Abstract

Tidal periodicities, in particular the neap/spring/neap cycle, can be recognized in the internal structure of large-scale cross-bedded sets. Another frequently occurring periodicity is the diurnal inequality of the tide. This is a regular variation of the water levels reached by successive high and/or low waters in semi-diurnal and mixed tidal systems. As a result, the strength of successive flood and ebb currents fluctuates. A particular depositional site in a tidal environment is frequently dominated by one of these two tidal currents. This can be reflected in the sediments in a bundle-wise building up of unidirectional cross-strata. During a dominant current stage, the bedform advances, and its displacement is reflected in the thickness of the forest deposits (bundles). The neap/spring sequence is reflected in variations of the bundle thickness over a period of 14 days. Variations in the strength of successive dominant currents due to the diurnal inequality of the tide are reflected in thick-thin variations in the thicknesses of successive bundles. The diurnal inequality of the tide occurs in semi-diurnal tidal regimes and in the semi-diurnal parts of mixed tidal systems. When the tidal currents within a semi-diurnal or mixed tidal regime drop to such low magnitudes that bedform migration ceases, for example around neap tide, the number of bundles within a neap/spring cycle may be (much) less than 28, and the depositional system can be incorrectly interpreted as a "diurnal tidal system." Then the reflection of the diurnal inequality of the tide in the bundle thicknesses may reveal that bedform migration was subject to semi-diurnal tides and that standstill periods in the bedform migration must have caused a reduction in the number of bundles per neap/spring cycle. Several examples from the recent literature are used to demonstrate this principle. In present-day diurnal systems the difference between low and high water is generally small and, moreover, the timespan between low and high water is twice as long as in semi-diurnal systems. The low tidal amplitude and the low frequency result in generally small tidal current velocities, which are rarely strong enough to produce large-scale megaripples with bundle cycles. Apart from the recognition of tidal influences in fossil sedimentary deposits, measurements of lunar cycles offer a tool for differentiating diurnal from semi-diurnal and mixed tidal systems in the geological past. Changes in the Earth-Moon rotational system may also be recognized.

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