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Revision of the trace fossil Megagrapton Książkiewicz, 1968 with focus on Megagrapton aequale Seilacher, 1977 from the lower Eocene of the Lesser Caucasus in Georgia

Uchman Alfred, Lebanidze Zurab, Beridze Tamar, Kobakhidze Nino, Lobzhanidze Koba, Khutsishvili Sophio, Chagelishvili Rusudan, Makadze Davit, Koiava Kakha, Khundadze Nino

Geological Quarterly

2022

Vol. 66, No. 1

art. no. 8

Megagrapton Książkiewicz, 1968 is a characteristic deep-sea trace fossil belonging to the group of graphoglyptids and mostly preserved as a network of irregular meshes in hypichnial semirelief. So far, eleven ichnospecies have been distinguished under this ichnogenus, though commonly on weak evidence. The so-far poorly known ichnospecies Megagrapton aequale Seilacher, 1977 is described here on the basis of the numerous, newly discovered specimens from deep-sea siliciclastic deposits of the Bolevani Subsuite (lower Eocene) in the Lesser Caucasus of Georgia, together with other collections and published examples. A neotype of this ichnospecies is designated and the diagnosis emended. M. aequale occurs in lower Cambrian to upper Miocene deep-sea turbiditic deposits, mostly in the Paleogene. It is characterized by relatively small, variable meshes, which have mostly irregular sub-pentagonal, sub-hexagonal or sub-heptagonal shapes that are variable in size and are bordered by curved or straight semicircular ridges. It has been mistaken for Paleodictyon, which forms regular hexagonal nets. Paleodictyon imperfectum Seilacher, 1977 is included in M. aequale as the ichnosubspecies M. a. imperfectum, which is characterized by relatively thin bordering ridges. After critical analysis of all ichnospecies, only M. irregulare Książkiewicz, 1968, M. submontanum (Azpeitia Moros, 1933), and M. aequale are recommended for further use. These are distinguished on the basis of the prevailing morphology of the meshes, irrespective of large differences in morphometric parameters within the ichnospecies. Irredictyon chaos Vialov, 1972 is included in M. irregulare as the ichnosubspecies M. i. chaos, which is characterized by relatively thick bordering ridges. Megagrapton is interpreted as a cast of a subsurface open burrow network with a few connections to the sea floor. The burrows probably functioned as a trap for small organisms (ethological subcategory irretichnia).

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

Olivero E., Lopez C. M., Torres Carbonell P., Malumian N.

Acta Geologica Polonica

2010

Vol. 60, no. 1

77-91

Graphoglyptids are diagnostic ichnofossils of the Paleodictyon ichnosubfacies (Nereites ichnofacies), which is well represented in deep-marine Mesozoic.Cenozoic thin-bedded turbidites. However, unusual shallow-water records of Mesozoic-Cenozoic Paleodictyon and particular preservational restrictions of graphoglyptid burrows introduce the question of whether graphoglyptids are reliable bathymetric indicators. We document and discuss another unusual graphoglyptid association preserved in shallow-marine, high-energy, organic-rich, and bioturbated turbidites of leveed channels in the upper middle Eocene CCa member, Cerro Colorado Formation, Fuegian Andes. The member includes 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 three horizons with Desmograpton, Glockerichnus, Helicolithus, Helminthorhaphe, Megagrapton, Paleodictyon and Urohelminthoida. Associated beds are lenticular, channeled sandstone turbidites with marked basal erosion surfaces and variable proportions of interbedded mudstone-sandstone with high content of plant debris. Trace fossils in the channeled sandstones 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 beds within a highly energetic and bioturbated interval seems to support the concept that preservational restrictions on graphoglyptid burrows could be locally more important than bathymetric constraints.