Mesophotic normality: Lower Miocene coral bioherms pioneering the incipient Red Sea

The southernmost Miocene coral bioherms in western Saudi Arabia (Wadi Waqb Member) record a short episode of coral growth in the incipient Red Sea that was terminated by a salinity crisis. Here, we investigate the paleoecological significance of these coral-dominated deposits to better understand ecosystem dynamics during the early stages of Red Sea basin evolution. The coral bioherms form mound stacks on a ramp-like slope morphology predefined by an unconformity on the underlying Precambrian to Cenozoic basement complex. Thus, they are distinctly different from modern tropical coral reefs with their typical geometry of shallow fringing reef and back-reef lagoons. The Miocene bioherms are embedded in fine-grained sediment containing planktonic foraminifera, suggesting an open marine setting below storm wave base, consistent with meso- to oligophotic conditions. Euphotic allochems such as Halimeda plates and benthic foraminifera (including abundant epiphytic taxa) are interpreted to have been shed downslope from shallow water areas such as seagrass meadows. Notably, the large benthic foraminifer Miogypsina globulina is recorded for the first time in Saudi Arabia, allowing us to assign the lower investigated succession to the Burdigalian and to identify the Burdigalian–Langhian transition, thus refining the previous age determination of the Wadi Waqb Member based on planktonic foraminifera. The precise age enables correlation of the investigated deposits in a context with upper Oligocene to Lower/Middle Miocene successions in the Mediterranean, revealing strong paleoenvironmental similarities, including the dominance of massive coral growth, similar to modern meso-oligophotic settings. As the Wadi Waqb Member was deposited before the global cooling trend of the Middle to Late Miocene, our findings support the notion that the ecology of early Cenozoic coral ecosystems was fundamentally different from modern corals reefs that migrated upward in the water column into shallower water environments during the Tortonian. • First description of southernmost archive in KSA of a critical time of coral evolution. • First description of southernmost archive in KSA of critical time of Red Sea opening. • First strict biostratigraphical definition of the pioneering coral bioherm formation. • Ecological interpretation of non-actualistic coral ecosystems. • Relating this non-actualistic appearance to biological evolution and conditions in incipient Red Sea.