Late quaternary coastal stratigraphy on a platform-fringed tropical coast - a case study from Zanzibar, Tanzania

The coasts of Zanzibar's islands, in common with those of the adjacent Afri can mainland coast, are formed largely of Stage 5e Pleistocene limestones o f back-reef facies. The limestones form typically undercut cliffs and assoc iated wave-cut platforms, commonly more than 1km wide. Where not masked by the deposits of beach ridge plains, the platforms coincide with the contemp orary intertidal zone. This coincidence might suggest that the platforms ar e mid-late Holocene products, formed since the post late-Weichselian glacia l sea level attained its current highstand position. However, the present e xtremely slow rates of limestone cliff recession due to marine erosion, tog ether with the existence, at the landward margin of the platform, of well l ithified beach rocks of lithologies markedly different to those of the cont emporary beach sands, indicate that most of the platform erosion occurred p re-Holocene. A sea-level stillstand period, following the peak of the Stage 5e highstand, is suggested. The overwashing of a pre-existing platform as a consequence of Holocene sea-level rise would have significantly and abrup tly increased the area of intertidal to shallow subtidal habitats and thus the potential for the increased production of calcium carbonate sediment de rived from that biota. The impact on the platform environment of predicted sea-level rise over the next century would be to create an extensive shallo w subtidal environment promoting the growth of the calcareous green alga, H alimeda, coral mounds and small patch reefs. The beach ridge plains would b ecome increasingly vulnerable to erosion.