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Antidunes are nearly symmetrical, sinusoidal bed forms formed by near-critical flows, and comprise all bed profiles in phase with surface gravity waves in the overlying flow. This definition applies regardless of bed-form propagation direction or mode of formation. They occur in a wide range of depositional environments but are ubiquitous in the swash zones of fine-grained beaches. Antidunes are a common but little-recognized feature of washover fans, where they generate distinct sedimentary structures that are well preserved in the stratigraphic record.
On washovers at Seabrook Island, South Carolina, antidune trains form at the fan apex, in phase with an undular hydraulic jump. This jump marks the transition within the overwash surge from supercritical flow of the berm crest to subcritical flow on the fan itself. Bed-form distribution on the fan is controlled not only by this transition but also by downfan energy dissipation within subcritical portions of the flow. From berm crest to fan terminus (i.e., downstream) the lateral sequence of bed geometries is: flat, erosional bed; antidunes; flat bed with primary current lineation and rhomboid rill marks; flat, featureless bed; rhomboid to cuspate ripples; undulatory, linear ripples; washed-out ripples; flat bed. Heavy minerals are concentrated near the fan apex owing to selective down an transport of quartz. Landward fan migration results in a genetic sequence of sedimentary structures in which all of these features are preserved.
Streamwise boundaries of the antidune field are abrupt. Within the field itself, both crest amplitude and crest spacing decrease downfan with decreasing velocity. Bedding geometry is controlled by bed-form scale, by the pulsing nature of the flow (which tends to wash out bed forms at the end of each rock event), and by eolian deflation during neap tides and fair weather. Upstream antidunes produce thin lenses of backset laminae which are commonly truncated by flat beds or superposed backsets. These lenses grade downstream into thinner lenses with lower angle backsets, and finally into zones of very low-angle truncation surfaces.
Orthoquartzitic washovers in the Carboniferous of Kentucky and the Ordovician of South Africa display well-preserved antidune formset geometries. Although the very well-sorted, mono-mineralic nature of these sandstones renders internal stratification nearly invisible, backset laminae are apparent. Pleistocene washovers in South Africa exhibit antidune bedding types influenced by grain size. Fine to medium-grained sand is organized in en echelon lenses of backset laminae interbedded with flat beds. Coarse sand and granules occur as flow-perpendicular ribbons with lenticular cross sections and spacing similar to antidune spacings on modern fans.
Many published photographs of flat beds contain unrecognized examples of antidune stratification. In many places, the antidune cross-laminae would be impossible to detect without heavy minerals or other markers to accentuate bedding. However, in apparent massive beds, x-ray radiographic techniques have revealed cross-laminae with antidunal affinities. Moreover, antidunes may be responsible for much of the low-angle truncation normally associated with flat-bedded facies.
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