Pacific Coastal and Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 74. Structure deposited by straight-crested bedforms with two sets of straight-crested superimposed bedforms. The superimposed bedforms fluctuate in height, one set growing while the other set diminishes. The periods of height fluctuations are such that the superimposed bedforms experience 13 complete cycles during the time that the main bedform migrates one wavelength. Both sets of superimposed bedforms are migrating away from the viewer, one set obliquely up the lee slope and one set obliquely downslope. At the time that the bedform is depicted, the bedforms migrating up the main lee slope are decreasing in height, and the bedforms migrating down are increasing in height.
RECOGNITION: This example was created to determine the origin of the real structure shown in Figure 75. The upslope and downslope apparent dip directions of individual cross-beds in the outcrop were a clue that the structure was produced by upslope and downslope migration of superimposed bedforms or by along-crest migration of three-dimensional bedforms. The depositional situation shown here was developed by trial-and-error experimentation with different depositional situations that satisfied one of these conditions.
ORIGIN: The depositional situation simulated in this computer image requires reversing transport directions and an along-crest (longitudinal) component of sediment transport that is nearly as large as the across-crest (transverse) component of transport. The calculated trend of the transport vector relative to the main crestline is 54 degrees, which means that the main bedform is slightly more transverse than longitudinal. An example of eolian dunes with this kind of morphology is shown in Figure 76, but bedforms with similar morphology and behavior could also be expected to occur in tidal flows.