Cenozoic trace fossils of the Cruziana, Zoophycos and Nereites ichnofacies from the Fuegian Andes, Argentina
AbstractThe systematics, paleoenvironmental implications, and diversity of Cenozoic trace fossils from the Fuegian Andes are studied. The relatively complete Paleocene-Miocene stratigraphic column includes ichnoassemblages of the Cruziana (Leticia Formation, late Middle Eocene), Zoophycos and Nereites (Early Eocene-Early Miocene turbidite systems) ichnofacies. The last two ichnoasemblages contain the only known deep marine Cenozoic trace fossils in Argentina. The late Middle Eocene Leticia Formation represents a transgressive-regressive cycle and bears the ichnogenera Curvolithus, Diplocraterion, Gyrochorte, Rosselia, Patagonichnus, Asterosoma, Palaeophycus, Paradictyodora, Planolites, Rhizocorallium, Schaubcylindrichnus, Taenidium, and Teichichnus. Ichnogenera of the Early Eocene-Early Miocene turbidite systems include Scolicia, Chondrites, Gyrophyllites, Nereites, Phycodes, Phycosiphon, Phymatoderma, Stelloglyphus, Zoophycos, Ophiomorpha and graphoglyptids. Graphoglyptids are dominated by Paleodictyon, Helicolithus, Helminthorhaphe, Desmograpton and Megagrapton. They are recorded in thin-bedded turbidites and mudstones (lobe deposits) and assigned to the Paleodictyon ichnosubfacies (Nereites ichnofacies). Ophiomorpha rudis and O. annulata are common at the contact between thick-bedded turbidites and mudstones, with abundant plant fragments. In sandrich, proximal channel-lobe deposits, they characterize the Ophiomorpha rudis ichnosubfacies (Nereites ichnofacies). Scolicia prisca and Nereites isp. are common in rippled fine-grained sandstones interbedded with thin mudstones. Zoophycos ispp. are dominant in slope mudstones with synsedimentary slumping. The maximum ichnodiversity is recorded in the late Middle-Late Eocene; which is concomitant with a marked cooling trend. The basal Oligocene displays an abrupt drop in diversity, whereas the Early Miocene shows a moderate diversity. These data do not support the alleged control of increased Eocene ichnodiversity by global warming during the Cenozoic thermal maximum. Specialized food competition, particularly for the graphoglyptid organisms, and generalized oligotrophy seem to offer a better explanation.
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