Cloudina-microbial reef resilience to substrate instability in a Cadomian retro-arc basin of the Iberian Peninsula

Abstract The Navalpino anticline of the Central Iberian Zone provides a unique case study of well-exposed late Ediacaran reefs. Drone images show their spatial arrangement as tens of shelly-microbial patch, biohermal and complex reefs encased in a dominantly shaly succession. The latter was deposited on a perturbed block-platform located in a Cadomian retro-arc basin fringing West Gondwana. Reefs display the traditional core/flank/inter-reef subdivision and developed on generally low-energy, clear-water substrates, between normal and storm wave base. Cores consist of millimetre-sized irregular mesoclots of dense microbial microsparite, locally interrupted by laminated (stromatolitic) fabrics, containing abundant Sinotubulites and Cloudina shells, the last with upright preservation. Basement breakdown perturbations related to fissuring, hydrothermal influence and slope-apron breccia deposition led to nucleation and development of patch-reefs on unstable substrates. Their ecological succession is characterized by stabilization (pioneer)-to-colonization phases, the latter represented by Cloudina shells preserved upright and encased in microbial crusts, pointing to their active role as pioneer frame-building taxa.