Pacific Coastal & Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 40. Structures formed by stoss-depositional, transverse bedforms with superimposed symmetrical bedforms or longitudinal spurs.
RECOGNITION: Despite containing scour pits in their troughs, these bedforms do not deposit trough-shaped sets of cross-beds, because the entire depositional surface is preserved. The superimposed spurs trend parallel to transport (and therefore do not migrate laterally). The scour-pit and spur deposits are stacked vertically as shown in the crest-parallel vertical section. Stoss-depositional climb of this bedform assemblage causes scour pits in the bedform troughs to migrate upward relative to the depositional surface, thereby producing distinctive structures characterized by cross-beds whose traces appear as nearly concentric circles in horizontal sections.
ORIGIN: As with other examples of stoss-depositional climb, formation of this structure requires relatively rapid rates of deposition. Otherwise, it is not as unusual a structure as might be imagined (Figs. 41 and 64). This kind of structure is discussed in greater detail in the explanations of Figures 63 and 64, reversing analogs of the structure shown here.