Origin of the sandy-silty cover along the Oum Er Rabia River (Atlantic Morocco)
by
Khalil, Mohamed
Institut Agronomique et Vétérinaire Hassan II, B.P. 6202 Madinat El Irfane, Rabat, Morocco

1. INTRODUCTION The lower part of the Oum-Er-Rabia valley walls is often the setting of a sandy-silt accumulation that usually develops along the banks of the river. This sandy-silty cover has been considered previously as two low terraces of the river, namely the upper Pleistocene and the lower Holocene. According to our analyses and 14C dating, this deposit corresponds to recent aeolian sediments. 2. FIELD DATA Downstream Khenifra, on the left bank of the Oum-Er-Rabia River, the sandy-silty cover shows ripple-marks structures formed by recent active wind action. It marks an active aeolian accumulation fed from the deflation on the actual alluvial plain and adjacent slopes. This processus occurs during dry summers when the Oum Er Rabia discharge is at its lowest level. In addition, cross-sections show thin laminated structures containing some rounded pebbles derived from an upper outcropping Permo-Carboniferous conglomerate. Downstream Kasba Tadla, cross-sections clearly show aeolian ripple marks also fed by deflation reworking alluviums. In the Rehamnas area, the sandy-silty deposits expand on a low terrace towards a palaeo-marsh and a palaeo-lake corresponding to an abandoned arm of the braided Oum-Er-Rabia River. Here the wind supply appears with a marshy influence (traces of organic matter) and a lacustrine facies (cross-bedding). In short, an aeolian sandy-silty cover often occurs near and on the riverbanks, as well as above the foot of the slopes of the Oum-Er-Rabia River. Some slope deposits sometimes are interbedded into the sandy-silty formation. Such accumulations are not alluvial low terraces of the Oum-Er-Rabia. The wind is the main factor for these deposits. It seems like humans had intervened in this deposit hundred of years before as depicted by pictures where one can detect fossilized ashes and remnants of clay pottery within this accumulation. This is likely to be true since these benches encompass well-developed soils, a requirement for population settlements. 3. 14C DATING In the Rehamnas area, nearby the El Massira dam, a sample of wood found into the sandy-silty deposit has provided an age of 410 years ± 50 BP (Lv n° 2130). It indicates the modern character -upper Holocene- of these deposits. 4. ANALYSES OF HEAVY MINERALS WITH A MICROSCOPE Usually 100 g of sediments were used for each sample, the heavy minerals content being separated in bromoform by centrifugation of sorted material in three size fractions: 44-125 µm, 125 - 150 µm and > 150 µm. 30 samples of heavy minerals from the sandy-silty deposits have been analysed. The main heavy minerals are: augite, zircon, tourmaline, garnet, apatite, collophane, epidote and hornblende. The augite content is particularly interesting, two facies being distinguished: acicular augite of Neogene origin and rounded augite reworked from mainly Triassic sediments. Into the sandy-silty samples near the alluvial riverbed, as in the present day alluviums, the Neogene augite only occurs in weak proportion (2 to 4%). The Triassic augite predominates (30 to 58%). At the foot of the slopes cut in the Permo-Triassic sediment, the proportion of ubiquitous mineral increases and only the most resistant ferromagnesian minerals are present. The change in composition is a direct link with the origin: rich in acicular augite near the alluvial sediment, but richer in local grains near the slope supply. In short, the heavy mineral content of the sandy-silty cover links with its site. The alluviums of the Oum-Er-Rabia River are the main origin for the sandy-silty cover, but some light lateral influence clearly occurs near the slopes. 5. CONCLUSION The sandy-silty deposit is Upper Holocene sediment as indicated by 14C dating. Its main origin is aeolian, mostly fed by deflation of the Oum-Er-Rabia alluviums.

Date received: November 30, 2003