Pacific Coastal and Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 25. Structure formed by bedforms with parallel superimposed bedforms migrating in the same direction.
RECOGNITION: This structure closely resembles other kinds of cross-bedding with cyclic foresets, but is perhaps distinguishable because the cross-beds deposited by the superimposed bedforms consistently downlap along the bounding surfaces scoured by the superimposed bedforms. In contrast, cross-bedding formed by reversals in migration direction or fluctuations in bedform morphology typically contains basal wedges (FIG. 22A) or scalloped cross-bedding with relatively conformable upcurrent-dipping beds that immediately overlie the lower set boundary (FIG. 22B). In more realistic depositional situations, either real or computer-generated, superimposed bedforms are unlikely to be exactly parallel to the main bedforms for long distances along-crest, and recognizing the deposits of downslope-migrating bedforms is easier than in the simulation shown here. Local differences in orientation of the crests of the two sets of bedforms cause the cross-beds that are deposited by the superimposed bedforms to dip in a different direction from the bounding surfaces that are scoured by the superimposed bedforms (Figs. 65 and 66). The polar plot of this structure (not shown) is similar to the plots of all other two-dimensional cross- bedding: cross-bed and bounding-surface poles plot along a straight line through the center of the plot.
ORIGIN: All of the previous examples of structures that form by cyclic fluctuations in bedform morphology or path of climb require flows that cyclically fluctuate in strength or direction. In the structure illustrated in this example, however, bedform morphology fluctuates even in steady flows. The fluctuations in morphology result from the differing migration speeds of the two sets of bedforms. Cyclic passage of the superimposed bedforms over the main bedforms causes cyclic constructive and destructive interference, thereby generating cyclic foresets even in steady flows. Superpositioning of bedforms is a common phenomenon in most flows where bedforms are large enough for other bedforms to be accommodated on them. The structure illustrated here is an approximation of structures that are found in fluvial deposits (Banks, 1973; McCabe and Jones, 1977), tidal deposits (Dalrymple, 1984; Anastas et al., 1997), and eolian deposits (Brookfield, 1977; FIG. 26; Mountney et al., 1999), but in most natural examples the superimposed bedforms could be expected to have crestlines with a slightly different trend, sinuousity, or along-crest length relative to the main bedform, as illustrated in many of the following computer images (such as Figures 46 and 65).