ON THE ORIGIN OF PARTING-STEP LINEATION IN EOLIAN SANDSTONES

Parting-step lineation, a structure formed in sandstones by the breakage of fine parallel laminae, is traditionally associated with horizontally laminated sandstones deposited during upper-flow regime conditions in allvuvial environments. This structure is observed also in sandstones of eolian origin where it forms by interlaminar breakage in finely laminated beds in the toes of dune forsets. Deposition at the toes of dunes is dominated by grainfall and the migration of eolian ripples. The steps thus appear to form primarily by breakage across laminae controlled by differences in mechanical strength between very-fine grained grainfall laminae and coarser-grained ripple drift laminae. Abrupt changes in grain size between the very-fine grained grainfall and the coarser-grained ripple drift laminae, and the consequent variations in mechanical strength due to differential cementation and/or grain packing, make this possible. Importantly, the orientation of the steps is not controlled by grain orientation and therefore is not a paleocurrent indicator despite its common application in alluvial sandstones.