Eocene graphoglyptids from shallow-marine, highenergy, organic-rich, and bioturbated turbidites, Fuegian Andes, Argentina

E.B. OLIVERO; LOPEZ CABRERA, M.I; TORRES CARBONELL, P.

ACTA GEOLOGICA POLONICA

WYDAWNICTWO NAUKOWE INVIT

Lugar: Warzawa; Año: 2010 vol. 60 p. 77 - 91

0001-5709

Graphoglyptids are diagnostic ichnofossils of the Paleodictyon ichnosubfacies (Nereites ichnofacies), which is wellrepresented in deep-marineMesozoic–Cenozoic thin-bedded turbidites. However, unusual shallow-water records ofMesozoic–Cenozoic Paleodictyon and particular preservational restrictions of graphoglyptid burrows introduce thequestion of whether graphoglyptids are reliable bathymetric indicators.We document and discuss another unusualgraphoglyptid association preserved in shallow-marine, high-energy, organic-rich, and bioturbated turbidites of leveedchannels in the upper middle Eocene CCa member, Cerro Colorado Formation, Fuegian Andes. The memberincludes the facies associations: 1) mudstones, 2) interbedded mudstones and thin-bedded Tbc turbidites, and 3)thick-bedded sandstones andmudstones. Facies association 3), interpreted as channel deposits, records at least threehorizonswithDesmograpton,Glockerichnus,Helicolithus,Helminthorhaphe,Megagrapton, Paleodictyon andUrohelminthoida.Associated beds are lenticular, channeled sandstone turbidites withmarked basal erosion surfaces andvariable proportions of interbeddedmudstone-sandstone with high content of plant debris. Trace fossils in the channeledsandstones are dominated by Ophiomorpha rudis and O. annulata; mudstones within the thick-bedded mudstone-sandstone beds bear Nereites, Phycosiphon, Zoophycos and Paradictyodora, with subordinate Schaubcylindrichnus,Tasselia and Scolicia. The unusual preservation of limited graphoglyptid-bearing bedswithin a highlyenergetic and bioturbated interval seems to support the concept that preservational restrictions on graphoglyptidburrows could be locally more important than bathymetric constraints.