Cross-Bedding, Bedforms, and Paleocurrents (Concepts in by David M. Rubin

By David M. Rubin

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Extra resources for Cross-Bedding, Bedforms, and Paleocurrents (Concepts in Sedimentology & Paleontology 1)

Sample text

Superimposed bedforms can migrate up the lee slope of the main bedform in at least two situations, however. First, upcurrent migration of superimposed bedforms occurs locally in the 52 COMPUTER IMAGES-VARIABLE 2-0 upslope-migrating bedform that deposited a relatively thick set of foresets (B). The main bedform continued to migrate to the left after the superimposed bedforms disappeared (C). If found in the geologic record, these beds might be incorrectly identified as tidal deposits (Figs. 29 and 30), because of the flow reversals indicated by the reversals in ripplemigration direction.

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 crosslaminated 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 structure shown here is similar to those produced by bedforms that reverse migration direction (Fig. 19); both have erosion surfaces that are arranged in vertically zig-zagging sequences. The two kinds of structures can be distinguished, however, because reversals in asymmetry (shown here) cause cross-beds to offlap and onlap the erosion surfaces, whereas reversals in migration direction (Fig. 19) produce erosion surfaces with relatively concordant overlying beds. Reversals in asymmetry also produce zig-zags in the centers of the bedform troughs-a feature that is not produced by reversals in migration direction.

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