Investigation of reddening patterns of dune sands — The megabarchans of Al-ghord Lahmar (Khnifiss National Park, South-West of Morocco)

Abstract

The present paper aims to investigate the geological and environmental parameters controlling the redness of sand at the megabarchans located nearby Khnifiss lagoon (SW of Morocco). Mineralogical investigation either by laboratory or remotely sensed data showed the abundance of a mixed composition of Quartz, Carbonates and mafic minerals with a dominance of Hematite Quartz coated grains. Also, grain size analysis revealed the heterogeneity of the megabarchans sand and the dominance of fine and well to very well sorted sand with a mean grains size of (0.2mm). Mineralogical species were adequately distributed according to their size and density. We noted the dominance of iron bearing minerals including: Magnetite, Hematite, Ilmenite, Ulvospinel, and Pyroxenes, in the very fine fraction (<63μm). Yet, the coarser fractions (>63) μm knew a dominance of Hematite coated Quartz, Carbonates and Feldspars. These both fractions are carried by wind with almost the same threshold motion, which meant that the size could compensate the density. That explains their deposition in the same environment. The megabarchans migrated over the past 325years with a rate of 2m/year. Thus, grains may undergo many turnover cycles until they can be reactivated by wind. Climatic parameters such as precipitations with 164mm/year, the frequent stormy events that bring a flash flooding, in addition to the high humidity and important fog and dew provide a source of moisture that make the megabarchans a moistened environment encouraging the iron oxidation. Thus, the abundance of a source of iron and a source of moisture, in addition to the stability of the megabarchans, and their water holding capacity remained the effective factors that led to the megabarchans redness.