A taphonomic approach to coral reef study: examples from the Oligocene of the Piedmont Basin (NW Italy) and Gornji Grad Beds (Slovenia).

Taphonomy is the study of those post-mortem processes which affect organisms and their skeletons through biological, physical and chemical alterations. The taphonomic history of a fossil assemblage can provide a wide range of information concerning environmental conditions and ecological relationships of the living community including causes of death, biological and physical processes on the sediment surface, rates of transport, surface residence times before burial and diagenesis. Taphonomic analysis can ultimately reveal processes and patterns not readily recognizable by other palaeoecological or sedimentological approaches. A far as coral reefs are concerned, there has been the tendency to consider them as examples of preserved life assemblages. As a consequence, taphonomic approaches to the study of corals and coral reefs are relatively rare. Nevertheless, the development and demise of coral reef frameworks and the preservation potential of both in situ and rubble coral assemblages are strictly related to taphonomic processes such as, for example, physical disturbance (storm events and terrestrial run-off) and bioinfestation on both living and dead corals. Taphonomic signatures are especially significant in reconstructing environmental conditions associated to scleractinian-dominated turbid-water bioconstructions (TWBs) thriving in episodically or permanently turbid waters where marine environmental parameters often approach the threshold levels for tropical carbonate production.Coral assemblages from the Oligocene of the Piedmont Basin (NW Italy) and the Gornji Grad Beds (Slovenia) represent an example of “reefs” developed in nearshore deltaic turbid-water settings. Within these two case studies we present here preliminary results of a taphonomic approach using microfacies analysis as a tool to infer the incidence of controlling factors as terrigenous input, reduced light penetration and turbidity. Our analysis is particularly focused on primary processes occurring on coral remains before or during sedimentation, especially fragmentation and bioinfestation. Microfacies analysis shows: 1) generally low incidences of fragmentation; 2) low rates of abrasion of coral clast boundaries; 3) an inverse relationship between fragmentation and bioinfestation. Bioinfestation is a process involving both bioerosion and encrustation. In contrast to bioerosion, which represents a destructive process due to grazers and boring organisms (such as clionid sponges and boring bivalves), encrustation is a constructive and stabilizing process mainly represented in the studied material by coralline algae and encrusting foraminifera. This process can affect both living and dead substrates. With respect to bioinfestation: 1) Encrustation and bioerosion are typically sparsely distributed among coral clasts and also within the same coral fragment; 2) Bioerosion is inversely correlated with terrigenous input; 3) The degree of infestation by boring organisms is also dependent on coral morphology and skeletal architecture: in fact branching corals are more susceptible to bioerosion and among these, Stylophora is more susceptible than Acropora; 4) The dependence of encrustation on a specific controlling ecological factor is still unclear. Through this taphonomic approach it was possible to infer for both study areas a muddy environment with a generally low hydrodynamic regime, dominated at least occasionally by high turbidity and/or sedimentation. Sediment-resistants and fast-growing coral morphologies were promoted and the production of dead coral rubble was frequent, especially during phases of abundant terrigenous run-off from rivers. Nevertheless, the rate of transport for coral clasts was low, such that these assemblages were deposited almost in situ. Surface residence times of coral rubble before burial was very variable and was probably the main factor controlling incidences of bioinfestation, which is typically not homogenous.