Trace fossils as diagnostic indicators of deep‐marine environments, Middle Eocene Ainsa‐Jaca basin, Spanish Pyrenees

Abstract A quantitative study of trace fossil abundance in the Middle Eocene deep‐marine clastic systems, Ainsa‐Jaca basin, Spanish Pyrenees, shows that they are powerful discriminators of submarine fan and related environments. Sixteen fan and related environments are recognized from upper‐slope gully to the distal basin‐floor. For each environment, the degree of bioturbation (density), trace‐fossil diversity, number of pre‐depositional and post‐depositional trace fossils, as well as the number of graphoglyptid ichnospecies were quantified. In the more laterally confined and channel‐dominated Ainsa basin, there is a trend of increasing bioturbation intensity and trace‐fossil diversity away from channel‐axis to off‐axis environments. In the more unconfined and distal Jaca basin, there is a trend of increasing trace‐fossil diversity and number of pre‐depositional trace fossils including graphoglyptids from the channel‐lobe transition to the fan‐fringe. The trace‐fossil assemblages of the Ainsa‐Jaca basin are characteristic of a number of sub‐ichnofacies of the Nereites ichnofacies. In the distal Jaca basin, the Paleodictyon sub‐ichnofacies occurs in the lobe‐fringe and fan‐fringe, whereas the distal basin‐floor has a trace‐fossil assemblage typical of the Paleodictyon sub‐ichnofacies, but with a high proportion of post‐depositional fodinichnia. Trace‐fossil assemblages of proximal basin, axial, environments are characteristic of the Ophiomorpha rudis sub‐ichnofacies, whilst proximal off‐axis environments, have a mixed Paleodictyon‐Ophiomorpha rudis sub‐ichnofacies trace‐fossil assemblage.