Structure and Diagenesis of Recent Algal-Foraminifer Reefs, Fernando de Noronha, Brazil

Abstract

ABSTRACT The volcanic island of Fernando de Noronha, in the South Atlantic, is fringed by an emergent algal ridge rising from a subtidal volcanic or eolianite foundation. The ridge and adjacent reef flat result from aggradation by skeletal growth, mainly crustose coralline algae and foraminifer Homotrema rubrum, and deposition of volcanic gravel extending landward as beachrock. The aggrading units involve sand-filled skeletal and gravel frameworks with the detrital sediment representing about two-thirds of the reef volume. Distribution of radiocarbon dates suggests combined effect of marine abrasion and seaward progradation after an initial upward growth rate of 1 m/500 years prior to 3000 years B.P. At present, the ridge and reef flat are undergoing considerable diagenetic alteration. Pervasive generation of Mg-calcite micrite cement occurring simultaneously with bioerosion and deposition of new carbonate fabric produces dense and texturally complex reefrock. This early lithification is taking place in an extremely turbulent wave-swept environment. and its upper limit extends several meters above the intertidal zone.