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Dust and Loess |
1 Centre de Recherches en Géographie Physique de l’Environnement, Université de Caen, France
2 Département de Géodynamique des Milieux Continentaux, Université P. & M. Curie, Paris, France
Various surficial calcitizations and laminar calcretes overlie non-carbonate basement rocks in southern Morocoo (Ifni and Anti-Atlas areas) and basalt outcrops or biodetrital sands in the eastern Canary Islands (mainly Fuerteventura). Geomorphological, sedimentological and micromorphological data provide evidence for their origin by deposition of Pleistocene calcitic dust and syn- to post-depositional colluvial reworking. These dusts were derived from the comminution of marine and local biodetrital sands mobilized by the westerlies on the emergent offshore shelves during times of Pleistocene low sea-level.
An opposite aeolian system related to Holocene and present-day Saharan dusts explains the formation of silts resting on the slopes of Fuerteventura and the fine fraction of the colluvial soils of southern Morocco. These deposits often lie unconformably on the calcitic bodies and their sedimentological and micromorphological characters are similar to those of the modern Saharan dusts. This suggests opposite major climatic conditions for the two aeolian dust systems: the calcitic dust system was controlled by a southerly shift of the midlatitude circulation in the Pleistocene; the later Saharan dust contribution seems to be related to the re-establishment of the trade wind circulation and Saharan air outbreaks, particularly in the early Holocene when the continuance of blocking situations in the midlatitude Atlantic may have been notable.
