Intertidal clay-drape couplets (Gironde estuary, France)

Clay-drape couplets on subaqueous dunes have been regarded as a diagnostic feature of the subtidal environment since Visser's seminal paper (1980). Th e new observation of clay-drape couplets in the intertidal zone on a presen t day tidal bar of the Gironde estuary shows that they are not restricted t o the subtidal zone. In the intertidal zone, low-tide slack-water clay drapes are deposited in t he bottomsets of the dominant current dunes when the muddy water retained i n the troughs is absorbed into the sand during the emergence of the interti dal bar. They drape emergence run-off ripples generated by the drainage cur rents in the bottomsets. High-tide slack-water clay drapes are deposited ov er the entire dune surface and are preserved on the lee side of the dunes a nd 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-d rape couplets by the dominant current bundle. The clay-drape couplets deposited in the intertidal zone can be distinguish ed from their subtidal counterparts on the basis of two morphological diffe rences: 1. In the intertidal zone, the low-tide clay drape is only present in the b ottomsets 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 bottom sets 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 typicall y deposited over ripples oriented in the tidal-current direction (ebb or fl ood). There is a difference of polarity of 90 degrees between the intertida l and subtidal small-scale bedforms draped by the low-tide slack-water drap es.