An unusual poriferan community in the Albian of Islares (north Spain): Palaeoenvironmental and tectonic implications

The discovery of a well preserved Lower Cretaceous siliceous sponge community in Islares (Cantabria, north Spain) is documented. Lower Albian sponges crop-out in great abundance as a rich necrocoenosis embedded in a marly matrix. Demosponges make up the bulk of an 11 m thick succession, which is interpreted as having formed in a slope setting. The sponges show body preservation and were buried “in situ”. All types of growth forms are present; they grew so tightly that they often fused secondarily. Shape polarity suggests steady low energy, predominantly easterly palaeocurrents. The main factor limiting the community seems to have been space competition. A preliminary documentation of the systematic palaeontology is presented: only taxa from the families Tetracladinidae, Megamorinidae and Corallistidae were found. Geometrical restoration of the platform-basin transect allows estimation of a depth of approximately 40 m for the lithistid sponges. Sea-level variations driven by block-faulting controlled the available accommodation space. Sponges occur just below and above a sequence boundary and are associated with a major tectonic event linked with the opening of the Bay of Biscay.