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Pacific Coastal & Marine Science Center

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

Cross-Bedding, Bedforms, and Paleocurrents

Dip plots, static images, and captions

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Dip plot; see caption below.
Static image of cross-section; see caption below.

FIG. 29.  Simulated tidal bundles.  Input parameters were adjusted so that the bedforms would behave like the tidal bedforms described by Visser (1980), Boersma and Terwindt (1981), and Terwindt (1981).  With each diurnal tide cycle, the simulated sand waves vary in asymmetry and migration speed; superimposed ripples develop during times of weak currents.  The ripples migrate faster than the main bedforms and reverse migration direction when the tidal flow reverses.

RECOGNITION: Fluctuating flow is indicated by the reversals in migration direction of the superimposed ripples.  Cyclicity of the flow fluctuations is demonstrated by the cyclic spacing of bounding surfaces that define the bundles.

ORIGIN: This structure requires cyclic fluctuations in flow velocity and direction.  The best examples of this kind of structure occur in tidal deposits (FIG. 30), but cyclic foresets produced by annual wind cycles occur in eolian deposits (Figs. 23 and 24).

 

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