Pacific Coastal and Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 51b. Structures formed by dunes with along-crest-migrating superimposed dunes; Navajo Sandstone (Upper Triassic? and Jurassic), Zion National Park, Utah. Both Figure 51a and Figure 51b show different vertical sections through the same beds.
RECOGNITION: The origin of this kind of structure is most evident in block diagrams, because those illustrations relate the scour- pit migration paths (in the horizontal sections) to the scallop- and trough-shaped sets visible in vertical sections. The structure in this outcrop appears most similar to the computer images in Figures 46H, K, and N. In the first of these computer-generated examples (Fig. 46H), the angle between the crestline of the main bedform and the resultant transport direction is 81degrees; the bedform is 9 degrees from being perfectly transverse to flow. In Figure 46K the angle between the crestline and the transport direction is 66 degrees (24 degrees from transverse), and in Figure 46N the angle is 13 degrees (77 degrees from transverse). The similar appearance of these differently formed structures demonstrates the difficulty of precise paleocurrent determinations. This figure shows a second vertical section through the same beds as Figure 51a. In this figure, the scallops are nearly undetectable, because the outcrop is nearly parallel to the trough axes; as shown in Figure 46, the scallops are not visible in vertical sections parallel to their axes.