Snapshot of a lower Pliocene Dendropoma reef from Sant Onofre (Baix Ebre Basin, Tarragona, NE Spain)

Abstract

We study a vermetid–coralline algal buildup from the lower Pliocene deposits of the Baix Ebre Basin (Sant Onofre, NE Spain). The bioconstruction framework is made up by the intergrowth of Dendropoma and encrusting coralline algae, mostly Spongites fruticulosus with rare Neogoniolithon brassica-florida. Thus, it can be considered a fossil analog of the present-day Mediterranean Dendropoma reefs. The bioconstruction developed on top of a flat palaeotopographic high formed by pre-Pliocene substrate in a sheltered embayment. Submarine cliff sediments, consisting of big blocks and boulders, were deposited along the margin of the palaeohigh. Crusts of coralline algae, intergrowing with serpulids and vermetids, extend from the flat top downwards along the talus. Gray-bluish marl, yellowish silt and fine-grained sand, were deposited in the surrounding low areas. A rich and diverse dweller assemblage occurs in the buildup: bivalves (pectinids, venerids, Lithophaga), gastropods (muricids, nassarids, olivids, cancellarids), ahermatypic coral Cladocora ?, barnacles of the family Pyrgomatidae, Balanus trigonus and regular echinoids. In the talus deposits, clusters of Neopycnodonte cochlear, Hinnites ercolanianus and Balanus sp. dominate the faunal assemblage. Further, the blocks of the talus are densely bored, showing Gastrochaenolites, Entobia, Maeandropolydora and Caulostrepsis. Faunal and boring assemblages indicate that the buildup settled and developed in very shallow conditions (most likely less than 10m water depth). The whole fossil assemblage, framework and dweller community, are preserved in situ, maintaining their original growth positions. Development in a protected setting, together with rapid burial due to progradation of siliciclastics on top of the palaeotopographic high, accounts for this exceptional preservation in such a shallow environment.The vermetid bioconstruction of Sant Onofre represents the only carbonate deposits found in the lower Pliocene basins of NE Spain. Sequence stratigraphic architecture indicates that these carbonates represent the maximum flooding deposits. Thus, maximum carbonate production took place due to sediment starvation in the palaeohigh while terrigenous deposits were formed in the surrounded low areas.