Composite ichnofabrics in a fossil rocky shore: the Messinian-Pliocene unconformity of Bajo Segura Basin, southeastern Spain

An ichnofabric analysis was performed in the fossil shore platform and cliff recorded in the Messinian-Pliocene unconformity of the western Mediterranean (Bajo Segura Basin, southeastern Spain) with the aim of documenting trace-fossil distribution in the paleo-rocky shore, deciphering main controlling factors, and evaluating the dynamics of the Pliocene flooding after the Messinian Salinity Crisis (MSC). Firmground domiciles (i.e. Thalassinoides and horizontal U-shaped burrows) occur where the unconformity is resting on marlstone, and trace fossils are absent where the unconformity is resting on sandstone. Abundant borings characterize surfaces where the unconformity intersects limestone. Two bioerosion ichnofabrics have been identified. The autocomposite and monospecific Gastrochaenolites ichnofabric exhibits different expressions depending on its location within the rocky shore profile. In downward-facing surfaces of cliffs, particularly within the notches, Gastrochaenolites is abundant and deep. In contrast, on the shore platforms, borings are less abundant and commonly truncated by the recurrent wave-induced erosion. This pattern likely reflects longer colonization windows in the former and stronger wave erosion in the later, negatively impacting on the ability of the endolithic benthos to keep up with physical processes. In contrast to classic examples of autocomposite ichnofabrics that involve a single benthic community migrating upward under continuous slow sedimentation (herein termed ‘accretionally modulated autocomposite ichnofabrics’), the ones in the studied hardgrounds were generated by a single community affected by wave erosion and are herein termed ‘erosionally modulated autocomposite ichnofabrics’. The more diverse and heterocomposite Gastrochaenolites-Entobia ichnofabric consists of a monospecific ichnocoenosis of erosionally truncated Gastrochaenolites, cross-cut by borings of a higher diversity ichnocoenosis that do not show evidence of such truncation. This ichnofabric reflects high-energy conditions on the shore platform, followed by low-energy conditions under deeper water, showing the evolution of the Pliocene flooding in the Colmenares area.