Pacific Coastal & Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 6. Structure deposited by two-dimensional wind ripples climbing at a stoss-erosional, lee-depositional, net-positive angle of climb; modern eolian deposits from Padre Island, Texas, photographed by Ralph Hunter. This structure is a real example of the structure generated by computer in Figure 5.
RECOGNITION: This structure was deposited by wind ripples that migrated from right to left. The path of ripple climb was steady from near the bottom of the photograph to immediately below the buried ripples. A temporary change in flow conditions, possibly a change in the flow direction, caused the ripples to be preserved at one horizon. As the ripples migrated, they left behind cross-laminated beds. The beds have a mean thickness that is equal to approximately half the ripple height, which indicates that during the time that each ripple migrated one wavelength, deposition raised the bed elevation by approximately half the ripple height. In the plane of the photograph, the bounding surfaces that separate the cross-laminated beds dip toward the right or are relatively horizontal, whereas the generalized depositional surface (the bed surface if the ripples were smoothed off) dips at a low angle toward the left. The inclination of the depositional surface can be recognized by connecting the troughs or crests of the preserved ripple forms. The inclination of the depositional surface also can be recognized by connecting the distinctive dark-colored laminae that were deposited simultaneously on the lee sides of adjacent ripples (indicated by arrows). The change in sand color from light to dark occurred at approximately the same time that the ripples changed their path of climb, suggesting that a change in wind direction or wind strength simultaneously introduced sediment from a new source and changed the ripple morphology or behavior.