Basalt Mounds and Adjacent Depressions Attract Contrasting Biofacies on a Volcanically Active Middle Miocene Coastline (Porto Santo, Madeira Archipelago, Portugal)

Abstract

Small basalt mounds with encrusting corals and inter-mound carbonate sandy zones with abundant rhodoliths corresponding to an ancient intertidal to shallow-water sea floor are exhumed from overlying volcaniclastic deposits and basalt lava flows at Pedra de Água on Ilhéu de Cima off Porto Santo, one of the islands of the northeastern Atlantic Madeira Archipelago (Portugal). The mounds rise above the surrounding surface to attain a height of about half a meter. The mounds exhibit an in situ assemblage of hermatypic corals, dominated by Tarbellastrae and Solenastrea. They formed as massive (4.2 × 1.9 m average length), isolated patches in a protected bay close to shore eroded from an uneven basalt substrate dated to the Middle Miocene (14 to 15 Ma). The slightly deeper zones between basalt mounds, which alternate with them over a distance of more than 20 m, are covered mainly by coarse carbonate sand on which rhodoliths up to 14.8 cm in diameter are preserved in situ. Many rhodoliths have grown around a basalt core, which indicates a local, nearshore source for development. Complete burial of the elevated coral settlements and intervening low zones populated by rhodoliths occurred when volcanic lapilli and other tephra catastrophically buried this part of the rocky shore. The rhodoliths and coral assemblages exposed in an area of 12 m2 were canvassed systematically using census quadrats to quantify community relationships.