Pacific Coastal & Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 58. Structure formed by sinuous bedforms that fluctuate in migration speed and asymmetry. This depositional situation is virtually identical to the one shown in Figure 22B, except that those bedforms have straight crestlines, whereas the ones shown here have sinuous crestlines. The upper image shows the bedform morphology when asymmetry is a maximum in the asymmetry-fluctuation cycle; the lower image shows the morphology at a later time in the cycle, when bedform asymmetry is a minimum.
RECOGNITION: Although structures deposited by reversing bedforms can closely resemble structures produced by downslope migration of superimposed bedforms that exactly parallel the main bedforms, exact parallelism of the two sets of bedforms becomes less likely as either set of bedforms becomes more three-dimensional. Where the superimposed bedforms do not exactly parallel the main bedform, or where the superimposed bedforms do not have crestlines as long as those of the main bedform, the cross-beds deposited by the superimposed bedforms do not appear conformable in horizontal sections and in crestline-parallel vertical sections with the bounding surfaces scoured by the superimposed bedforms (Figs. 65 and 66). In contrast, conformable cross-beds and bounding surfaces (as shown here) suggest a fluctuating-flow origin. In this structure, and in many of the other structures deposited by variable three-dimensional bedforms, the bounding- surface dip pattern mimics the cross-bed dip pattern. This feature indicates that bounding surfaces have roughly the same shape as the cross-beds.
ORIGIN: This is the same depositional situation as that in Figure 22B, except that these bedforms have sinuous crestlines.