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Conference papers

Criteria for the identification of ventifacts in sedimentary successions: a review and reappraisal

Abstract : Ventifacts are phenoclasts fashioned by corrasion (abrasion by wind-carried particules). They are climate-sensitive sedimentary features testifying long periods without any vegetation in non-marine environments. Therefore, they can be used, with more confidence than aeolian dunes, to characterize desert conditions, under hot or cold palaeoclimate. However, long experience in studying Permian,Triassic and Quaternary ventifacts from various countries in Europe and Africa, shows that many clasts presented in the literature as ventifacts, are not, and conversely, that typical ventifacts often go unnoticed. That is why we set out to review critically the diagnostic properties used to distinguish them. Because, in the current nature, most of ventifacts are found in desert pavements (regs), thin gravel beds on outcrops are frequently supposed correspond to such deflation lags, and thus to be the best place to found ventifacts. The reality is often disappointing because such beds may be formed in more dynamic environments: on fluvial bars or on sea shore. Undisturbed paleoregs are very rare, and ventifacts are usually found scattered, at varied reworked states. As ventifacts are typical of hyper-arid climates, the sedimentary units were they formed are devoid of palaeosols. Nevertheless, in some cases, climate change could lead to incor-poration of ventifacts in soil profiles of semi-arid type. Surface features are not useful to recognize ventifacts, except a polarity resulting from the contrast between a smooth upper face and an irregular basal face. Ventifacts are often reputed to exhibit a polished aspect. On modern specimens, this is due to a varnish (coating) of variable chemical composition; on ancient ventifacts (and even other pebbles) enbedded in a clayey matrix, a true mechanical polish can occur, probably related to differential compaction. Mesoscale features, usually neglected, are very significant. While in water, gravels wear out almost exclusively by mutual friction, in air, sandblasting is able to reach the most depressed zones of their surface. Therefore, even if the outline of a ventifact shows reentrants, it is uniformly smooth, and whether pre-existent cavities appear, their wall is also smoothed. Shallow scour pits, initiated by removal of soft grains, formed on the same way; they may mutually interfere to produce a polygonal pattern. On the other hand, especially on ventifact edges, may appear large percussion scars, produced by the largest clasts rolled by sand-blast. At macroscale, general shape hardly allows distinguishing a ventifact from any other clast, especially if it is a first-cycle sedimentary element. What is characteristic is the occurrence of specific ridges limiting facets; some are sinuous, fading towards the base, others tend to follow the general outline. The number of ridges depends partly on the initial shape of the clast, but the classic dreikanter is the least typical shape, since it may have another origin.
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Contributor : Isabelle Dubigeon <>
Submitted on : Thursday, February 2, 2012 - 10:15:09 AM
Last modification on : Friday, April 5, 2019 - 8:17:31 PM

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M. Durand, Sylvie Bourquin. Criteria for the identification of ventifacts in sedimentary successions: a review and reappraisal. 28th IAS Meeting, Jul 2011, Saragosse, Spain. ⟨insu-00665496⟩

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