Pacific Coastal & Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 4. Structure formed by two-dimensional, stoss-depositional bedforms climbing at a subvertical angle. This structure is a two-dimensional example of the more general class of structures that have been called type 3 ripple-drift cross-lamination (Walker, 1963), type B and C ripple-drift cross-lamination (Jopling and Walker, 1968), supercritical climbing-ripple structure (Hunter, 1977), depositional-stoss climbing-ripple cross-stratification (Harms and others, 1982), and stoss- depositional climbing-ripple structure (Rubin and Hunter, 1982).
RECOGNITION: Like depositional surfaces in structures deposited by vertically climbing bedforms, depositional surfaces in this structure are not truncated. Bedform morphology and behavior are readily inferred from the completely preserved depositional surfaces.
ORIGIN: The conditions required to produce subvertical climb are not quite as unusual as the conditions necessary to produce vertical climb, because subvertically climbing bedforms do not remain stationary while sediment is transported to the depositional site. Even subvertical stoss-depositional climb requires somewhat unusual conditions, however. Net deposition on stoss slopes requires a rate of deposition that approaches or exceeds the rate of bedform migration. These conditions can be met either by a relatively rapid rate of deposition or by a relatively low rate of bedform migration. Rapid rates of deposition imply high transport rates and fallout from suspension, as explained by Ashley and others (1982), whereas slow rates of bedform migration imply low transport rates. An ideal situation for meeting these conflicting requirements is in a flow that undergoes a downcurrent decrease in transport rate, such as would occur where the near-bed velocity decreases. In such a situation, sediment can be transported to the depositional site at a rapid rate while bedforms migrate slowly. The conditions favoring this rapid deposition from suspension occur in fluvial flows (FIG. 7) and turbidity currents, and many excellent examples of this structure occur in deposits of such flows. Low rates of bedform migration can also occur where bedforms are poor traps for the sediment that is being transported (as is the case with antidunes) or where bedforms trend nearly parallel to the transport direction. Although a longitudinal bedform orientation can reduce the bedform migration speed and thereby increase the angle of climb (equation 2 and Rubin and Hunter, 1985), the alignment with the flow must be exceptionally exact before bedforms can climb at stoss-depositional angles without deposition from suspension (Rubin and Hunter, 1985).