Costras carbonatadas en la sucesión aluvial distal eocena de la Formación Hamada de Boudenib II al sur del Alto Atlas (Marruecos)

Abstract

On the southern edge of the High Atlas Mountains of Morocco, the Tertiary continental deposits form extensive tablelands, the so-called "Hamada", in the Boudenib area. Two units have been defined in' the Paleogene: a Lower unit "Hamada de Boudenib I" (lower Eocene) and an Upper one "Hamada de Boudenib II" of Middle/Upper Eocene age. This latter lies unconformably and covers most of the Lower unit; generally dips towards the south and thickens in this same direction. Their central basin deposits essentially consist of mudstones and sandy mudstones, deposited In distal are as (alluvial plain) of alluvial fans with sparse channels and ephemeral carbonate ponding. Intense processes of crust formation produced several fades of calcretes and dolocretes and the ubiquitous palygorskite. The duricrust features indicate that they were formed by continuous processes of carbonate precipitation in the vadose and capillary zones of soil profiles, developed on the alluvial plain, with alternative wetting and drying episodes in a semiarid climate