Crustose algal frameworks from the Eocene Alpine Foreland

Abstract

Upper Eocene algal limestones of the Eastern Alpine Foreland in Austria contain three different types of crustose algal frameworks. These reveal several similarities with the present-day coralligene de plateau of the Mediterranean Sea, but the framework types differ in the crust-forming algal taxa, the sedimentary regime, and the environmental setting. (1) The coralline-coral frameworks develop from siliciclastic sediments and show a succession from coverstones to bindstone to cruststone. Initiation of this framework in a siliciclastic regime is only possible because of the occurrence of Neogoniolithon sp., a species that is able to grow on fine-grained soft substrate. The onset of a dense crustose framework was related to the reduced sediment input and the occurrence of Lithothamnion sp., which is able to form cruststones. (2) Pure coralline algal frameworks are restricted to cruststones dominated by both Lithothamnion sp. and Neogoniolithon sp. They formed on top of maerl megaripples during calm periods and shed crustose fragments towards depressions, which allowed framework progradation. (3) Peyssonneliacean dominated frameworks occur with intercalations of coralline algal crusts. They formed in muddy environments interpreted as protected depressions between maerl dunes. We found that crustose frameworks are controlled by three factors: (1) Sediment input. Low input favors the growth of frameworks, while coarser grained siliciclastics cause burial of initial algal crusts. Terrigenous mud influences water turbidity and thus favors the occurrence of peyssonneliacean algae. (2) Taxonomic composition. Framework formation requires the existence of crustose algal taxa, which are able to grow directly on fine-grained soft substrate ( Neogoniolithon sp., Polystrata alba), or to encrust and stabilize coarse-grained particles ( Lithothamnion sp.), and/or to form multiple successions of dense crusts ( Lithothamnion sp., P. alba). Phymatolithon sp. characteristically encrusts corals, but it does not contribute to the frameworks. Surprisingly, algae do never form protuberances within the frameworks, even if they show protuberant growth forms in maerl sediment. (3) Substrate stability as a function of substrate composition and hydrodynamic energy. Due to their occurrence on maerl bottoms and the lack of evidence for submarine hardgrounds, the settlement of crustose buildups requires low tidal current activity and/or wave agitation.