Gyrolithes as a proxy in the evolution of a river-influenced bay: an integrated ichnological and micropalaeontological approach in the Lower Pliocene of the Bajo Segura Basin, SE Spain

Gyrolithes is a well-known trace fossil characterized by a typical corkscrew appearance and particularly common in Cenozoic shallow-marine deposits. In this paper, the variation of Gyrolithes abundance in a Lower Pliocene section (southern sector of the Bajo Segura Basin, SE Spain) deposited after the flooding following the Messinian Salinity Crisis is analysed, in order to define the main palaeoecological parameters driving the presence of this trace fossil in a marginal marine scenario. An integrated study based on ichnological, sedimentological and micropalaeontological data was carried out, these latter focused on planktic and benthic foraminifera, with the aim of a better definition of the palaeoenvironment and its evolution. The section is made up of interbedded sandy mudstones, where the Gyrolithes-Thalassinoides composite burrow develops, and fine-grained hummocky cross-stratified sandstones, colonized by the producer of Ophiomorpha nodosa. A progressive shifting from a relatively more confined bay to a more open one connected to marine environment is shown by foraminifera. In this context the decreasing-upward abundance of Gyrolithes in background sediments can be related to an increase in bottom-water salinity, which is consistent with the affinity of this ichnogenus for brackish-water conditions. The higher abundance of this trace fossil in the lower part of the section would indicate higher freshwater discharge due to more restricted conditions related to the configuration of the coastline. Towards the top of the section, the development of a more open environment would favour the dilution of the freshwater influx and the establishment of more normal marine conditions that strongly limits the development of Gyrolithes. This study supports therefore the use of Gyrolithes as a proxy for even slight salinity anomalies in marginal marine settings.