NEW DATA ON THE LAMINAR HORIZON GENESIS OF CALCRETES DEVELOPED ON MOROCCO COARSE QUATERNARY ALLUVIUM - CONSEQUENCES ON THE DESERTIFICATION PROCESS

The laminar horizon, Klm, is frequently found in soils of semiarid reg ions of Morocco. It is thin, bedded, and located either on the surface or under soft horizons. Its origin continues to be debated. The lamin ar horizon was examined in three basins of north-eastern Morocco by us ing macromorphological, micromorphological and nanomorphological metho ds, and by examining its mineralogical characteristics. Results concer ning the Klm horizon showed the following: rooting components were pre sent in the : upper layers of the horizon; crystallization of calcite rods in light-beige layers was dominant; an elevated phyllosilicate co ntent was present in the red-brown layers; degradation by deflation of the laminar horizon was evident in the oldest stages of an increasing ly arid basin; and in basins with increasing humidity there was an abs ence of the klm horizon under thick soft horizons but a presence under thin soft materials. The following hypothesis regarding genesis of th e laminar horizon is proposed During periods of precipitation, drained water accumulates at the surface of indurated horizons having coarse elements and they become saturated with calcium carbonate. During the following dry period plant roots spread at the surface of indurated ho rizons. Roots dissolve calcite, causing a concentration of phyllosilic ates, whereas calcite is precipitated in the root cellular structure ( endorooting calcite). There is also precipitation of calcite in the rh izosphere (perirooting calcite). The consequence of calcite dissolutio n and phyllosilicate concentration is the creation of red-brown layers . The precipitation of perirooting and endorooting calcite contributes to the creation of light-brown layers. The presence of Klm horizon in dicates that the capacity for water reserves in soft horizons of soil is minimal. The system advances toward plant cover and subsequent soil degradation. Presence of Klm horizon may be considered as a desertifi cation marker. Its pedological origin may indicate a risk of long-term desertification.