Blue mesophotic coral bioherms in the pre-evaporitic Messinian of the western Mediterranean (Almería–Níjar Basin, SE Spain)

Abstract

Bioherms are common in the pre-evaporitic Messinian in Neogene basins in SE Spain. Coral bioherms reported to date in the region have been interpreted as shallow-water reefs based on coral morphology (stick-like Porites and Tarbellastraea domes) and surrounding sediments. However, at the western margin of the Almería–Níjar Basin, coral bioherms grew in relatively deep waters with low turbidity on the eastern shelf of the upland precursor to the Sierra de Gádor. Bioherms occur in the lower part of a reef unit, in some cases growing directly on the erosion surface separating this unit from the underlying older Miocene rocks. They are formed mainly by Porites, with minor Tarbellastraea, encrusted by foraminifera, coralline algae and microbialite. The thickness of bioherms ranges from less than 1 m to about 20 m in the largest bioherm. Mesophotic conditions can be inferred from direct palaeodepth measurements at the outcrop, showing that bioherms started to develop at several tens of metres water depth. The dominance of thin, laminar, contorted Porites colonies with few finger-like projections is also indicative of low light levels. Facies laterally equivalent to the bioherms—namely algal/bivalve wackestone to packstone, rhodolith rudstone rich in coralline algae of the order Hapalidiales, and Halimeda floatstone to rudstone—are all characteristic of relatively deep waters in the Mediterranean Neogene. The low siliciclastic content of bioherms and surrounding deposits (<8%) suggests that low light conditions in which bioherms grew were primarily due to water depth. The Almería bioherms are the first record of Upper Miocene blue mesophotic reefs in the world.