Pacific Coastal & Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 9. Structure formed by bedforms climbing at a stoss- erosional, lee-depositional, net-negative angle of climb.
RECOGNITION: When bedforms climb at a negative angle, they scour a wavy surface into the underlying substrate. The spacing of the undulations on the bounding surface is equal to the bedform spacing, but the complete bedform height is not necessarily preserved. Bedforms climbing at negative angles have cores that are composed in part of older substrate. As shown in Figure 5, bounding surfaces scoured by positively climbing bedforms dip upcurrent relative to the depositional surface. In contrast, bounding surfaces scoured by bedforms climbing at an angle of 0 are parallel to the depositional surface, and bounding surfaces scoured by negatively climbing bedforms dip downcurrent (relative to the depositional surface).
ORIGIN: In areas undergoing net erosion, bedforms migrate downward relative to the generalized depositional surface. This downward scouring - climbing at a negative angle - is probably just as common as climbing at a positive angle, but, because the process is erosional rather than depositional, the resulting structures are under-represented in the geologic record. This kind of structure can originate at all scales and in any environment where two-dimensional bedforms exist. Small-scale examples formed by ripples can be preserved where the bedforms are later buried by deposition. Large-scale examples can be shown to occur at the surface of some bedform fields such as in the Strzelecki and Simpson deserts in Australia, where dunes are composed in part of non-eolian substrate (Folk, 1971; Breed and Breed, 1979), and large erosional tidal sand ridges with similar internal stratification have been reported by Berné et al. (2002, fig. 10b).