Paleoenvironment and taphonomy of lower Miocene bivalve and macroid assemblages: the Lagos Biocalcarenite (Lagos-Portimão Formation, southern Portugal).

Between Lagos and Albufeira, the Algarve coast of southern Portugal is marked by outcrops of the lower Miocene Lagos-Portimão Formation (LPF) consisting of yellow sandstone and coarse skeletal-rhodolithic limestone. This contribution focuses on the rhodoliths, their paleoecology, taphonomy, and biological composition, in the Lagos Biocalcarenite, the lower member of the LPF. Special attention is paid to the unusual occurrence of numerous rhodoliths nucleated around articulated bivalve shells, as well as to the nature of their biological interactions and taphonomic features. The calcareous algae of the rhodoliths (Phymatolithon calcareum and Spongites sp.) are commonly interlayered with thin bands of bryozoans and serpulids. Thick beds of non-nucleated spheroidal rhodoliths first appear at approximately 5-6 m above the base of the LPF as a result of a storm event that shifted rhodoliths in a shoreward direction. The bioeroded surface at the top of the Cretaceous Porto de Mós Formation, at the base of the overlying LPF succession, is a wave-cut platform representing the Miocene transgressive surface. [ABSTRACT FROM AUTHOR]