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Pacific Coastal & Marine Science Center

Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”

Cross-Bedding, Bedforms, and Paleocurrents

Photographs

Photo of rock or sand showing pertinent structure or structures; see caption below.

FIG. 61.  Zig-zag structure resembling herringbone cross-bedding; Lamb Point Tongue (Upper Triassic?) of the Navajo Sandstone, Kanab Creek, Utah.

RECOGNITION: The bounding surfaces in this zig-zag structure are inclined at relatively low angles, and, consequently, the outcrop resembles the structures produced within zig-zagging scour pits, as illustrated in the right-hand vertical sections of Figure 59. Although the zig-zags have the appearance of herringbone bedding, herringbone bedding is commonly believed to form by transverse bedforms that reverse their direction of migration frequently, such as with reversing tidal currents.  Unless the reversing bedforms migrate large (and relatively equal) distances during each flow reversal, reversing bedforms will produce structures that are grossly different from herringbone bedding (Figs. 18, 19, 21, 63, 64, 69, 77, and 78).  Other processes that are likely to produce structures that might be mistaken for true herringbone bedding include: zig-zagging of spurs and scour pits (Figs. 59 and 71), vertical stacking of trough-shaped sets of cross-beds produced by bedforms with out-of-phase scour pits (Fig. 34), and long-term shifts in channel geometry that cause reversals in locations of ebb- and flood-dominated channels.

 

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