BIOEROSION OF MAGMATIC ROCKS IN A COASTAL ENVIRONMENT - THE EXAMPLE OF THE CAP-VERT PENINSULA (WESTERN SENEGAL)

Magmatic rocks (mainly dolerites and basanites) and thick ferruginous crusts (ironpans) outcroping on the shore of the Cap Vert peninsula (W estern Senegal) are not uniformely abraded by borers and scrapers. Bio abrasion and bioattack are faint in basanites and thick ferruginous cr ust. Conversely dolerites are penetrated by microorganisms, essentiall y fungi, in particular related to the genus Ostracoblabe and actively bored by the sea-urchin Echinometra lucunter. Fungi are rather numerou s throughout the intertidal zone, particularly in its upper half-part, and persist in the lower part of the supralittoral zone actively mois tened by sprays. E. lucunter inhabits the lower part of the intertidal zone and the upper part of the subtidal zone. The study of thin secti ons showed that fungi first behave like chasmoendoliths before to act as euendoliths. Pyroxenes and peridots can be intensively permeated, w hereas plagioclases are generally poorly affected by fungal attacks. A lthough very scarce, the cyanobacteria Hyella sp. can invade plagiocla ses situated at the sample surface, at least up to the half level of l ow tide. Frequencies of borers and scrapers were determined in the dif ferent biological horizons recognized by Sourie (1954). Boring activit y of E. lucunter was surveyed during 21 months. The deepening of their dwelling holes is about 1 mm/year in the lower half of the intertidal zone whereas it is only about 0.4 mm/year in the associated thick fer ruginous crusts. The calculated rate of bioerosion can reach annually about 2 050 g/m(2) in the lower horizon of low tide where the populati on of E. lucunter is particularly important. The rate of biological er osion decreases rapidly in the overlying horizons. The bioerosion of t he dolerites of the Cap Vert peninsula is an active process, contribut ion of which to the destruction of the rocky shore and to the coastal morphogenesis during the Quaternary is certainly significant.