Pacific Coastal and Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 68. Structures with compound cross-bedding interpreted to have formed both by fluctuating flow and superimposed bedforms; eolian deposits, Navajo Sandstone (Upper Triassic? and Jurassic), Zion National Park, Utah. Note the person for scale near the lower right corner.
RECOGNITION: The lowest two sets of cross-beds in this photograph have bedding characteristics that are best explained by fluctuating-flow processes like those illustrated in Figures 22 and 58: basal wedges (A) and internal bounding surfaces that are relatively conformable with underlying foresets (B). In contrast, the set of cross-beds that occupies most of the upper half of the photograph contains subsets of cross-beds (C) that were deposited by superimposed bedforms, evident from the differing dip directions of the cross-beds and subset bounding surfaces and from the trough-shaped bounding surfaces of some of the subsets (D). Although this photograph shows compound cross-bedding formed by reversing of the main bedforms (lower part of photograph) and formed by migration of superimposed bedforms (upper left), the structures are in different sets of cross-beds and were therefore deposited at different times and by different dunes, not simultaneously on the same bedform as is illustrated in Figure 67. The same set that contains the subsets deposited by the superimposed bedforms, however, also has other subsets (E) with relatively conformable set boundaries. These subsets resemble reversing-bedform deposits. If such an interpretation is correct, then superimposed bedforms and fluctuating flow both affected this dune, although not necessarily at the same time.