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Pacific Coastal & Marine Science Center

Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”

Cross-Bedding, Bedforms, and Paleocurrents

Photographs

Photo of rock or sand showing pertinent structure or structures; see caption below.

FIG. 28.  Structure produced by a migrating bedform with superimposed bedforms that migrated down and up its lee slope; fluvial deposits, Colorado River, Grand Canyon National Park, Arizona.  Area shown is 35 cm from top to bottom.  This structure is a combination of the structures simulated in Figures 25 and 27 and also includes structures produced by bedforms climbing at stoss-depositional angles.

RECOGNITION: While the main bedform that deposited these beds migrated from right to left, ripples that were superimposed on its lee slope repeatedly reversed their direction of migration (A).  Eventually the ripples were replaced with a larger 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 ripple-migration direction.  The real cause of the flow reversals was probably the formation and decay of eddies at the depositional site.  The eddies may have been restricted to the lee side of the main bedform or may have been more widespread eddies in the lee of river-channel constrictions (Schmidt, 1986).

 

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