of from 30° to 60°, and approximately horizontal hinge lines. The trends of hinge lines vary greatly depending on position within an individual cross-bed set. Along the margins of troughs, hinge lines are at high angles to those near trough apices. All of the folds have upper contacts whic have been produced by erosion before deposition of the overlying layers. In general, the lower part of the folded bed is not deformed and the cross-stratification is tangential to the underlying layer. Each unit of deformed cross-bedding consists of one overturned fold, but in many places the hinge area and/or the upper limb of such a fold contains a series of smaller folds also with flat-lying axial planes. Considerations of kinematics of fold formation show that fold geometry is controlled largely by the shape of the shear profile, and by the initial inclination of the bedding.

The distribution of the folds suggests that the process which caused the deformation occurred on several occasions during deposition of the Floral sands. The consistent pattern of the deformation and the lack of other types of deformed bedding suggest that liquefaction of the sediment was not common, that downslope slumping did not occur, and that ice-thrusting did not cause the deformation. It is believed that the folding was caused by frictional drag as a result of the passage of a mass of saturated sand over the surface of the cross-bedded sand. Such an interpretation is favoured also because this is the only mechanism which has produced overturning of cross-beds in laboratory experiments.