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Recently, a new suite of tide-generated sedimentary structures has been described, principally from sandy shoals and large excavations associated with flood-control structures in tidal estuaries of the Dutch coast. These sedimentary structures can be less ambiguous than criteria previously used to recognize tidalites. Structures and sequences of structures like those recognized in the North Sea can be applied to the rock record, in this case the Curtis Formation (Jurassic), San Rafael swell, Utah, to significantly enhance our ability to interpret tidal facies.
Our discussion centers on the recognition of tidal bundles, the lateral succession of cross-strata generated by the migration of a large-scale bedform during one dominant tidal episode. Tidal bundles in the Curtis consist of two gently dipping sigmoid-shaped pause planes which enclose avalanche foresets. They are up to 80 cm (30 in.) thick and 11 m (36 ft) long. Pause planes may be accentuated by erosion of the megaripple by the subordinate tide, by generation of ripples or small megaripples with opposed inclinations, and/or by a drape of fine sediment which settles during slack water. Systematic variability that occurs within bundles is due to increasing then waning current velocity during a tidal episode. Systematic variability among tidal bundles results from regular fluctuations o tidal current velocities during a lunar month (neap/spring cycles). These include changes in bundle thickness, dip of foresets, internal geometry, and lateral extent. Recognition of these features in the Curtis leads to an uncontestable interpretation of its tidal origin.
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