Aeolian accumulations in Tombua region (SW Angola). Structural control on sand composition and dune morphology

Abstract

<p>Tombua is a small fishing village located in SW Angola that was founded during the former colonial period, when it was called Porto Alexandre. Regional climatic conditions are typical of desertic environments, with less than 20 mm of annual precipitation. Low rainfall and persistent southerly winds promoted the advance of aeolian accumulations and the formation of the Moçamedes dune field. This dune field widens to the north until it meets the first major river valley, the Curoca, less than 10 km to the north of Tombua. Hence, coastal landscape is markedly different on the two banks in the vestibular position of this river. Because of aridity and, specially, aeolian processes, Tombua is the southernmost location with paved roads along the coast of Angola, but great human effort has been required to limit the advance of wind sands.</p><p>Two sectors with different dune features, bounded by a broadly strait line striking NNW-SSE from the eastern limit of Tombua village southward for about 25 km, can be distinguished in the region. To the east prevail barchan dunes that frequently display a clear pink hue imposed by garnet enrichment. Individual dunes can reach 7 m in height and approximately 50 m in length (along wind direction). To the west occur transverse and parabolic dunes dominated by quartz with secondary amounts of feldspar and substantially less heavy mineral contents. Individual transverse dunes lie on previous sand accumulations, and usually do not reach more than 50 m high. Parabolic dunes, which prevail near the coastline, are usually smaller (~30 m high).</p><p>Barchan dunes of the eastern sector lie on a southward tilted surface, rising in elevation from the sea level to approximately 45 meters at the latitude of Tombua. On this platform, with relative sand scarcity, wind deflation is responsible for the preferential removal of more voluminous and lower density particles (such as quartz), leaving behind a residue enriched in heavy minerals. This process culminates with the generation of granatiferous placers, recognized for some decades in the Province of Namibe.</p>