Are Miocene-age enigmatic carbonates in the incipient Red Sea of microbial origin? A morphological study

Abstract Hitherto undescribed fine-grained carbonate deposits of inferred microbial origin unconformably rest atop a truncated basement complex along the western coastline of northwestern Saudi Arabia. These carbonates form drapes on the basement complex as well as mounds aligned along a dominant fracture direction (ENE-WSW strike). They are directly overlain by uppermost Burdigalian to Langhian strata of the Wadi Waqb Member (Jabal Kibrit Formation), which feature the earliest coral bioherms associated with the nascent Red Sea, suggesting an early to middle Miocene age for the underlying putative microbialites. Outcrop geometries reveal that the carbonate unit has undergone no significant post-depositional tilting and is largely preserved at its original depositional angle (up to 35°). Thin-section petrography and X-ray diffraction (XRD) analysis confirm that the carbonates are predominantly composed of dolomite. Micromorphological features, including textures consistent with mineralized extracellular polymeric substances (EPS), and capsule-like forms reminiscent of bacterial remains, closely resemble those found in microbialites from modern hypersaline or highly alkaline lacustrine environments. Similarly, crystal splays resemble occurrence in the fossil record that have been interpreted to be indicative of high-alkalinity conditions. Taken together, the observations suggest a restricted environment characterized by elevated levels of salinity and/or alkalinity during the early stages of Red Sea rifting and underline that microorganisms played a significant role in carbonate deposition during this stage. The stratigraphic relationship between the microbialites and the overlying coral-bearing deposits indicates that the basin evolved into a fully marine environment in the late Burdigalian.