The Ellipsactinia Limestones of the Marsica area (Central Apennines): A reference zonation model for Upper Jurassic Intra-Tethys reef complexes

This study presents a new and innovative distribution model of the main reef builders that thrived on bioconstructed margins of Upper Jurassic Intra-Tethys platforms. The proposed model is based on the zonation of an Upper Jurassic reef complex presently exposed in the central Apennines. Excellent exposures allow the reconstruction of the reef profile across the platform margin. Biota are mainly represented by corals and calcified demosponges, distributed with different amounts and aggregating structures along the reef complex. Microencrusters represent a secondary but important component that actively contributes to the reef construction process. Reef builders are distributed in variable quantities and with variable aggregating structures along the reef complex, revealing a strong reef zonation. Like the biota, the sedimentary features (grain size, sorting and reworking degree, bioerosion, cement crusts and cavities) show a distinctive distribution along the reef complex. Three main zones have been recognized along the studied reef complex: an internal, more protected and deeper zone, corresponding to a relatively deep back-reef/lagoon, is dominated by corals and chaetetids; a central zone (reef flat), where well-preserved coral bioconstructions give rise to a true, but discontinuous framework, and a more external zone, corresponding to the reef crest, characterized by the shallowest depths recorded across the reef complex and dominated by stromatopores. The latter forms mound-shaped structures, playing a role essential for the zoned character of the reef complex. These structures actively contribute to the construction of a moderately high topographic relief in the reef crest.