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Intertidal clay-drape couplets (Gironde estuary, France)

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Abstract

Clay-drape couplets on subaqueous dunes have been regarded as a diagnostic feature of the subtidal environment since Visser’s seminal paper (1980). The new observation of clay-drape couplets in the intertidal zone on a present day tidal bar of the Gironde estuary shows that they are not restricted to the subtidal zone.

In the intertidal zone, low-tide slack-water clay drapes are deposited in the bottomsets of the dominant current dunes when the muddy water retained in the troughs is absorbed into the sand during the emergence of the intertidal bar. They drape emergence run-off ripples generated by the drainage currents in the bottomsets. High-tide slack-water clay drapes are deposited over the entire dune surface and are preserved on the lee side of the dunes and in the bottomsets. They drape the subordinate current ripples. Low-tide and high-tide slack-water clay drapes enclose one thin rippled sand layer (the subordinate current bundle) and are isolated from other adjacent clay-drape couplets by the dominant current bundle.

The clay-drape couplets deposited in the intertidal zone can be distinguished from their subtidal counterparts on the basis of two morphological differences:

1. In the intertidal zone, the low-tide clay drape is only present in the bottomsets of the dunes, whereas in the subtidal zone equivalent clay drapes are also present on the lower part of the lee side of the dunes.

2. In the intertidal zone, low-tide clay drapes are deposited in the bottomsets of the dunes over emergence run-off ripples oriented in the direction of the drainage currents (i.e. in a direction normal to the tidal currents). Conversely, in the subtidal zone, the equivalent clay drapes are typically deposited over ripples oriented in the tidal-current direction (ebb or flood). There is a difference of polarity of 90° between the intertidal and subtidal small-scale bedforms draped by the low-tide slack-water drapes.

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