Halimeda-rich beds in a mixed carbonate system from a pre-evaporitic Messinian reef (Crete Island, Greece)

Halimeda bioherms are gaining increasing attention in the scientific community due to their significance as carbonate producers and as a stratigraphic and paleoecological archive. Currently, the genus Halimeda is found in both tropical and temperate environments, playing a key role in carbonate sediment production owing to its rapid calcification processes. Halimeda can give rise to localized or extensive multi-kilometric bioherms, such as those observed in the northwestern part of the Australian Great Barrier Reef and other tropical areas in the Caribbean and Indian Ocean. Despite their widespread presence in modern oceans, Halimeda bioherms are poorly documented during the Cenozoic, showing a discontinuous stratigraphic distribution. Specifically, these bioherms are primarily described in carbonate Messinian pre-evaporitic successions in a few locations around the Mediterranean Sea, even though their first occurrence as reef builders has been documented since the Oligocene. In this study we present data carried out from stratigraphic and sedimentological analyses conducted on Halimeda-rich deposits belong to the Varvara Formation (Tortonian-Messinian) which crops out predominantly in the Neogenic Heraklion Basin (central Crete Island). Field observations integrated with thin section analysis and taphonomy assessments on algal thalli, allowed the identification of the main facies, primarily represented by Halimeda rudstones-floatstones, along with abundant encrusting organisms like crustose red algae, serpulids and acervulinids. These Halimeda-rich deposits consist of both small Halimeda bioherms, embedded inside clay-rich intervals, and resedimented layers characterized by bed sets with floatstone textures, approximately 1.5 meters thick, alternating with layers dominated by epiphytes-rich packstones and thin clay-rich beds. The remarkable presence of Halimeda in the lower Messinian successions on Crete Island is more significant than previously documented. We deduce that the occurrence and development of Halimeda within the Heraklion Basin are intricately tied to turbid-water nutrient-rich environments. The abundance of nutrients is correlated with the structural context, given that this basin represents a graben surrounded by prominent mountain chains. Furthermore, it is noteworthy that the potential impact of upwelling currents cannot be dismissed, as evidenced by the extensive modern analogues of Halimeda bioherms in the Australian Great Barrier Reef.