Aeolian sand dunes deposits are very well developed in Lanzarote and Fuerteventura Islands. There, active and fossil (palaeodunes) aeolian sand formations can be distinguished. These have been used as sand quarry showing very interesting cross-sections. An accuracy survey allows differentiate silty layers, pale brown coloured, with presence of land snails remains and insects nets. The literature defined these layers as paleosoils issued from the weathering of bioclastic sands; others authors explained them as Saharan clays. The geochemical, mineralogical and grain-size study carried out on a cross-section located on Corralejo (Fuerteventura) point to a mixture between bioclastic sand-dunes and Saharan dust, composed by silt fraction including quartz as the most important mineral. The study include grain-size, geochemical and mineralogical analysis (XRD) and two radiocarbon ages got on shells of terrestrial snails. These, included on silty layers, has been dated 16 ka and 13 ka BP. The comparison of these data with grain-size parameters of actual sand-dunes from Corralejo Natural Park allow presume a weak wind velocity during the arid phases of Late Pleistocene times; indeed, the presence of silty layers, interbedded between sandy layers is an evidence of important deposition of dust airborne from Sahara desert; geochemical and mineralogical evidences point to frequents arrival of African fine fraction. On the other hand, the close association between sahara dust and animal remains (insect nets and shell of Theba geminate) is an evidence of more dense vegetation cover. We suggest that the presence of these silty layers can be related with more humid palaeoclimates producing a massive deposition of Saharan dust as blood rain in a time when the polar front move southward in relation with changes in the atmosphere dynamic.
