Pacific Coastal & Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 55. Structure produced by perfectly longitudinal (nonmigrating) bedforms with along-crest-migrating sinuosities.
RECOGNITION: In vertical sections perpendicular to the bedform trend, these structures resemble zig-zag structures formed by reversing bedforms (Fig. 20 or formed by bedforms with reversing lee-side spurs (Fig. 59). The structures can be distinguished in horizontal sections, because the migrating sinuosities cause cross-beds to dip with an along-crest component. Cross-beds and bounding surfaces lack dips in the direction of sediment transport (toward 0 degrees). In a natural flow, this gap would be filled in to some extent by the migration of superimposed bedforms (Fig. 56) or by the advancing downcurrent noses of the main bedforms. Because the bedforms that produce this structure do not migrate laterally, the mean cross-bed dip direction varies from location to location.
ORIGIN: Longitudinal eolian dunes with sinuous plan forms were studied by Tsoar (1982, 1983). He documented that the sinuosities migrated along-crest, and he did not detect lateral migration of the dunes. Other examples of this bedform morphology could be expected to include oscillation ripples, tidal sand waves, and tidal ridges. The examples could be expected to be rare, because the reversing flows that produce such bedforms must be exactly balanced to preclude lateral migration of the bedforms. If the individual flow reversals transport too large a volume of sediment relative to the size of the bedforms, then the bedforms will reverse asymmetry, as simulated in Figure 77.