Pacific Coastal and Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 77. Structure produced by reversing, nonmigrating (perfectly longitudinal) bedforms with sinuosities that migrate along-crest. This structure is a reversing analog of the structure illustrated in Figure 55.
RECOGNITION: Vertical climbing of these bedforms is obvious in any section except those parallel to the bedform trend. Along- crest migration of crestline sinuosities induces an along-crest component of dip to the cross-beds (visible in the horizontal section and in sections parallel to the bedform crestline). Bounding surfaces in this structure are produced by two processes: along-crest migration of crestline sinuosities and reversals in bedform asymmetry. Along-crest migration of sinuosities produces trough-shaped bounding surfaces with axes that trend parallel to the bedform crestline. Reversals in bedform asymmetry cause the bounding surfaces to be scallop- shaped in horizontal section; each reversal in asymmetry produces one scallop along the bounding surface. Because of the complexity of these two processes, dips of bounding-surface planes plot in scatter patterns.
ORIGIN: This kind of structure is produced by reversing longitudinal bedforms. Reversing longitudinal dunes with migrating crestline sinuosities have been described by Tsoar (1982, 1983), and their reported internal structure is roughly similar to that shown here. Other possible origins include symmetrical tidal sand waves and ridges, and ripples in oscillatory flows that reverse by less than 180 degrees.