Palaeoecology of the Pleistocene (MIS 5.5) outcrops of Santa Maria Island (Azores) in a complex oceanic tectonic setting

Santa Maria is the oldest island of the Azores archipelago and the only one that possesses late-Miocene early-Pliocene and Pleistocene fossils. Two Pleistocene outcrops Lagoinhas and Prainha, from the Marine Isotopic Substage 5.5 (MIS 5.5) are known from Santa Maria Island. We studied the palaeoecology of the Lagoinhas fossil assemblages and we also provide detailed lithological sections from both outcrops, allowing for more detailed palaeoecological reconstruction.

Ten bulk samples of approximately 1 kg each were collected from the sand facies at Lagoinhas. Most of the gastropod taxa represented in these samples occur on the present Azorean shores, the exceptions being Conus sp., Gibbula cf. umbilicalis, Polynices lacteus and Trachypolia nodulosa, which have locally disappeared from the Azorean malacofauna during the course of the last glaciation. Most of the fossil gastropods were epifaunal and crawlers. About half of them were herbivores, about one quarter was carnivores, and only a few (less than 10%) were detritivores or parasites. The preferred type of substrate was rocky substrate and/or algae.

The palaeoecological evidence that results from the analysis of the fossiliferous beds (the bioerosion structures, the coralline algal and mollusc assemblages and their bathymetrical zonation) suggests a marine environment depositional setting from shallow infralittoral (shoreface) to foreshore/beach. A single highstand episode including minor oscillations is apparent during MIS 5.5 in the Azores.