Provenance analysis of Oligocene autochthonous and allochthonous coralline algae: a quantitative approach towards reconstructing transported assemblages

Abstract

The Oligocene shallow-water carbonates of the Lower Inn Valley (Tyrol, Northern Calcareous Alps) contain a rich coralline algal flora. These carbonates are known from two distinct settings: (1) autochthonous limestones and (2) debris flows intercalated with deeper water marls. The carbonate facies are dominated not only by crustose coralline algae (Corallinaceae, Rhodophyta), but also by smaller and larger benthic foraminifers, bryozoans, corals as well as lithoclasts of Triassic origin. Five species of coralline algae are identified and described in detail: Lithoporella melobesioides, Lithothamnion sp. A, Lithothamnion sp. B, Mesophyllum sp., and Sporolithon sp. The distribution of algal taxa shows distinct relationships to non-algal biota and substrates: L. melobesioides preferably encrusts other coralline algae, Lithothamnion sp. A is found encrusting bryozoans, Lithothamnion sp. B encrusts corals, while Mesophyllum sp. grows directly on fine-grained, soft substrates. Besides the fact that algal taxa reveal close relationships to other components, it can also be shown that algal assemblages are highly correlated to carbonate facies types. Hierarchical cluster analysis based on the relative abundance of taxa reveals five coralline algal assemblages showing distinct distribution patterns among carbonate facies. While two of the assemblages are restricted to the debris flows, the other three were found in both debris flows and autochthonous occurrences. A correlation between algal assemblages and carbonate facies allows the following conclusions to be made: (1) Both coralline algal taxa in particular and algal assemblages as a whole show distinct relations to water depth (herein referred to as a summary of light conditions and hydrodynamic energy) and substrate relationships. This makes them potentially valuable indicators for palaeoecological reconstructions. (2) Most of the coralline algal assemblages in allochthonous occurrences (i.e. debris flow) can be traced back to facies patterns in autochthonous occurrences. This indicates that a thorough microfacies analysis combined with a systematic study of specific components can be very useful in the reconstruction of palaeoenvironments, even if these are not preserved in their original context. (3) Some of the coralline algal assemblages within debris flows cannot be traced to autochthonous facies and do not seem to reflect primary facies compositions as they contain an atypical mixture of algal taxa and other components. These are characterised by a relatively high abundance of lithoclasts (usually >20% of the components) and a high degree of fragmentation of coralline algae thalli.