Pacific Coastal & Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 64b. Structure formed by reversing ripples with nonmigrating (longitudinal) spurs or superimposed bedforms; fluvial deposits, Colorado River, Grand Canyon National Park, Arizona. This is an example of the structure simulated in Figure 63.
RECOGNITION: In this structure reversals in the direction of ripple migration occurred at three scales, all of which are visible in the vertical section (Figure 64a). At the largest spatial scale and longest temporal scale is a reversal in migration direction throughout the entire bed. Migration directions are right to left at the bottom of the bed and left to right at the top. At a smaller spatial and shorter temporal scale are the several-centimeter-displacement back-and-forth oscillations of the ripples, most clearly visible in the center of the bed, where the angle of climb was vertical. At the smallest and shortest scale, the reversals in migration direction are represented by lamina-to-lamina zig-zags at the ripple crests. This bed was deposited during high discharge in 1983. Initial deposition was during downstream flow, but as flow receded and a topographic obstruction emerged upstream from the depositional site, an eddy formed, and local flow was directed upstream. Thus, the largest reversal represents one depositional episode (a flood). Smaller reversals may have been caused by the passage of small eddies in the flow or by instabilities in the main eddy. After the vertical section was photographed, a horizontal section was excavated through the same beds. The horizontal section is at a stratigraphic horizon slightly above where the angle of climb was vertical. The section shows circular cross-bed traces that were deposited by stoss-depositional climb of the scour pits in the bedform troughs.