Pacific Coastal & Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 59. Structure formed by migrating bedforms with spurs that reverse asymmetry and migration direction but have no net along-crest displacement. Vertical sections perpendicular to the trend of the main bedforms contain scallops, whereas sections parallel to the main bedforms contain zig-zag structures. Because the zig-zag structures have cross-beds that reverse in their direction of dip in the outcrop plane, the structures might be called herringbone cross-beds by some workers. In a strict sense, however, the structures are not true herringbones because the dip directions are not diametrically opposed.
RECOGNITION: In sections parallel to the trend of the main bedforms, the bedding appears similar to that produced by reversing two-dimensional bedforms (Fig. 18), and in sections perpendicular to the trend of the main bedforms, the bedding is scalloped. The key to identifying this structure is to recognize that the back-and-forth migration of the scour pits and intervening spurs is in a direction that is normal to the migration direction of the main bedform. This behavior is recognizable in horizontal sections or in three-dimensional blocks mapped from vertical sections. In horizontal sections, the trough-shaped sets deposited by the scour pits have a double appearance, most noticeable near the fingertips, where the scour pits complete their migration upward through the horizontal plane. This double appearance results from reversals in asymmetry of the lee-side system of spurs and scour pits. The deepest point in each scour pit reverses from one side of the scour pit to the other side each time the spurs reverse asymmetry. Before these computer images were generated, these zig-zag structures were thought to form only on the crests of lee-side spurs (Rubin and Hunter, 1983), but the computer images demonstrate that similar structures can form at the bottoms of scour pits. Because scour pits are topographically lower than spur crests, zig-zag structures deposited within scour pits have a higher preservation potential. Where spurs migrate back and forth without changing shape, zig-zags form only at the crests of the spurs.
ORIGIN: This structure is produced by transverse bedforms where the flow direction varies slightly, thereby causing the lee-side spurs to reverse direction of along-crest migration. These structures are useful indicators of paleocurrent direction because they are deposited by transverse bedforms, and, consequently, the trough axes and mean cross-bed dip directions are precise indicators of the paleocurrent direction. Real examples are shown in Figures 60-62.