Trace fossils from the Middle and Upper Eocene (Bartonian–Priabonian) molasse deposits of the Pamplona Basin (Navarre, western Pyrenees): palaeoenvironmental implications

Abstract

An ichnological study was carried out in the Middle–Upper Eocene sedimentary succession of the Pamplona Basin. This succession represents the molasse stage of the western part of the South Pyrenean peripheral foreland basin and extends from deep-marine turbiditic (Ezkaba Sandstone Formation) to deltaic (Pamplona Marl, Ardanatz Sandstone and Ilundain Marl formations) and marginal marine deposits (Liédena Sandstone Member, Gendulain Formation). The tectonosedimentary evolution of the basin can also be recognized by means of the ichnoassemblages preserved throughout the succession and by the ichnofacies analysis. A total of 23 ichnogenera and at least 28 ichnospecies have been identified. The assemblage of the Ezkaba Sandstone is characteristic of the deep-sea Nereites ichnofacies. The Ophiomorpha rudis and the Paleodictyon ichnosubfacies can be differentiated in this unit, which are typical of channel axis and off-axis environments. Trace fossils in the Ardanatz Sandstone and Ilundain Marl formations, with dominance of Thalassinoides and especially Ophiomorpha, could be related to the Cruziana ichnofacies, developed in relatively low-energy environments, in semicohesive muddy substrates with intercalated silt and sand. The ichnofauna of the Liédena Sandstone Member is composed of bird tracks, and invertebrate horizontal and vertical structures. Microbial mat impressions also occur. The evidence suggests that microbial mats acted as a taphonomic bias favouring the preservation of vertebrate tracks and conditioning the presence of other trace fossils. Consequently, the assignation of this ichnofauna to a specific ichnofacies is tentative, most likely a combination of the Mermia and Scoyenia ichnofacies.