Dunes Around Khnifiss Lagoon (Tarfaya, SW of Morocco): Composition, Itinerary In Dune Fields, Effects on Dunes’ Colours and Morphodynamic

Abstract:

Dunes around Khnifiss lagoon (28° 3'N, 12°13'W) show different colors ranging from black at the beach, whitish yellow in transverse dunes near the beach to reddish at the mega barchans situated few kilometers in the SW. The scientific question is about the abundance of different dunes in the same environmental conditions. The present work aims to investigate the factors that influence dunes color change, and then at which degree these factors could control dunes stability.

To highlight the difference in color observed at the dune fields then to characterize dunes mineralogy, Landsat TM images were used in addition to mineralogical analysis that was carried out for the black grains samples originated from megabarchans. Optic Microscope and SEM- EDS data was adopted, in addition to physico-chemical analysis provided by Electronic Microprobe. Grain size and shape analysis were conducted to characterize the different types of grains of sand.

3/1 Landsat image band ratio allowed iron oxide distinction, the results revealed the importance of iron oxide concentration. Furthermore, mineralogical and physico-chemical analysis revealed (i) a high grade of oxides (Rutile, Ilmenite, Magnetite, Ulvöspinel) in samples, (ii) silicates (Quartz, Clinopyroxene, feldspar, Zircon), (iii) phosphate (apatite) and (iv) carbonate (calcite). The grain size analysis of the sand originated from the megabarchans reveals that there are three populations of sand. Black grains with a diameter less than 100µm and dominated by the magnetite, red ones composed mainly by the quartz with diameter between 100 and 180 µm and grains with diameter more than 180 µm are white and composed by carbonates.

The threshold of motion of these different grains was calculated. It shows that these different grains have the same threshold of motion, which means that the grain size compensates the density. This explains the abundance of different populations of sand in the same environment. The dominance of iron oxides justified the color black in sand. However, the whitish yellow and reddish color observed in dunes could be attributed to iron oxide clay coating, produced under weathering conditions, covering the grains of quartz.

Key words: black sand, Landsat, Iron Oxide, Khnifiss beach, megadunes, Tarfaya, Morocco