Neogene rhodoliths in the mediterranean basins

Abstract

Rhodoliths are common components in Neogene shallow-water marine deposits from the Mediterranean and Paratethys regions. Rhodolith-rich deposits (rhodolith beds) appear to be most widespread in the Langhian (middle Miocene) and late Tortonian-early Messinian (late Miocene) intervals and their occurrences decline in the Piacenzian (late Pliocene). Numerous shallow-water platforms surrounded relatively small islands in the Mediterranean region from the Burdigalian to the early Messinian. In the Miocene subtropical to warm-temperate climatic context of the region, these platforms with limited siliclastic input were favourable places for carbonate production by coralline algae. Rhodoliths were major components of carbonate factories on ramps, with maximum concentrations in middle to proximal outer zones. Rhodoliths may occur among coral patches (mainly Porites) in inner-ramp deposits, but most rhodolith beds formed basinwards of corals, seagrass meadows or submarine dune fields (shoals) on the middle and proximal outer ramp. They also occur in lagoons, on reef tops, and on fore-reef slopes in rimmed platforms; and can be locally abundant in siliciclastic systems, in intervals of interrupted or reduced sediment influx. The occurrence of rhodolith concentrations is not indicative of any precise palaeodepth range within shallow-water platforms (from the shoreline to about 120 m), although most commonly formed at palaeodepths of several tens of meters. No clear patterns of variation in rhodolith shape and internal growth forms of coralline thalli can be detected along palaeodepth gradients, although loose branching growths occur mainly in the deepest concentrations. Rhodoliths formed in intermediate palaeodepths tend to comprise more complex combinations of coralline growth forms than shallower and deeper nodules. The taxonomic composition of algal assemblages in rhodoliths usually changes with palaeobathimetry. Spongites or Lithophyllum, and minor Neogoniolithon are abundant in the shallower rhodoliths, whereas they are less common or absent in deeper-water assemblages, which comprise mainly species of Mesophyllum, Lithothamnion, and Sporolithon. The number of species of Spongites (and probably of other genera) apparently decreases throughout the Neogene in the region. By the early Pliocene, components of coralline assemblages are similar to those forming rhodoliths in the present-day Mediterranean (about 90 % of species are extant ones).