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A quantitative study of trace fossil abundance in the Middle Eocene deepmarine 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 postdepositional trace fossils, as well as the number of graphoglyptid ichnospecies were quantified. In the more laterally confined and channeldominated 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 tracefossil diversity and number of pre-depositional trace fossils including graphoglyptids from the channel-lobe transition to the fan-fringe. The tracefossil assemblages of the Ainsa-Jaca basin are characteristic of a number of subichnofacies 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 subichnofacies trace-fossil assemblage.

International audienceA holistic view of the Bengal–Nicobar Fan system requires sampling the full sedimentary section of the Nicobar Fan, which was achieved for the first time by International Ocean Discovery Program (IODP) Expedition 362 west of North Sumatra. We identified a distinct rise in sediment accumulation rate (SAR) beginning ∼9.5 Ma and reaching 250–350 m/Myr in the 9.5–2 Ma interval, which equal or far exceed rates on the Bengal Fan at similar latitudes. This marked rise in SAR and a constant Himalayan-derived provenance necessitates a major restructuring of sediment routing in the Bengal–Nicobar submarine fan. This coincides with the inversion of the Eastern Himalayan Shillong Plateau and encroachment of the west-propagating Indo–Burmese wedge, which reduced continental accommodation space and increased sediment supply directly to the fan. Our results challenge a commonly held view that changes in sediment flux seen in the Bengal–Nicobar submarine fan were caused by discrete tectonic or climatic events acting on the Himalayan–Tibetan Plateau. Instead, an interplay of tectonic and climatic processes caused the fan system to develop by punctuated changes rather than gradual progradation